1995 O1 (Hale–Bopp): Evidence for an Extended Source in Hale–Bopp

ICARUS

135, 377–388 (1998) IS985990

ARTICLE NO.

Carbonyl Sulfide in Comets C/1996 B2 (Hyakutake) and C/1995 O1 (Hale–Bopp): Evidence for an Extended Source in Hale–Bopp Neil Dello Russo and Michael A. DiSanti Laboratory for Extraterrestrial Physics, NASA Goddard Space Flight Center, Code 690.2, Greenbelt, Maryland 20771; Department of Physics, The Catholic University of America, Washington, DC 20064 E-mail: [email protected]

Michael J. Mumma Laboratory for Extraterrestrial Physics, NASA Goddard Space Flight Center, Code 690, Greenbelt, Maryland 20771

Karen Magee-Sauer Department of Chemistry and Physics, Rowan University, Glassboro, New Jersey 08028

and Terrence W. Rettig Department of Physics and Astronomy, University of Notre Dame, Notre Dame, Indiana 46556 Received January 14, 1998; revised May 5, 1998

INTRODUCTION Carbonyl sulfide was detected in Comet Hale–Bopp (C/1995 O1) using high dispersion infrared spectroscopy at the NASA Infrared Telescope Facility on Mauna Kea, Hawaii. Six lines (P10, P11, P12, P14, P15, and P17) of the n3 fundamental band near 4.85 mm were detected on UT April 16.0, April 30.0, and May 1.2, 1997, and two additional lines (P18 and P22) were detected on April 30.0. Water was detected directly through emission lines of its n1 – n2 hot band on UT April 16.0, April 30.0, and May 1.2, with a rotational temperature of 93 6 16 K on May 1.2. Adopting a rotational temperature of 100 K for OCS, our derived production rates are (3.80 6 0.67) and (1.77 6 0.85) 3 1028 molecules s21 on April 16.0 and May 1.2, respectively. The measured production rates imply mixing ratios (QOCS /QH2O) of (4.58 6 0.77) and (2.76 6 1.34) 3 1023 on April 16.0 and May 1.2, respectively. Comparison of spatial profiles for OCS, H2O, and the dust continuum derived from spectra taken less than 1 h apart suggests that on April 16.0 a significant amount of OCS (p70% of the total) was released as an extended source. On April 30.0 and May 1.2 the quality of the data prevent a firm conclusion regarding an extended source contribution. If the nature of the OCS source did not change between April 16.0 and May 1.2, the weighted mean mixing ratio is (4.13 6 0.77) 3 1023. We searched for lines P1 through P8 of the n3 band of OCS in Comet Hyakutake (C/1996 B2) on UT March 24.5, 1996. Our upper limit (3s) for the OCS production rate in Hyakutake is 1.0 3 1027 molecules s21 (QOCS /QH2O , 5.3 3 1023).  1998 Academic Press Key Words: comets; spectroscopy; infrared; carbonyl sulfide; production rates.

Analyses of dust in Comet 1P/Halley and comparison with possible sulfur compounds in cometary ice (e.g., H2S, CS2 , and OCS) demonstrated that most sulfur is contained in the refractory CHON grains (Mumma et al. 1993, Delsemme 1991, Jessberger and Kissel 1991, Jessberger et al. 1988). About 10% of cometary sulfur may reside in the ice, with the sulfuretted ices together comprising p0.5% abundance relative to water (Meier and A’Hearn 1997, Roettger 1991). Since ices are more easily modified than are refractory grains, the present sulfur chemistry may reveal much about the origins of these ices and their subsequent processing history. The determination of the sulfur inventory in comets is thus an important piece to the puzzle of cometary origins. Molecular chemistry in dense interstellar cloud cores suggests several possible sources of volatile sulfur in comets, including H2S, H2CS, SO2 , and OCS. These should be present in cometary nuclei if they formed from icy-mantled interstellar grains as was inferred for Hyakutake (Mumma 1996, Greenberg 1982). H2S was first detected at radio frequencies in Comet Austin 1989c1 (Colom et al. 1990) and Comet Levy 1990c (Bockelee-Morvan et al. 1990), both with abundances p2 3 1023 relative to water. It was detected in Comets Hyakutake (Woodney et al. 1997a) and Hale–Bopp (Biver et al. 1997) with similar relative abundances. Although H2S is a major source of cometary

377 0019-1035/98 $25.00 Copyright  1998 by Academic Press All rights of reproduction in any form reserved.

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atomic sulfur, accounting for about half the inventory for volatile sulfur, recent modeling of the S I triplet near ˚ in several comets demonstrates that at least one 1814 A additional source of sulfur is needed (Meier and A’Hearn 1997). H2CS appears to be only a minor contributor to the inventory of cometary sulfur, with an abundance of p5.4 3 1026 molecules s21 (p1024 relative to water) in Comet Hale–Bopp (Woodney et al. 1997b). Upper limits on SO2 abundance (relative to water) range from ,5 3 1024 in Comet Bowell to ,8 3 1027 in Comet Iras–Araki– Alcock; SO2 /H2O , 2 3 1025 for P/Halley (Kim and A’Hearn 1991), suggesting that SO2 is not a major reservoir of sulfur in comets. However, recent data at millimeter wavelengths suggest that SO and SO2 are more abundant in Comet Hale–Bopp than in previous comets and SO2 may approach an abundance of 1023 relative to water (Wink et al. 1997, Lis et al. 1997, Bockelee-Morvan et al. 1997). Based on IUE observations of S and CS (Feldman et al. 1985, Smith et al. 1980, Feldman et al. 1980, Feldman and Brune 1976), Jackson et al. (1982) proposed that cometary CS should be the daughter of a very short lived parent such as CS2 . CS2 has a photodissociation lifetime of about 350 s at 1 AU (Huebner et al. 1992). Dissociation of CS2 should also produce atomic S. However, analyses of IUE spectra show that the abundance of CS2 (as inferred from CS) can account for only a small fraction of the observed volatile sulfur (Meier and A’Hearn 1997, Azoulay and Festou 1986). Carbonyl sulfide (OCS) is routinely detected in molecular clouds associated with star forming regions (Palumbo et al. 1995, Turner 1989, Jefferts et al. 1971). It is abundant, and is thus a strong candidate for the missing reservoir of volatile sulfur in comets; however, early searches for cometary OCS yielded only upper limits. Combes et al. (1988) reported QOCS /QH2O , 8 3 1023 (at the 2.5s confidence level) in Comet 1P/Halley, based on the Vega-1 IKS spectrum. Bockelee-Morvan et al. (1990) derived a mixing ratio for Comet Levy of ,2 3 1023 (3s), based on a search for the J 5 18–17 rotational transition at 219 GHz. DiSanti et al. (1992) found a mixing ratio of ,5.5 3 1023 (3s) in Comet Austin, based on a search for the n3 band at 4.85 em (2060 cm21). Cometary carbonyl sulfide was detected for the first time in Comet Hyakutake, at 145.95 GHz (Woodney et al. 1997a). Subsequently, OCS was detected in both rotational (Woodney et al. 1997b, Woodney pers. commun., Lis et al. 1997) and vibrational (Dello Russo et al. 1997, Reuter et al. 1997) emission in Comet Hale–Bopp. We report here the first quantitative study of OCS in comets at infrared wavelengths and present evidence that it has both direct and extended sources.

OBSERVATIONS AND DATA ANALYSIS

High dispersion infrared spectroscopy provides favorable prospects for studies of OCS because of high fluorescence efficiency (the g-factor for the n3 band at 4.85 em is p3 3 1023 s21 at 1 AU; Crovisier 1987), and the possibility of detecting multiple ro-vibrational lines in a single grating setting. The high spatial resolution obtained at infrared wavelength permits investigation of the spatial distribution close to the nucleus where both direct (nuclear) and extended sources may contribute to the production of OCS. We used the cryogenic echelle spectrometer (CSHELL) mounted on NASA’s IRTF 3-m telescope on Mauna Kea to search for OCS in Comets Hyakutake and Hale–Bopp. CSHELL has a 256 3 256-pixel InSb array detector with pixel size of 0.20, and provides one-dimensional spatial coverage along the 300 (150 detector rows) slit length. For our comet observations, we used a 10 wide slit, for which the spectral resolving power n /Dn p 2–3 3 104 (the exact value depends on the sharpness of the cometary emissions within the slit). On UT March 24, 1996, we searched for OCS n3 emission in Comet Hyakutake with the grating centered at 2060 cm21 (4.854 em; Table I). The heliocentric distance of the comet for this date was Rh 5 1.06 AU, while the geocentric distance D 5 0.106 AU. This grating setting encompassed the P1 through P8 lines, at a dispersion (Dnpix) of 0.0158 cm21 pixel21. The spatial sampling was about 16 km pixel21 at the comet. We searched for OCS in Comet Hale–Bopp on three dates: UT April 16.0 and 30.0 and May 1.2, 1997 (Table I). We set the central wavelength at 2056.3 cm21 (4.863 em), with the spectral grasp encompassing frequencies of the P10 through P17 lines. On April 30 we added a grating setting centered at 2053.1 cm21 (4.871 em) which covered the P17 through P24 lines. For Hale–Bopp, the spatial sampling ranged from 200 to 300 km pixel21 at the comet. Data at each grating setting were acquired using a sequence of four scans (source, sky, sky, source). Flat-fields (using an internal continuum lamp) and dark frames were obtained immediately following each sequence. Sky spectra were obtained by nodding the telescope 29 from the source. This technique provides sky cancellation via pixelby-pixel subtraction (source-sky-sky 1 source). We used 1 min total integration time on-source for Comet Hyakutake and 2 min total on-source for Comet Hale–Bopp per sequence of scans. Images of the comet at 3.5 em were taken before and after each sequence of scans to monitor and correct for cometary drift; these images of comet Hale– Bopp show the morphology of the dust coma (Fig. 1). For each grating setting, spectra of infrared standard stars obtained through a 40 wide slit were used for absolute flux calibration of the comet spectra.

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TABLE I Log of OCS Observations in C/1996 B2 Hyakutake and C/1995 O1 Hale–Bopp Rh (AU)

D (AU)

D˙ km s21

Spectral range (cm21)

1.06

0.106

214.5

2058.0–2062.0

0.947

1.53

125.7

2054.4–2058.3

Apr. 30.0 1997

1.05

1.75

129.7

2051.2–2058.3

May 1.2 1997

1.06

1.77

129.7

2054.4–2058.3

UT Date Hyakutake Mar. 24.5 1996 Hale–Bopp Apr. 16.0 1997

a

n3 OCS lines detected

No. of scans

1 P10, P11, P12, P14, P15, P17 P10, P11, P12, P14, P15, P17, P18, P22 P10, P11, P12, P14, P15, P17

2 3a 1

Due to poor seeing conditions only one scan sequence was used for quantitative analysis.

We developed an extensive set of IDL algorithms, specifically tailored for processing our CSHELL comet observations. Initial processing includes flat-fielding, removal of high dark-current pixels and cosmic ray hits, and registration of spectral frames such that the spectral dimension falls along a row and the spatial dimension is orthogonal to this. The HITRAN 1992 atmospheric model (Rothman et al. 1992) is used to assign a wavelength scale to the spectra and to establish absolute column burdens of the component absorbing species in the terrestrial atmosphere. Subtraction of the synthetic model (binned to the instrumental sampling interval, degraded to the spectral resolution of the comet scans, and normalized to the cometary continuum) from the comet spectra yields the cometary molecular emissions (still convolved with the atmospheric transmittance function; see Fig. 2A). In order to obtain the line flux for a molecular emission, the atmospheric transmittance at the Doppler-shifted line position for the molecular emission is determined from the fully resolved synthetic model. Division of the observed line flux by this monochromatic transmittance yields the true line flux incident at the top of the terrestrial atmosphere. For Comet Hale–Bopp, spatial profiles were determined for each OCS line. In a spectrum that was not continuumsubtracted, we summed over the spectral extent of each OCS line (typically 5 columns). Each strip for an individual OCS line contains emissions from the cometary continuum (plus any uncanceled sky) in addition to the cometary molecular emission. Two additional strips containing no cometary molecular emission were generated, one to either side of the OCS emission. The strips were scaled via division by the mean atmospheric transmittance across each strip. The average profile of the two adjacent continuum strips was subtracted from the OCS-plus-continuum strip to obtain the emission profile for each cometary OCS line. The profile for the line was then multiplied by the mean transmittance across the strip, and divided by the transmit-

tance at the Doppler shifted position of the OCS line. A single OCS profile with improved signal-to-noise ratio was obtained by combining profiles for individual OCS lines measured for each sequence of scans on UT April 16.0 (Fig. 3A). For UT April 30.0 and May 1.2, only a single scan sequence from each date was useful, and these were combined to create a single profile. Slight offsets due to uncanceled sky emission were removed through comparison with the continuum profiles. QUANTITATIVE ANALYSIS: DETERMINATION OF PRODUCTION RATES

Six lines of carbonyl sulfide were detected on UT May 1.2, 1997, in Comet Hale–Bopp (Fig. 2). Although the signal-to-noise ratios in the OCS line emissions are high (p9s at the peak of an OCS emission), the prolific production of dust makes the line to continuum contrast very small (2–3% on May 1 and 1–2% on April 16; Fig. 2B). In order to show these molecular emissions clearly in an onchip spectral extract, a continuum subtraction was performed (Fig. 2A). Random errors in OCS spatial extracts (Fig. 2B) are dominated by photon noise from the telescope, object, and sky. In a 10 3 80 spectral extract taken on May 1.2, the 1s photon noise level was p90 counts in regions of good atmospheric transmittance, higher in regions of strong atmospheric absorptions (Fig. 2C). Systematic errors in the fit of the atmospheric model to the spectrum are not included, but they are small in the region of OCS emissions (Fig. 2B). Since the emission lines are weak relative to the continuum, determination of an accurate rotational temperature is difficult. Analysis of water lines on UT May 1.2 implies a rotational temperature of 93 6 16 K for H2O (Dello Russo et al., in preparation); therefore, when extracting production rates for OCS (Table II), we adopted a rotational temperature of 100 K. When extracting production rates, it was noticed that

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DELLO RUSSO ET AL.

FIG. 1. The morphology of the dust coma of Comet Hale–Bopp as seen at 3.5 em on UT April 16.0 (A, B) and May 1.2 (C, D). East is up, north is toward the left, and the position of the slit and the sunward direction are indicated. The images were obtained through a circular variable filter, which has a spectral resolving power n /Dn p 80. (A, C) Gray-scale images. (B, D) Corresponding isophote maps. The count levels of successive isophotes are given by the expressions: 1250 2 394 (ln x) for B, and 825 2 252 (ln x) for D, where x denotes the isophote number (and x 5 1 for the central isophote). One count corresponds to a flux density of p1 3 10219 W m22 (cm21)21.

the apparent production rate increased for extracts centered off the nucleus compared to extracts centered on the nucleus. Possible reasons for this effect are discussed later. Production rate growth curves were generated by stepping a 10 3 10 box outward from the nucleus in each direction along the slit (Figs. 3B, 3C, 3D). A small correction factor was applied (15%) to account for line flux that falls outside

a five-column (10) extract. The line intensity is a measure of the column density of a species at different spatial positions along the slit, and the global production rates are inferred following model assumptions. Derived production rates are proportional to the signal summed within the box, divided by the fraction of all molecules expected to be included in the box. For this analysis, we assume that all

CARBONYL SULFIDE IN COMET HALE–BOPP

381

FIG. 2. OCS molecular line detections in Comet Hale–Bopp on UT May 1.2. (A) Continuum-subtracted on-chip spatial-spectral image showing the molecular emissions. Extended emission from CN(v 5 1 2 0; R3) is also evident. A spectrum was extracted by summing the signal in Fig. 2A (prior to continuum subtraction) over the rows encompassed by the horizontal bars on the right of Fig. 2A. (B) Cometary spectrum (solid line) extracted from a box (10 3 80, with spatial extent of p10,400 km) centered on the peak continuum intensity (the ‘‘nucleus’’). Comparison with a synthetic atmospheric spectrum (dashed line) reveals six OCS ro-vibrational lines from the n3 band. (C) The 1s stochastic error due to photon noise as a function of wavenumber for the spectrum displayed in B. One count corresponds to a flux density of p8 3 10219 W m22 (cm21)21.

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DELLO RUSSO ET AL.

FIG. 3. Spatial profiles and production rate growth curves for dust, OCS, and H2O in Hale–Bopp on UT April 16.0 (A) Distribution of OCS (solid line), H2O (dashed line), dust (dotted line), and a star (alternating dots and dashes) along the slit. The dust profile is from the OCS spectral region (the dust profile in the H2O spectral region is nearly identical), and the H2O and dust profiles are scaled to the OCS profile in arbitrary units. East and west directions are indicated. A bar corresponding to a distance of 10 is displayed in the top right. (B) Production rate growth curve for the dust, scaled in arbitrary units. Extracts taken east and west of the nucleus are shown (east extracts are solid squares, west extracts are open squares). Production rate growth curves for (C) OCS and (D) H2O (east extracts are solid squares, west extracts are open squares). All points represent 10 3 10 extracts (where 10 p 1100 km) except for the points at p1.1 3 104 km from the nucleus, which are binned 10 3 50 extracts. The horizontal arrows on these points indicate the spatial extent over which the production rates for these binned points were determined.

the molecules are released at the nucleus and flow outward with spherical symmetry (see appendix in Hoban et al. (1991) for a discussion of the formalism). Although cometary material generally does not flow from the nucleus with spherical symmetry, Monte-Carlo models of axisymmetric cometary outflow suggest that production rates determined from a weighted average of extracts taken east and west of the nucleus (Fig. 4) are a good approximation to the true value (Xie and Mumma 1996). When converting line intensities from a spatial profile (Fig. 3A) into production rates, systematic errors due to uncertainties in rotational temperature, flux calibrations, outflow velocity, etc., were evaluated. Stochastic errors in the spatial profiles were determined from the standard

deviation of the molecular line profile obtained by fitting a smooth curve (gaussian 1 polynomial) to the profile. For OCS, stochastic error dominated the systematic errors, so errors in production rates were determined from stochastic errors in the OCS spatial profiles. Stochastic errors in the spatial profiles remain fairly constant along the slit (Fig. 3A); however, errors in derived production rates increase as the line intensity decreases with distance from the nucleus (Figs. 3C, 3D). Although signal-to-noise ratios of spectral extracts centered on the nucleus are high (Fig. 2B), errors in production rates for extracts centered off the nucleus can be large. OCS production rates and abundances relative to water are given in Table II for Comets Hyakutake and Hale–

383

CARBONYL SULFIDE IN COMET HALE–BOPP

FIG. 4. Mean production rate growth curves for OCS (open diamonds), H2O (open squares), and the dust (solid circles). These curves are the weighted average of 10 3 10 extracts taken east and west of the nucleus on UT April 16.0.

Bopp. Our production rates for Hale–Bopp are calculated from a weighted average of 10 3 10 extracts extending over the range 3–120 east and west of the nucleus on April 16.0, and over 2–120 east and west of the nucleus on April 30.0 and May 1.2. The OCS production rate (QOCS , molecules s21), can be expressed in terms of observing geometry, atomic parameters, and the flux contained in one or more emission lines:

OF 5 hcn St OgD N

4fD2

QOCS

i

i51

.

N

OCS

i

i 51

f (x)

1AU

The photodissociation lifetime tOCS (s) and line fluorescence efficiency gi (g-factor, photons s21 molecule21) are

TABLE II Molecular Production Rates for Comets Hyakutake and Hale–Bopp Comet

Species

UT date

Rh (AU)

Hyakutake Hyakutake Hyakutake Hale–Bopp Hale–Bopp Hale–Bopp Hale–Bopp Hale–Bopp Hale–Bopp Hale–Bopp

OCS OCS H2O OCS OCS OCS OCS OCS H2O H2O

Mar. 19 Mar. 24.5 Mar. 24.5 Jan. 29 Feb. 21 Mar. 22 Apr. 16.0 May 1.2h Apr. 16.0 May 1.2

1.164 1.06 1.06 1.41 1.15 0.932 0.947 1.06 0.947 1.06

(1)

Q (molecules s21)

(3.80 (1.77 (8.30 (6.41

6 6 6 6

3.9 3 1026a ,1.0 3 1027c 1.9 3 1029d,e 4.2 3 1027f 6.2 3 1027f 6.1 3 1027g 0.61) 3 1028c 0.85) 3 1028c 0.40) 3 1030e 0.45) 3 1030e

QOCS /QH2O 2.1 3 1023b ,5.3 3 1023

(4.58 6 0.77) 3 1023i (2.76 6 1.34) 3 1023i

Note. Our derived production rates assume tOCS 5 1.05 3 104 s at 1 AU, vgas 5 1.1 3 (R2h 0.5) km s21 (based on outflow velocities measured by Biver et al. (1997)), Trot 5 100 K. a Woodney et al. (1997a). b Using OH production rates derived by Millis and Schleicher (1996) for March 19, QOCS /QOH 5 3 3 1023. c This work. d This is a slight upward revision of the value reported by Mumma et al. (1996). e Dello Russo et al. (Submitted to Science). f Woodney (pers. commun., 1997). g Woodney et al. (1997b). h Data from April 30.0 is also included. i The weighted mean for April 16.0 and May 1.2 is (4.13 6 0.77) 3 1023.

384

DELLO RUSSO ET AL.

both calculated for Rh 5 1 AU, D is the geocentric distance in meters, hcn is the energy (J) of a photon with wavenumber n (cm21), f (x) is the fraction of OCS molecules included in the sampled region (x being the fraction of a photodissociation scale length subtended by the beam radius), and Fi is the flux (W m22) from line i incident atop the terrestrial atmosphere. To derive QOCS , we summed over the P10, P11, P14, and P15 lines, which fell in regions of good atmospheric transmittance for the geocentric velocity of Comet Hale– Bopp on the dates in question (D˙ 5 125.7 km s21 on April 16.0 and 129.7 km s21 on May 1.2). The P12 line was not used because it was blended with a CN emission (Fig. 2A). To evaluate the fraction f (x), we refer to the Appendix of Hoban et al. (1991) which addresses the problem for a rectangular aperture. Using a photodissociation lifetime of 1.05 3 104 s at 1 AU (Crovisier 1994), we estimate f (x) 5 1.49 3 1022; i.e., only p1.5% of all OCS molecules in the coma should lie within a rectangular aperture 10 in width and 100 in length, extending 2.5–12.50 off the nucleus in either the east or west direction. Our derived production rates are relatively insensitive to the adopted rotational temperature (100 K); for example, QOCS 5 (3.51 6 0.62) and (4.14 6 0.74) 3 1028 molecules s21 for rotational temperatures of 80 and 120 K, respectively, on UT April 16.0 (water production rates are similarly insensitive to rotational temperature). Our derived production rates for Comet Hale–Bopp are larger than values derived from millimeter measurements at comparable heliocentric distances (Table II); the reason for this difference is unknown. The production rates in Table II show that OCS is a major contributor to the volatile sulfur inventory in Comet Hale– Bopp and, along with H2S (and for some comets SO2), can account for the inferred volatile sulfur inventory in comets (Meier and A’Hearn 1997). Production rate growth curves for other volatiles detected in Comet Hyakutake show behavior similar to that of Hale–Bopp growth curves; therefore, in determining an upper limit for OCS production in Hyakutake extracts centered off the nucleus were used. We obtain an upper limit for QOCS in Comet Hyakutake from the maximum line flux consistent with a 3s noise level in an average of 10 3 30 extracts centered 20 east and west of the nucleus. This value (,1.0 3 1027 molecules s21 on UT March 24.5) is higher than, and is therefore consistent with, the value reported by Woodney et al. (1997a) for UT March 19, 1996. DISCUSSION: PRODUCTION RATE GROWTH CURVES

The production rate growth curve for OCS on UT April 16.0 shows some interesting effects which may have implications for the way OCS is released from the nucleus. Figures 1A, 1B, 3A, and 3B illustrate the asymmetry of dust outflow from Comet Hale–Bopp on UT April 16.0.

The dust as measured from thermal emission shows a greater extension to the west than to the east (Fig. 3B; the dust extract shown in Figs. 3A and 3B are taken from the OCS grating setting; dust extracts taken from water grating settings are nearly identical). The production of OCS was also greater in the west direction (Fig. 3C) with QOCS 5 (4.77 6 1.00) and (3.01 6 0.90) 3 1028 for west and east extracts, respectively. However, an east–west asymmetry is not apparent in H2O on this date (Fig. 3D). The strong correlation between the spatial distributions of OCS and dust, and the lack of spatial correlation with the dominant volatile released from the nucleus (H2O), suggests that OCS production is associated with the dust—or that H2O and OCS are not uniformly mixed in the nucleus. Other differences were found among OCS, H2O, and dust, and these are most clearly revealed by the rate of increase in the derived production rates with distance from the nucleus (Fig. 4). To generate scaled production rates for the dust, Qdust is set equal to QOCS on the nucleus, and tdust @ tOCS is assumed. The production rates for water and dust increase by about the same ratio, and they reach their steady values at about the same distance from the nucleus (Fig. 4); this is consistent with H2O being released primarily as a parent, directly from the nucleus (Dello Russo et al., in preparation). The production rate for OCS increases by a much higher factor and reaches its steady value farther from the nucleus than the production rate of the dust (Fig. 4). OCS reaches a steady Q value about four times higher than that seen in the dust profile in Fig. 4, but that steady Q value is only reached p3000–3500 km from the nucleus compared with p1500 km for the dust (and water). The slower increase to a steady production rate in OCS is also clearly seen in the broader extent of the OCS profile compared with the more sharply peaked water and dust profiles (Fig. 3A). The apparent production rate increases for all species for extracts centered off the nucleus compared to extracts centered on the nucleus (Fig. 4). Several factors which could cause apparent increases in the production rate include rotational temperature and outflow velocity variations, atmospheric seeing and cometary drift, optical depth effects, and release of an extended source. We next consider these factors as possible reasons for the differences in the production rate growth curves for OCS, H2O, and the dust (Fig. 4). Could variations of rotational temperature and outflow velocity in the coma affect our derived production rates and reproduce the observed growth curves? We assigned the same rotational temperature and outflow velocity to each spatial position along the slit. A variation of OCS rotational temperature within the coma could cause errors in the derived production rates. However, for the lines measured, the derived production rates for OCS and H2O are relatively insensitive to the adopted rotational temper-

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CARBONYL SULFIDE IN COMET HALE–BOPP

ature (see earlier discussion). Relative line intensities for OCS and H2O extracts centered off the nucleus were not significantly different from extracts centered on the nucleus, indicating that the rotational temperature variations in the coma were not extreme, and cannot explain the different behavior of OCS and H2O growth curves (Fig. 4). A change in outflow velocity with distance from the nucleus could also affect derived production rates. However, in order to explain the different OCS and H2O growth curves, the outflow velocity would have to be several times higher near the nucleus, and much higher for H2O than for OCS in this region, which is very unlikely. Therefore, variations in outflow velocity in the coma are unlikely to be a major cause of differences in production rate growth curves for OCS and H2O. Could turbulence in the atmosphere and cometary drift cause the OCS and H2O production rate growth curves to differ? Auto-tracking with a CCD-guider was used for Comet Hyakutake, but this method could not be used for Comet Hale–Bopp because those observations were conducted during daylight. Drift perpendicular to the slit causes loss of signal, while drift parallel to the slit smears the spatial profiles. Comet images taken before and after the OCS sequence of scans were examined; these showed that the comet drifted ,0.50 during a sequence of scans. Differences in seeing and drift effects for OCS and H2O scans were assessed by comparing the continuum profile generated from an OCS spectrum with continuum profiles generated from water spectra at other grating settings near 5 em taken within p1 h. Continuum profiles generated from H2O settings at 5 em are nearly identical in intensity, width, and shape to generated OCS continuum profiles, indicating similar seeing and drift conditions. Since the OCS profile is much broader than the H2O and dust profiles on UT April 16.0 under similar conditions of seeing and drift (Fig. 3A), atmospheric seeing and cometary drift cannot explain the differences in the OCS and H2O profiles and growth curves. The maximum extent of seeing and drift effects can be estimated by determining the distance from the nucleus where the scaled production rate of the dust (Qdust) reaches a ‘‘steady’’ value (Fig. 3B). This occurs p1000–1500 km (p10) from the nucleus, indicating that atmospheric seeing and cometary drift effects are probably only important within p10 of the nucleus on April 16.0. The apparent production rate will be reduced when the medium becomes optically thick, and this effect may become important close to the nucleus. However, we now show that optical depth effects cannot explain the different growth curves observed for OCS and H2O. We approximate the cometary gas as a population at rest relative to the nucleus (no net outflow) since this represents the worst case (overestimates the optical depth); velocity dispersion along the line-of-sight leads to reduced optical depths. dA(l) is the optical depth at wavelength l for absorption

of a photon and can be expressed dA(l) 5 K0 N, where K0 is the absorption coefficient at line center (cm2 molecule21) and N is the column density of absorbers (molecules cm22). Since cometary infrared lines are seen in emission, dtot(l) 5 dA(l) 1 dE(l), where dE(l) is the optical depth at wavelength l for reabsorption of an emitted photon along the line of sight. The absorption coefficient at line center is expressed as (Pugh and Rao 1976) K0 5

!lnf2 SDn D, Sline

(2)

D

where Sline is the line strength (cm molecule21) and DnD is the Doppler line width (cm21). The Doppler line width is determined from the expression (Pugh and Rao 1976) DnD 5

S DF n0 c

(2kNAT ln 2) M

G

1/2

.

(3)

Here n0 is the position of the line center in wavenumbers, c is the speed of light (cm s21), k is Boltzmann’s constant (erg K21), NA is Avogadro’s number (molecules mole21), T is the gas temperature (K), and M is the molecular weight (g mole21) of the absorbing gas. Column densities for H2O and OCS were determined for a rotational temperature of 100 K. For OCS, the column density can be determined for a given dtot(l) (for dtot(l) 5 1, NOCS p 1.3 3 1015 molecules cm22 for the P14 line). Since the OCS lines result from resonance fluorescence (in a fundamental vibrational band), we assume dE(l) p dA(l). The H2O 101–212 line (n1 – n2 band) emission, although pumped by the n1 fundamental and the n1 1 n3 combination band (Bockelee-Morvan and Crovisier 1989), results from nonresonance fluorescence (a ‘‘hot’’ band); therefore, we assume dE(l) p 0 since optical depth effects in the pumps dominate. For dtot(l) 5 1, NH2O p 3.7 3 1017 molecules cm22. Once an optically thick column density is determined, the distance (R0) from the nucleus (along a line of sight centered on the nucleus) where the coma becomes optically thick can be estimated for each species from N5

Q 4fv

E

e2R/L dR, R0 R 2 y

(4)

where Q is the production rate (molecules s21), v is the outflow velocity (cm s21), and L is the path length (cm). Solving Eq. (4) for dtot(l) 5 1 gives R0 p 200 km for both OCS and H2O on UT April 16.0 (where 10 p 1100 km); therefore, beyond 0.50 from the nucleus optical depth effects are negligible. Also, the water profile and the weighted average water growth curve closely track the dust profile and growth curve, respectively (Figs. 3A, 4),

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suggesting that optical depth effects for H2O, and therefore OCS, can be important only close to the nucleus. Therefore, optical depth effects cannot explain the different growth behavior for OCS, compared with dust and water. Since rotational temperature and velocity variations, seeing, drift, and optical depth effects cannot explain the differences in the OCS and H2O growth curves, we must examine other possible explanations. Could an outburst have occurred at the time of the April 16.0 observations, causing a temporary enhancement of OCS? The dust (measured simultaneously) with OCS did not show an enhancement at this time, and measurements of H2O and dust taken within 1 h of OCS show no evidence of an outburst (Figs. 3A, 4). The dust profiles derived from the OCS and H2O spectral regions are virtually identical, demonstrating that no unusual activity occurred during this time interval. OCS also shows an enhancement in both the east and the west directions relative to the dust (Fig. 3A). Although an outburst rich in OCS and poor in H2O and dust is not ruled out, it is considered unlikely. RELEASE OF OCS AS AN EXTENDED SOURCE

It is possible that OCS is released as an extended source. The production rate growth curves and spatial profiles (Figs. 3A, 4) suggest that a significant amount of OCS might be produced off the nucleus on April 16.0. The possible correlation of OCS release with dust release and the fact that most sulfur is contained in the refractory CHON grains suggest that an extended source of OCS may be derived from refractory organic grains. However, since the chemical identity of a parent species and the mechanism for extended OCS release is unclear, an extended contribution from a gaseous species cannot be ruled out. A production rate growth curve for a molecule released exclusively as a nuclear source should track the scaled dust growth curve in the absence of optical depth effects and grain fragmentation (e.g., H2O; Fig. 4). Using this technique, the nuclear source contribution to the total OCS production rate on April 16.0 was p1 3 1028 molecules s21 (Fig. 4). An extended source of OCS will generally be released at some distance from the nucleus, and if released from grains, OCS so produced may have a different outflow velocity from the native population. An outflow model for the extended source is needed to quantify the contribution of an extended source of OCS (and to quantify the total production rate). If the scale length over which the extended component is released is small compared with the scale over which the production rate is determined, then a spherically symmetric, uniform outflow model should give a good approximation when calculating production rates. We derive an extended source contribution of p(2.8 6 0.7) 3 1028 molecules s21 for OCS on April 16.0 by subtracting the inferred native OCS production

FIG. 5. Spatial profiles for OCS (solid line) and scaled dust (dashed line) for UT April 30.0/May 1.2 along the slit (10 p 1300 km). A bar corresponding to a distance of 10 is displayed in the top right.

rate from the total derived OCS production rate. This implies that p70% of the total OCS produced on April 16.0 comes from an extended source. The combined spatial profile for OCS on UT April 30.0/ May 1.2 was compared with the spatial profile of the dust to determine if an extended source was present on this date (Fig. 5). Although a contribution from an extended source on this date is possible, the large stochastic error in the OCS profile prevents a firm conclusion. If an extended source of OCS is correlated with dust production, it is possible that any extended source contribution would decrease from April 16.0 to May 1.2 due to about a twofold decrease in dust intensity between these dates. The expected decrease in dust temperature (p5%, assuming T p Rh20.5) can account for a decrease of p35% in intensity, the remaining change in intensity may imply reduced dust production. SUMMARY

Carbonyl sulfide was detected in Comet Hale–Bopp. Six lines of the n3 fundamental band (P10, P11, P12, P14, P15, and P17) were detected on UT April 16.0, April 30.0, and May 1.2, 1997, and two additional lines were detected on UT April 30.0 (P18 and P22). Adopting a rotational temperature of 100 K for OCS (based on a derived rotational temperature of (93 6 16) K for water on May 1.2), our production rates are (3.80 6 0.67) and (1.77 6 0.85) 3 1028 molecules s21 on April 16.0 and May 1.2, respectively. From the measured production rates, we report a mixing ratio (QOCS /QH2O) of (4.58 6 0.77) and (2.76 6 1.34) 3 1023 on April 16.0 and May 1.2, respectively. If the nature of OCS production was unchanged over this interval, the weighted mean mixing ratio is (4.13 6 0.77) 3 1023. The

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P1 through P8 lines of the n3 band of OCS were searched in Comet Hyakutake (C/1996 B2) on UT March 24.5, 1996. We derive a 3s upper limit for the OCS production rate in Hyakutake of 1.0 3 1027 molecules s21 (QOCS /QH2O , 5.3 3 1023) for UT March 24.5. The apparent production rates for OCS, H2O, and dust increase to steady values at a certain distance from the nucleus in Hale–Bopp. H2O and dust reach steady production rate at about the same distance from the nucleus, but OCS production is much more extended than either dust or water on April 16.0. Comparison of OCS and H2O obtained from spectra taken within 1 h of each other show that their spatial distributions differ within the coma of Hale–Bopp; this effect is intrinsic to the comet and is not an artifact of the observing conditions (the dust profiles were virtually unchanged over this interval). Our observations suggest that a significant amount of OCS (p70% of the total) was produced from an extended source on April 16.0, which may be correlated with the dust, while H2O was released primarily as a parent, directly from the nucleus, and shows no clear correlation with dust production. Data on April 30.0 and May 1.2 are not of sufficient quality to determine the existence of an extended source for OCS; however, an extended source contribution on these dates is not ruled out. This work shows that production rates determined solely from apertures centered on the nucleus may be misleading: the total production rate may be underestimated when the aperture is smaller than the extended source region, but it may be overestimated when the aperture is larger than the extended source region and a nuclear source model is applied. Production rates derived from nucleus-centered extracts are more susceptible to problems such as seeing, drift, and optical depth effects than extracts taken off the nucleus. Examination of the apparent spatial dependence of production rates helps to assess and avoid these problems. Growth curves derived from spatial profiles can then provide important new insights regarding the release of cometary ‘‘parent’’ volatiles. An OCS abundance relative to water of about 2–5 3 1023 on these dates demonstrates that OCS is a major source of volatile cometary sulfur. The presence of abundant OCS in comets and star-forming regions supports the view that comets were at least partially formed from icymantled interstellar grains. ACKNOWLEDGMENTS This work was supported by the NASA Planetary Astronomy Program under RTOP 344-32-03 to M. J. Mumma. We thank D. Schleicher for helpful comments which improved the manuscript. We thank T. Reyes for assistance with figure production. We are grateful for the outstanding support form the staff of the NASA Infrared Telescope Facility. The NASA IRTF is operated by the University of Hawaii under contract to NASA.

REFERENCES Azoulay, G., and M. C. Festou 1986. The abundance of sulfur in comets. In Asteroids, Comets, Meteors II: Proceedings of a Meeting Held at the Astronomical Observatory of the Uppsala University, June 3–6, 1985 (C. I. Lagerkvist, B. A. Lindblad, H. Lundstedt, and H. Rickman, Eds.), pp. 273–277. Uppsala Universitet Reprocentralen HSC, Uppsala. Biver, N., D. Bockelee-Morvan, P. Colom, J. Crovisier, J. K. Davies, W. R. F. Dent, D. Despois, E. Gerard, E. Lellouch, H. Rauer, R. Moreno, and G. Paubert 1997. Evolution of the outgassing of Comet Hale–Bopp (C/1995 O1) from radio observations. Science 275, 1915– 1918. Bockelee-Morvan, D., and J. Crovisier 1989. The nature of the 2.8-em emission feature in cometary spectra. Astron. Astrophys. 216, 278–283. Bockelee-Morvan, D., J. Crovisier, P. Colom, D. Despois, and G. Paubert 1990. Observations of parent molecules in Comets P/Brorsen–Metcalf (1989o). Austin (1989c1), and Levy (1990c) at millimetre wavelengths: HCN, H2S, H2CO, and CH3OH. In Proceedings of the 24th ESLAB Symposium on the Formation of Stars and Planets, and the Evolution of the Solar System, pp. 143–148. ESA SP-315. Bockelee-Morvan, D., J. Wink, D. Despois, N. Biver, P. Colom, J. Crovisier, E. Gerard, B. Germain, E. Lellouch, R. Moreno, G. Paubert, J. K. Davies, W. R. F. Dent, and H. Rauer 1997. Molecular observations of C/1995 O1 (Hale–Bopp) with the IRAM Telescopes near perihelion: Interferometric maps and new parent molecules. Bull. Am. Astron. Soc. 29, 1047. Colom, P., D. Despois, D. Bockelee-Morvan, J. Crovisier, and G. Paubert 1990. Millimetre observations of Comets P/Brorsen-Metcalf (1989o) and Austin (1989c1) with the IRAM 30-m radio telescope. In Workshop on Observations of Recent Comets (W. F. Huebner, P. A. Wehinger, J. Rahe, and I. Konno, Eds.), pp. 80–85. Southwest Research Institute, San Antonio, TX. Combes, M., V. I. Moroz, J. Crovisier, T. Encrenaz, J.-P. Bibring, A. V. Gringoriev, N. F. Sanko, N. Coron, J. F. Crifo, R. Gispert, D. BockeleeMorvan, Yu. V. Nikolsky, V. A. Krasnopolsky, T. Owen, C. Emerich, J. M. Lamarre, and F. Rocard 1988. The 2.5- to 12-em spectrum of Comet Halley from the IKS-VEGA experiment. Icarus 76, 404–436. Crovisier, J. 1987. Rotational and vibrational synthetic spectra of linear parent molecules in comets. Astron. Astrophys. (Suppl.) 68, 223–258. Crovisier, J. 1994. Photodestruction rates for cometary parent molecules. J. Geophys. Res. 99, 3777–3781. Dello Russo, N., M. J. Mumma, M. A. DiSanti, K. Magee-Sauer, R. Novak, and T. Rettig 1997. Int. Astron. Union Circ. 6682. Delsemme, A. H. 1991. Nature and history of organic compounds in comets: An astrophysical view. In Comets in the Post-Halley Era, 1, (R. L. Newburn, Jr., M. Neugebauer, and J. Rahe, Eds.), pp. 377–428. Kluwer Academic, Dordrecht/Norwell, MA. DiSanti, M. A., M. J. Mumma, and J. H. Lacy 1992. A sensitive upper limit to OCS in Comet Austin (1989c1) from a search for n3 emission at 4.85 em. Icarus 97, 155–158. Feldman, P. D., and W. Brune 1976. Carbon production in Comet West 1975n. Astrophys. J. 209, L45–L48. Feldman, P. D., H. A. Weaver, M. C. Festou, M. F. A’Hearn, W. M. Jackson, B. Donn, J. Rahe, A. M. Smith, and P. Benvenuti 1980. IUE observations of the UV spectrum of Comet Bradfield. Nature 286, 132–135. Feldman, P. D., H. A. Weaver, and M. C. Festou 1985. The ultraviolet spectrum of periodic Comet Encke (1980 XI). Icarus 60, 455–463. Greenberg, J. M. 1982. What are comets made of? A model based on interstellar grains. In Comets, (L. L. Wilkening, Ed.), pp. 131–163. Univ. of Arizona Press, Tucson.

388

DELLO RUSSO ET AL.

Hoban, S., M. J. Mumma, D. C. Reuter, M. DiSanti, and R. R. Joyce 1991. A tentative identification of methanol as the progenitor of the 3.52-em emission feature in several comets. Icarus 93, 122–134. Huebner, W. F., J. J. Keady, and S. P. Lyon 1992. Solar photorates for planetary atmospheres and atmospheric pollutants. Astrophys. Space Sci. 195, 1–294. Jackson, W. M., J. B. Halpern, P. D. Feldman, and J. Rahe 1982. Production of CS and S in Comet Bradfield (1979 X). Astron. Astrophys. 107, 385–389.

Mumma, M. J., P. R. Weissman, and S. A. Stern 1993. Comets and the origin of the Solar System: Reading the rosetta stone. In Protostars and Planets III (E. H. Levy, J. I. Lunine, and M. S. Matthews, Eds.), pp. 1177–1252. Univ. of Arizona Press, Tucson. Palumbo, M. E., A. G. G. M. Tielens, and A. T. Tokunaga 1995. Solid carbonyl sulphide (OCS) in W33A. Astrophys. J. 449, 674–680. Pugh, L. A., and K. Narahari Rao 1976. Intensities from infrared spectra. In Molecular Spectroscopy: Modern Research (K. Narahari Rao, Ed.), Vol. 2, pp. 165–227. Academic Press, New York.

Jefferts, K. B., A. A. Penzias, R. W. Wilson, and P. M. Solomon 1971. Detection of interstellar carbonyl sulfide. Astrophys. J. 168, L111–L113.

Reuter, D., D. Jennings, P. Sada, K. Hinkle, S. Ridgway, L. Wallace, P. Bernath, and J. Waller 1997. Int. Astron. Union Circ. 6681.

Jessberger, E. K., and J. Kissel 1991. Chemical properties of cometary dust and a note on carbon isotopes. IN Comets in the Post-Halley Era (R. L. Newburn, Jr., M. Neugebauer, and J. Rahe, Eds.), Vol. 2, pp. 1075–1092. Kluwer Academic, Dordrecht/Norwell, MA. Jessberger, E. K., A. Christoforidis, and J. Kissel 1988. Aspects of the major element composition of Halley’s dust. Nature 332, 691–695. Kim, S. J., and M. F. A’Hearn 1991. Upper limits of SO and SO2 in comets. Icarus 90, 79–95. Lis, D. C., D. Mehringer, D. Benford, M. Gardner, T. G. Phillips, D. Bockelee-Morvan, N. Biver, P. Colom, J. Crovisier, E. Gerard, D. Gautier, D. Despois, and H. Rauer 1997. New molecular species in Comet C/1995 O1 (Hale–Bopp) observed with the Caltech Submillimeter Observatory. Bull. Am. Astron. Soc. 29, 1046. Meier, R., and M. F. A’Hearn 1997. Atomic sulfur in cometary comae ˚ . Icarus 125, 164–194. based on UV spectra of the S I Triplet near 1814 A Millis, R., and D. Schleicher 1996. Int. Astron. Union Circ. 6344. Mumma, M. J. 1996. Hyakutake’s interstellar ices. Nature 383, 581–582. Mumma, M. J., M. A. DiSanti, N. Dello Russo, M. Fomenkova, K. MageeSauer, C. D. Kaminski, and D. X. Xie 1996. Detection of abundant ethane and methane, along with carbon monoxide and water, in Comet C/1996 B2 Hyakutake: Evidence for interstellar origin. Science 272, 1310–1314.

Roettger, E. E. 1991. Comparison of Cometary Comae Using Ultraviolet Spectroscopy: Composition and Variation. Ph.D. dissertation, Johns Hopkins University. Rothman, L. S., R. R. Gamache, R. H. Tipping, C. P. Rinsland, M. A. H. Smith, D. Chris Benner, V. Malathy Devi, J.-M. Flaud, C. Camy-Peyret, A. Perrin, A. Goldman, S. T. Massie, L. R. Brown, and R. A. Toth 1992. The HITRAN molecular database: Editions of 1991 and 1992. J. Quant. Spectrosc. Radiat. Transfer 48, 469–507. Smith, A. M., T. P. Stecher, and L. Caswell 1980. Production of carbon, sulfur, and CS in Comet West. Astrophys. J. 242, 402–410. Turner, B. E. 1989. Recent progess in astrochemistry. Space Sci. Rev. 51, 235–337. Wink, J. E., D. Bockelee-Morvan, N. Biver, P. Colom, J. Crovisier, E. Gerard, and H. Rauer 1997. Int. Astron. Union Circ. 6591. Woodney, L. M., J. McMullin, and M. F. A’Hearn 1997a. Detection of OCS in Comet Hyakutake (C/1996 B2). Planet. Space Sci. 45, 717–719. Woodney, L., J. McMullin, M. A’Hearn, and N. Samarasinha 1997b. Int. Astron. Union Circ. 6607. Xie, X., and M. J. Mumma 1996. Monte Carlo simulation of cometary atmospheres: Application to Comet P/Halley at the time of the Giotto spacecraft encounter. II. Axisymmetric model. Astrophys. J. 464, 457–475.