1996 B2) and implications for nuclear properties

Planet. Space Sci., Vol. 45, No. 6, pp. 735-139, 1997 0 1997 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0032-0633/97 $17.00+0.00

Pergamon

PII: S0032-0633(97)00079-2

X-Band VLA* observations of comet Hyakutake (C/1996 B2) and implications for nuclear properties”f Yanga R. FernBndez,’ Arunav Kundu,‘.’ Carey M. Lisse’ and Michael F. A’Hearn’ ‘Department of Astronomy, University of Maryland, College Park, MD 20742-2421, U.S.A. *Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, U.S.A. Received

30 August

1996; accepted

3 January

1997

Introduction

The nuclei of comets represent the most pristine objects remaining from the birth of the Solar System; detailed studies of these objects can yield information on the dynamics and chemistry of the young system. UnforCorrespondence to: Y. R. Fernandez (Tel: 301-405-1551; Fax: 301-314-9067 ; E-mail : [email protected]) *VLA (Very Large Array) is part of the National Radio Astronomy Observatory, which is a facility of the National Science Foundation, operated under a cooperative agreement by Associated Universities, Inc. tBased on a presentation given at the conference “Asteroids, Comets, Meteors ‘96” at Versailles, France, during 8-12 July, 1996.

tunately, the nucleus of a comet is typically one of the hardest aspects to observe. When the comet is close to the Sun and active, and relatively close to Earth, the nucleus is shrouded in the gas and dust coma. When the comet is far from the Sun, and the coma is weak, the nucleus is faint and hard to discern. As a result, we know just simple properties of the nuclear population, and then the numbers are well constrained for only a handful (roughly a dozen) of the several hundred known comets. The nucleus of comet lP/Halley, from in situ measurements, is the only object for which it might be said a great deal is known, but this means little when trying to understand the ensemble properties of nuclei. Moreover, most of what we do know concerns properties of the short-period population. Reviews of the sparse collection of nuclear knowledge are given by A’Hearn (1988), Belton (1991), and Meech (1997). Clearly, it is extremely important to take advantage of any chance cometary apparition that passes rather close to Earth, especially if the comet is of long period. The long-period comet C/1996 B2 (Hyakutake) presented such a fortuitous situation when it passed 0.10 AU from Earth on UT 25 March, 1996, the closest cometary approach in 13 years. We mounted an intensive, multiwavelength observing campaign in order to image the nucleus and derive its physical properties: e.g. size, temperature, albedo, and rotation rate. Our observations covered the optical, near-infrared, thermal-infrared, and microwave. In this paper we will present our results from the attempt to observe the thermal microwave continuum from the comet’s nucleus with NRAO’s Very Large Array (VLA) ; we are preparing a paper on our detection of the nucleus in the thermal-infrared and our upper-limit to detection in the optical (Lisse et al., 1997).

Observations

and reductions

Our observations consisted of one 12 h track from 0700 to 1900 UT on 27 March, 1996, about two days after the

736

Y. R. Fernandez

comet’s perigee. We observed at 3.55 cm, with a 100 MHz bandwidth, using all of the 27 telescopes (in C array) during the vast majority of the observation. Our flux calibrator was the quasar 3C 286 (IAU name 133 1+ 305, flux of about 5.2Jy), and our phase calibrator was the radio galaxy 4C 76.03 (IAU name 0410+769, flux of about 2.2 Jy) (Perley and Taylor, 1996). Individual integrations 10 s long ; the total amount of time spent integrating on the comet was about 10 h. Phase stability during the track was good. During the observation, the comet itself was between 0.121 and 0.13 1 AU from Earth, 1.OO and 0.989 AU from the Sun, and the phase angle was between 85.7” and 90.2” (“third-quarter” phase). The ephemeris that was used at the time of the observation was provided by D. K. Yeomans (private communication), but, owing to the proximity of the comet, had formal positional uncertainties of 8 arcsec in right ascension and 2 arcsec in declination (I-(T). The proper motion of the comet ranged from 4.7 to 5.5arcsec per integration time, and decreased by about 0.065 arcsec per integration time per hour. The VLA telescopes can only track the linear proper motion of a source. But, due to the rapid motion of the comet, we had to take into account the second-order motion. We updated the linear tracking rates every 33.5min so that the array was always pointed within one synthesized beam (3.5 arcsec) of the most accurate formal position of the comet available at the time. The standard data reduction, using the Astronomical Image Processing System (AIPS), yielded no significant signal from the comet. One problem we considered was that, due to the positional uncertainty of the comet, the signal might have been smeared during the course of our observation. A more accurate ephemeris of the comet, produced by D. K. Yeomans a few weeks after perigee, revealed that our tracking rates were not significantly in error (off by 10.01 arcsec per integration time), but the center of pointing was indeed offset from the true position of the comet by up to 6.6 arcsec at times during our observing run. It should be pointed out here that the HPBW of the VLA telescopes at our observing frequency is 5.3 arcmin and primary beam attenuation is negligible for these offsets. In essence, the amplitude and phase of the complex integration data points were correct, but the projected baselines (or uu-spacings) of the telescopes are slightly offset, because of the difference in the observed and actual position of the comet. We recalculated the correct uv-spacings for each baseline for every 10s integration, and inserted these corrected spacings into the dataset. After the correction, we obtained a more robust map.’ ‘This problem is important to consider for any interferometric observation of a fast-moving object, because of the inherent uncertainty in the ephemeris and in the method data are taken at the observatory. To stress this latter point, it was important that, in order to correctly reduce our data, we understand intimately where the telescopes point at a given time, and what information pertaining to that are stored as “data”. In this case the “data” were not correct, and we were forced to alter them by hand. Since VLA (or any radio interferometric array) typically does not perform observations of this sort, we emphasize the special requirements of the data analyst in this situation. A description of some of the problems with interferometric observations of close, fast objects is given by de Pater et al. (1994).

rt al.: X-Band

VLA observations

of comet Hyakutake

Our CLEAN map, with 2- and 3-0 contours, is shown in Fig. 1. Since we had directly manipulated the data (via the corrections), we would expect the signature from the comet to appear in the middle of the map. The synthesized beam-3.5arcsec wide-is shown in the lower left. The field of view in this map is 4.2arcmin by 4.2arcmin, slightly smaller than the HPBW of the telescopes’ primary beam. With such a synthesized beam size, the nucleus would have appeared as a point source. The r.m.s. noise in our map is 7.6 pJy per beam. There appear to be some linear structures or streaks running generally northeast-southwest in the map. These may be extended background sources passing through the field of view as the array tracked the comet, or due to a few anomalous visibility-data points. It is important to stress, however, that the structures appear only on the land 2-a, level, and as such are not statistically significant. A histogram of pixel values is well described (to better than 2%) by a Gaussian of g = 7.6pJy and shows no excess of pixels l-, 2-, or 3-a, from the mean. In addition, the structures have a low intensity. Integrating over the roughly several hundred synthesized beams that they cover, the flux density is about I-1OmJy. Had the hypothetical structures been tracked, they would have covered around 30-l 00 synthesized beams, based on the thickness of the streak. This implies their surface brightness was only - lo-1OOpJy per beam.

Implications

and conclusions

Our measurement of the r.m.s. flux in the synthesized beam from the comet allows us to place a 3-o upper limit on the thermal microwave continuum flux of 22.8 ,uJy at 3.55cm. There have been no detections of the thermal microwave continuum from comets at VLA; our 3-o upper limit is much lower than all previous 3-o upper limits : comet Austin (C/1982 Ml = 19828 = 1982 VI) was observed at 6cm (Snyder et al., 1983) with an upper (C/ 1983 limit of 140 pJy ; comet IRA,!-Araki-Alcock Hl = 1983d = 1983 VII) was observed at 2 and 6cm (de Pater et al., 1985) with upper limits of 750 and 9OpJy, respectively; comet Crommelin (27P = 1983n = 1984 IV) was observed at 2cm (Schenewerk et al., 1986) with an upper limit of 136pJy; and comet Halley (lP/1982 Ul = 19821 = 1986 III) was observed at 2cm (Hoban and Baum, 1987) with an upper limit of 100 pJy. To obtain an estimate of the nuclear size, we use (1) where Sj, is the measured flux density from the nucleus (I 22.8 pJy), i the wavelength (3.55 cm), A the geocentric distance (averaging 0.126 AU during the observation), k the Boltzmann constant, T the nuclear temperature, E;,the emissivity at 3.55 cm, and R the nucleus’ effective radius. Radar measurements of comet Halley have indicated that the microwave emissivity is quite high, at least 0.95 (at 12.6 cm (Campbell et al., 1989)) near values measured for Earth-approaching asteroids (0.8-0.9, reviewed in Goldstein et al. (1984)). Since the emission originates from material about one skin-depth down from the actual sur-

Y. R. Fe1-nindez et al.: X-Band VLA observations CLEAN Contour

131

of comet Hyakutake

Mop of C/1996

82. 3.6 cm, 12 hr track,

240 210 180 150 120 90 Relative Right Ascension (arcsec)

60 (East

27 Mar 96

30 0 at left)

Fig. 1. CLEAN contour map showing the null-result from our observations of comet Hyakutake. Contours sh0.w the 2- and 3-g levels. The synthesized beam size is at lower left. Since we had inserted the correct uvspacings by hand into the data, the comet should appear as a point source at the center of the map. R.m.s. noise is 7.6pJy per beam. Some of the linear structures running northeast to southwest are possibly background sources passing through VLA’s field of view during the track

face of the nucleus, and the skin-depth (dependent on the material’s composition and proportional to the wavelength modulo the complex component of the refractive index) is on the order of a few decimeters, the nucleus must be smooth on length scales of this size. The temperature is ; the smoothness of the surface less well-constrained implies the thermal behavior a few decimeters deep is important. Data from the most recent Halley apparition indicate the nucleus is porous and thermally resistive (review by Keller (1990)), so the internal temperature does not change much over either a diurnal or an orbital period. In situ measurements of the near nuclear region of Halley by the Giotto and Vega 2 spacecraft imply that the thermal conductivity of the nucleus is low : of the multitude of dust “jets” identified apparently emanating from the nucleus (Smith et al., 1986; Thomas and Keller, 1987), there is only a hint of one of them coming from the night-side (identified by Smith et al.). The antisolar hemisphere is too cold, with too little residual heat to drive sublimation once the Sun has set. Lclcations of the nucleus where sublimation is occurring, however, will have a temperature closer to that al: which water ice sublimates, 200 K (Lien, 1990). Considlering all this, a temperature of 1.50 K for the temperature ‘of the nucleus a few decimeters deep is not unreasonable. It is important however to note the limiting cases. If the nucleus were a fast rotator with high conductivity (i.e. isothermal), the temperature would be like that of a black body at 0.995AU, 280K. A very rough surface that does not emit well in centimeter wavelengths (i.e. superheated) but is still very conductive could have an even higher temperature, 300 or 350K. If the nucleus had extremely low conductivity, we might expect a temperature closer to the value the nucleus had upon

formation in the trans-Neptunian region of the Solar System, around 50 K (Rickman, 1991). We have combined all of this information into Fig. 2. Effective radius is plotted vs. temperature, with the four solid curves representing four possible emissivities. The black body temperature is on the dotted line, while the sublimation temperature is indicated by the dashed line. For the true emissivity and observed temperature of the nucleus, the radius could lie anywhere to the left of the appropriate point. For E = 0.9 and T = 150K, the 3-a upper limit to the radius is 3.0 km. This is consistent with the result obtained by Lisse et al. (1997) of 2.4kO.5 km (via their thermal-infrared data), and by Harmon and Ostro (1997) of 1.5 50.5 km (via their Goldstone radar experiment). Our estimate of the radius is not very sensitive to the temperature when T-200-300 K, but it becomes 5 km for the unlikely (but possible) 50 K limit. Our observations were simultaneous with many of the ROSAT observations of Hyakutake that occurred a few days after perigee (Lisse et al., 1996). Since non-thermal radio emission is frequently coincident with X-ray emission from other astrophysical sources (e.g. supernova remnants, quasars, etc.), we were motivated to place an upper limit on the microwave emission from the location of the X-rays in Hyakutake’s coma; the VLA field of view overlaps a portion of the X-ray emitting region reported by Lisse et al. (1996). The center of brightness of the Xray emission with respect to the nucleus is about 2’ north and 2.5’ east of the nuclear position (on 27.7 March UT), whereas our map extends only 2.1’ from the nucleus. If we approximate the size of the X-ray emitting region as a 4’-by-8’ oval, with the short axis on the Sun-comet line, then the overlap between the VLA field of view and the

738

Y. R. Fernandez Size and Temperature

of C/1996

El2 Nucleus

et al.: X-Band VLA observations

Implied

by VLA X-band

A=O.126

AU

(avg.

S=22.6E-6

1.5

2.0

2.5

3.0 Effective

3.5 Radius

4.0

12

hr

27

Mar

4.5

over

of comet Hyakutake

Observotions

track)

_

Jy track 96

5.0

5.5

(km)

Fig. 2. Constraints on the radius, temperature, and emissivity of the nucleus based on the calculated upper limit to the flux density. Dotted line shows the black body temperature at 0.995 AU from the Sun ; dashed line shows sublimation temperature of water ice. The true effective radius of the nucleus lies anywhere to the left of the intersection of the true emissivity and the true temperature. For E = 0.9, and given the range of likely temperatures, we set the maximum effective radius to be 3 km

X-ray emitting region is a wedge that contains about 9% of the total X-ray flux, and about 13% of the total VLA field of view. Assuming the microwave flux from the coma follows the same spatial distribution as the X-ray flux, the 3-a upper limit to the microwave flux from the X-ray emitting region is 223mJy. This value is more than a factor of two lower than that reported by Minter and Langston (1996) using the same-size X-ray emitting region of the coma. This result indicates that the mechanism responsible for the production of X-rays is not able to produce much microwave radiation. Lisse et al. (1996) found an X-ray luminosity of 4 x lOI ergs-’ from the coma as measured by the ROSAT HRI, which is sensitive to 0.1-2.0 keV photons. Assuming a power law spectrum, where the intensity is proportional to P (where v is the frequency and CIthe spectra1 index), and using the largest possible effective bandpass (1.9 keV) and our microwave upper limit, we find that 51must be less (i.e. flatter) than 0.59, fairly flat. Some of the mechanisms postulated by Lisse et al. (1996) to explain the X-ray emission (e.g. magnetic field-line reconnection and magnetospheric disruption) would necessarily produce some microwave radiation, but these theories are not yet developed enough to provide a prediction of the microwave flux to compare with our results. However, the derived flatness of the spectrum does rule out synchrotron emission (which typically has a spectra1 index of l-2) as the source of the Xrays, as would be expected from the typically low strength of the interplanetary magnetic field. Summary We have observed the near-Earth comet C/1996 B2 (Hyakutake) with VLA at 3.55 cm as part of an intensive

multiwavelength campaign to constrain the nuclear properties of this object. Our aim was to observe the thermal microwave continuum from the nucleus. The short geocentric distance during the observations (- 0.126 AU) presented special problems with the data reduction, since the ephemeris for the fast-moving comet was not accurate enough at the time we observed it. Corrections to the uv-spacings in the data were necessary, using a more reliable ephemeris produced after the observation, to remove any smearing effects that may have occurred. The nucleus of Hyakutake was too small to detect, but our flux density upper limit is lower than any other from an attempted VLA observation of a comet’s thermal microwave continuum. With some assumptions about the physical nature of the nucleus, we can place an upper limit of 3.0 km on the nucleus’ effective radius, consistent with the thermal-infrared results of Lisse et al. (1997) and the radar results of Harmon and Ostro (1997). In addition, since our VLA track occurred during some of the ROSAT observations of the comet, we can set an upper limit on the microwave flux coming from the X-ray emitting region in the comet’s coma of about 0.22 Jy.

Acknowledgements. The authors appreciate the help extended to us by Brian Butler (NRAO) before and during the observations. Pat Palmer (Chicago) and Ken Sowinsky (NRAO) have played a large part in configuring VLA to observe fast, nearby objects. We are also indebted to Donald K. Yeomans (JPL) for answering our multiple requests for ephemerides. Jim Morgan (UMD) was invaluable in helping us write a reduction routine that would merge with our data. Stephen White (UMD) helped us prepare an observing script that would account for our target’s high proper motion. The authors appreciate the support of a grant from the Planetary Astronomy program of the National Aeronautics and Space Administration. Y.R.F. was supported for

Y. R. Fernandez

et al.: X-Band

part of this work through UMD.

VLA observations

a Graduate

of comet Hyakutake

School Fellowship

from

References A’Hearn, M. F. (1988) Observations of cometary nuclei. Ann. Rev. Earth Planet. Sci. 16, 273-293. Belton, M. J. S. (1991) Characterization of the rotation of cometary nuclei. In Comets in the Post-Halley Era, eds R. L. Newburn Jr, M. Neugebauer and J. Rahe, pp. 691-721. Kluwer Academic, Boston. Campbell, D. B., Harmon, J. -K. and Shapiro, I. I. (1989) Radar observations of comet Halley. Astrophys. J. 338, 1049-l 105. Goldstein, R. M., Jurgens, R. F. and Sekanina, Z. (1984) A radar study of comet IRA!;&Araki-Alcock 1983d. Astron. J. 89, 174551754. Harmon, J. and Ostro, S. J. (11997) in preparation. Hoban, S. and Baum, S. (19:37) A VLA search for 2-cm continuum radiation from comet Halley. Icarus 70, 264-268. Keller, H. U. (1990) The nucleus. In Physics and Chemistry of Comets, ed. W. F. Huebner, pp. 13368. Springer, Berlin. Lien, D. J. (1990) Dust in comets-I. Thermal properties of homogeneous and heterogeneous grains. Astrophys. J. 335, 680-692. Lisse, C. M., Dennerl, K., Englhauser, J., Harden, M., Marshall, F. E., Mumma, M. J., Petre, R., Pye, J. P., Ricketts, M. J., Schmitt, J., Trtimper, J. and West, R. G. (1996) Discovery of X-ray and EUV emission from comet Hyakutake C/1996 B2. Science 274,205-209. Lisse, C. M., Fernandez, Y. R., Kundu, A., A’Hearn, M. F., Dayal, A., Hoffmann, W. F., Deutsch, L. K., Fazio, G. and Hora, J. (1997) The nucleus of comet Hyakutake (C/1996 B2). Icarus (in preparation). Meech, K. J. (1997) Physical properties of comets. In Asteroids, Comets, Meteors ‘96 : Proceedings qf the 10th COSPAR Col-

739

loquium, eds A.-C. Levasseur-Regourd and M. Fulchignoni. Elsevier, Oxford (in preparation). Minter, A. H. and Langston, G. (1996) 8.35 and 14.35GHz continuum observations of comet Hyakutake C/1996 B2. Astrophys. J. Lett. 467, 3740. de Pater, I., Wade, C. M., Houpis, H. L. F. and Palmer, P. (1985) The nondetection of continuum radiation from comet IRAS-ArakiiAlcock (1983d) at 2- to 6-cm wavelengths and its implication on the icy-grain halo theory. Icarus 62, 349% 359. de Pater, I., Palmer, P., Mitchell, D. L., Ostro, S. J., Yeomans, D. K. and Snyder, L. E. (1994) Radar aperture synthesis observations of asteroids. Icarus 111,489.-502. Perley, R. A. and Taylor, G. B. (1996) The VLA Calibrator Manual. National Radio Astronomy Observatory, Socorro, New Mexico. Rickman, H. (1991) The thermal history and structure of cometary nuclei. In Comets in the Post-Halley Era, eds R. L. Newburn Jr, M. Neugebauer and J. Rahe, pp. 733-760. Kluwer Academic, Boston. Schenewerk, M. S., Palmer, P., Snyder, L. E. and de Pater, I. (1986) VLA limits for comets Austin (1982 VI) and P/Crammelin (1983n)evidence for a diffuse OH halo. Astron. J. 92, 166-170. Smith, B., Szego, K., Larson, S., Merenyi, E., Toth, I., Sagdeev, R. Z., Avanesov, G. A., Krasikov, V. A., Shamis, V. A. and Tarnapolski, V. 1. (1986) The spatial distribution of dust jets seen at Vega-2 fly-by. In Proceedings of the 20th ESLAB Symposium on the E.xploration of Halley’s Comet, eds B. Battrick, E. J. Rolfe and R. Reinhard, pp. 3277331. ESA Publications Division, Noordwijk, The Netherlands. Snyder, L. E., Palmer, P. and Wade, C. M. (1983) An upper limit to the microwave continuum radiation from comet Austin (19828). Astrophys. J. Lett. 269, 21-23. Thomas, N. and Keller, H. U. (1987) Fine dust structures in the emission of comet P/Halley observed by the Halley Multicolour Camera on board Giotto. Astron. Astrophys. 187,843846.