IRAS observations of nearby main sequence stars and modeling of excess infrared emission

Adv. Space Res. Vol. 6, No. 7, pp. 43-46, 1986

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IRAS OBSERVATIONS OF NEARBY MAIN SEQUENCE STARS AND MODELING OF EXCESS INFRARED EMISSION D. E. Backman,* F. C. Gillett* and F. J. Low** *Kitt Peak National Observatory, National Optical Astronomy Observatories, P.O. Box 26732, Tucson, AZ 85726, U.S.A. **Departmentof Astronomy, University of Arizona, Tucson, AZ 85721, U.S.A. ABSTRACT IRAS observations of the 50 stellar systems within 5.2 PC reveal 6 systems with significant flux in excess of that expected from photospheric emission at wavelengths of 25 to 100 ~m. No star in this sample has a significant excess at 12 ,~m. Ground-based measurements of the near-infrared flux were used to determine the brightness of the stellar photospheres for extrapolation to the far infrared. Examples of far-infrared excess in the 5-pc sample, in addition to the previously known case of En (Gliese 144), are: r Ceti (Gliese 71), which has an excess at 60 ~m but was not detected at 100 ,um; Ross 128 (Gliese 447), with excess at 60 and 100 ~sm; 61 Cygni (Gliese 820) with excess at SO and 100 rem; and a CMa (Gliese 244) and BD+43’ 4305 (Gliese 873), with excess at 100 sm only. There is cold extended emission from “infrared cirrus” near the line of sight to the latter three stars that may contribute to the apparent excesses. Bri~1i~ far-infrared excesses associated with the main sequence stars a Lyr, a PsA, fi Pic, and En have been interpreted as emission from shells of orbiting particles, possibly connected to the process of planet formation. The excesses determined here are all much fainter than those four. Precise measurement of the source profile shapes and peak positions of the fainter excesses was not possible. For that reason and because of the possibility that background ISM emission is involved, they are considered candidates rather than definite discoveries of new circumstellar particle clouds. The excesses associated with Ross 128 and 61 Cygni are discussed in terms of the characteristic radii and effective radiating areas of clouds of particles that could produce the observed emission. INTRODUCTION a Lyr was the first main sequence star found to have a large infrared excess from cool circumstellar material /1/. Similar excesses around a FaA, fi Pic, and ~ En were discovered during the IRAS mission /2/. Weissman and Harper et al. /3,4/ have proposed that the particles aroufld a Lyr are debris from large bodies like comet nuclei, released by collisional fragmentation or sublimation. The frequency of occurrence and properties of these particle clouds are therefore relevant to discussions about how often and in what manner do planetary systems form. This paper is concerned with (1) results of a systematic search in the IRAS data for excesses from stars in the solar neighborhood, and (2) models of the spatial extent and density of particle clouds capable of producing the observed excesses. The nearby stars provide a typical sample of main sequence stars in our galaxy, dominated by K and M dwarfs. The limit of IRAS sensitivity to the dimmest stellar photospheres was roughly 5 pc, so this sample provides most of the available far-infrared information on individual low-luminosity stars. The normal properties of the stars in this sample will be the subject of a later paper. RESULTS OF THE SEARCH The distance of 5.2 Pc (17 light years) has conventionally been used to define the nearby stars /5,6,7/. Within this volume 50 stellar systems are known in addition to our own, containing 71 stars plus several suspected astrometric companions. The present completeness of the 5-pc sample for objects of known types has been estimated at —~80% /8/. The distribution of spectral types of the stars within 5.2 Pc ~5 shown in Table 1. Our sample consists of all stars with trigonometric parallax r~+ p.e. < 0.192 listed by Gliese or Gliese and Jahreiss /9,10/, pIus LHS 288 /11/ and LHS 292 /5/. The IRAS data considered here consisted of pointed observations for 29 of the 50 stellar systems in the sample, and coadded survey scans for 20 systems. Wolf 359 (Gliese 406), the third nearest stellar system to the sun, lies in the 5% of the sky not covered by the IRAS survey, and was also not the subject of any pointed observation. Table 2 compares the limiting sensitivities of IRAS pointed observations, coadded survey data, and the Point Source Catalog (PSC) at 12 and 60 sm. Near-infrared (1-5 em) photometry of these objects was required to specify the photospheric flux for extrapolation to the IRAS bands. The most sensitive IRAS observations of bright nearby stars have photometric uncertainties relative to standards of
44

D. E. Backman, F. C. Gillett and F. J. Low

TABLE 1

5 Pc stellar sample Spectral type distribution A F G

K M wd TOTAL TABLE 2

2 1 2 9 51 6 71

IRAS sensitivity limits to stellar photospheres at d 12 microns

f pair of pointed observations coadded survey Point Source Catalog

25 mJy 75 ** 250

spec. type* ——

M8 MS

I

5 pc

(S/N

=

3)

60 microns

I f

I I

=

40 mJy 125 ** 400

spec. type* MO KO GO

*latest spectral type detectable **dependent on position Five star systems in the 5-pc sample have significant far-infrared excesses in one or more IRAS bands. They are alpha CMa — Gliese 244 (100 pm only), r Ceti — Gliese 71(60 pm only), En — Gliese 144 (25, 60, and 100 pm) /2,12/; 61 Cygni — Gliese 820 (60 and 100 pm), BD+43 ‘4305 — Gliese 873 (100 pm only), and Ross 128 — Gliese 447 (60 and 100 pm). All of these except BD+43 ‘4305 were observed in pointed mode. In comparison, Aumann /12/ conducted a search of the PSC for excesses associated with stars in the Gliese and Wolley et al. catalogs /9,13/ (d < 25 pc), supplementing the PSC measurements with coadded survey data in cases where excess was suspected. That search resulted in detection of 6 objects in common with our sample: a CMa, a AqI, a CMi, a Ccii, r Ceti, and En. The magnitudes in Aumann’s Table 1 show excesses for a CMa, a Cen, r Ceti, and En, but only En is noted as having a significant excess, defined in that paper as 1121-160] > 1. Excess flux in the present paper is defined as departure from the narrow band on the (M ,L-[far-IR]) plane containing most of the objects detected. This band represents the far-infrared properties of normal stars. For example, the locus of presumed photospheric emission runs from L-[601 — -0.11 at M — +1.42 (Al) to L-]60] — +0.31 at M~ +9.59 (M2), and was extrapolated linearly in magnitudes to dimmer M to evaluate the 60 pm excess of Ross 128. The significance of excesses were calculated using the combined uncertainty resulting from: (1) the noise in the individual measurement; (2) relative photometric uncertainty, which equals approximately 7% for all 4 bands in a pair of pointed observations /14/ and 10-13 % for faint sources in a pair of survey HCONs /15/, and (3) uncertainty introduced by the color correction (<4%) and detector non-linearity (<3%). These excesses are an order of magnitude dimmer than the ones associated with a Lyr, a PsA, ~ Pie, and En. Slow-scan observations of the bright excesses resulted in precise measurement of the centroid positions of the excesses and the source profile shapes, allowing firm identification of the excesses with the stars and calculation of the clouds’ boundary radii and characteristic particle sizes /2/. Such measurements were not possible with the faint excesses considered here. Thus, uncertainty about the amount of background interstellar material in the line of sight to these objects and lack of detailed information about the faint excesses leads us to consider them candidates rather than definite examples of stars with orbiting particle clouds. Future far-infrared studies using, e.g., SIRTF, should be directed at these objects. We will next discuss the individual objects and calculate parameters of particle clouds consistent with the excesses present in two wavelength bands, i.e. for Ross 128 and 61 Cygni. Ross 128 Ross 128 is a single M5 star with a bolometric luminosity of —‘3 x io-~LG at a distance of 3.4 pc. It is the best new case for infrared emission caused by particles around a nearby star. The excess above the expected photospheric flux is 0.12 Jy at 60 pm and 0.22 Jy at 100 pm. It is unlikely that the observed excess is due to superposition of an interstellar cloud because Ross 128 is located in a sparse region of the infrared sky (b — +60’). The SO and 100 pm data indicate a source consistent in size with a point source, peaked 61 arc-sec from the star’s position. Most of the position difference is in the cross-scan direction, in which 61 arc-sec represents a 2.3~discrepancy /14/. The excess has a 60/100 pm color temperature of 46 K that is warmer than isolated cirrus knots /16/. This color temperature corresponds to a distance from the star of --‘2 AU (0.6 arc-see) for large particles with zero albedo and 1, i.e. like ISM grains, Thus the grain sizes 26 arc-see) for inefficient particles with emissivity e oc V observations of En (K2) reveal a cloud are AU not(—.8 constrained by small, the observations. For comparison, the slow-scan extending from r 1

—~

1 AU to r2

200 AU with a lower limit to the particle size of order 10 microns.

Nearby Stars with Infrared Excess

45

2) of the particles around Ross 128 is —2 x 1026 cm2, with a luminosity at X The radiatin~area < 120total pm effective of 3 x i028 erg s . For (4~r~a comparison, the particles around En have a 60/100 pm color temperature of 58 K, an effective area of 6 x i027 cm2, and a luminosity at X < 120 pm of 2 x 1029 erg 5~1/2/. The ratio of the luminosity radiated at X < 120 pm by the particles to the star’s luminosity is 3 x 10~in the Ross 128 system versus 2 x io~for En. 61 Cygni This system is a binary star with a projected component separation of 29 arc-sec (100 AU at d 3.4 pc). The components have spectral types of K5e and 1<7 and luminosities of 0.13 and 0.07 L 0, respectively. 61 Cygni is close to the galactic plane (b -6°), and the surrounding sky shows substantial structure at 60 and 100 pm. Some of the flux attributed.to 61 Cygni at these wavelengths may be from cirrus in the line of sight. If point-source filtering is used, the source at the position of 61 Cyg shows an excess of 0.12 Jy at 60 pm and 1.8 Jy at 100 pm relative to the expected sum of the two photospheres. Point-source filtering yields a lower limit to the flux associated with the star by eliminating a constant background level and essentially counting only the flux within a single beam position. The 60 pm peak is 24 arc-sec from component A in the cross-scan direction, with a 3c uncertainty radius of —.70 arc-sec. This excess has a color temperature of 23 K that is consistent with the presence of interstellar cirrus in the beam. A profile across the source from short scans by single detectors, however, seems to show a stronger source than that which appears in the point-source processing. The 60 and 100 pm background contribution can be estimated from a quadratic function fit to the flux observed farther than 4 arc-mm from the source position. If all the flux above this fit is attributed to the 61 Cygni system, then the excess is 1.8 and 4.1 Jy at 60 and 100 pm, respectively, with a color temperature of 39 K. This can be considered an upper limit to the flux from 61 Cygni. The source is apparently resolved at 60 and 100 pm, with a deconvolved size HWHM 75 arc-sec (--.N-S). In summary, the lower limit to the flux is consistent with the sum of the stellar photospheres plus either a discrete background interstellar cloud or cool material in the 61 Cygni system. If the excess represents superposition of a background source, it is aligned with the 61 Cygni system to a surprising precision. On the other hand, the upper flux limit is consistent with a strong and possibly resolved source with a temperature high enough that it is unlikely to represent part of the ISM. If we assume that the observed excess is entirely emission from material in the 61 Cygni system, then a source color temperature 23 < T < 39 K corresponds to a distance of 50 > r > 20 AU from component A or 40 > r > 15 AU from component B for large particles with zero albedo. Grains with an emissivity ~ are consistent only with the flux upper limit/high temperature case; such grains could be heated by both stars and be located —250 AU from the system center. Inefficient grains can be ruled out for the low temperature limit because the characteristic cloud radius would be —.900 AU (250 arc-sec), much larger than the IRAS beam. The observations are thus consistent with a range of possible situations for the particles, which may be close to either or both components, may envelope the entire system, or be collected near the Lagrange points of the binary orbit. Examination of data from individual detectors does not allow sufficient spatial resolution to identify the centroid of the excess with either one of the binary components because the projection of the component separation on the direction perpendicular to the scans is only 19 arc-sec. 2 > A > 6 x 1027 cm2, the range corresponding to the temperature of thesystem star’s flux is re-radiated at X < 120 pm by The total effective radiating area of the limits. particlesThe in fraction the 61 Cygni is 3 that x 1028 cm particles is in the range 2 x io-~to 9 x io-~assuming component A is the heating source, or 1 x i~-~ to 6 x io-~if both stars are considered together. r Ceti r Ceti is a single G8 star with a luminosity of 0.47 L 0 at a distance of 3.6 pc. This star and e En were the subjects of Project Ozma, the first systematic search for signals from extraterrestrial technical civilizations /17/. The star lies at a galactic latitude of -73°. There is an excess of 0.14 Jy at 60 pm peaked ~ 19 arc-sec from the star. The flux upper limit at 100 pm places a limit on the color temperature of T > 80 K (cf. a Lyr’s excess T —-. 85 K /1/). a CMa and BD+43 ‘4305 These objects present interpretation problems similar to 61 Cygni. They lie at low galactic latitudes of b — -9 (a CMa) and b -13° (BD+43°4305), and there is cirrus emission near the line of sight to both stars. However, the excess appears centered on the star position in both cases. a CMa. This star has a spectral type of Al and a white dwarf companion < 10 arc-sec away. The system has an apparent 100 pm excess of 2.4 Jy. The lower limit on excess at 60 pm implies a color temperature limit of T < 43 K. Thermal emission from solid grains heated by the star would be inconsistent with the observations because even large (“blackbody”) grains would have to be located at r > 200 AU (> 80 arc-sec) to satisfy the temperature limit, and would not have been included in the IRAS beam. BD+43 ° 4305. Also known as EV Lac, this star has a spectral type of M4.5e and is an astrometric binary with a primary orbit amplitude of —.0.04 arc-sec — 0.2 AU at d — 5.0 Pc /7/. The system has an apparent 100 pm excess of 1.93 Jy in coadded survey scans. The lower limit on excess at 60 pm implies a color temperature limit of T < 23 K.

46

D. E. Backman, F. C. Gillett and F. J. Low

SUMMARY Using the most sensitive IRAS observations, we have searched the 50 nearest stellar systems for evidence of excess flux in the far-infrared that could be caused by solid particles around these stars. Five new instances of excesses are found in this volume to the limit of IRAS sensitivity. Ross 128’s excess is detected in two bands and the star is located at high galactic latitude, making it the most likely new candidate for excess interpretable as emission from orbiting particles. r Ceti is also at high galactic latitude, but the presence of excess in only one band prevents identification of the physical process responsible. Three instances of excess might be partly or completely caused by background cirrus: 61 Cygni, BD+43’ 4305, and a CMa. However, the close position coincidence between the stars and the peaks of their respective excesses is evidence that some of the excess flux may be associated with the stars. The upper temperature limit on the excess in the direction of a CMa combined with the star’s distance and the size of the IRAS detectors make it unlikely that the excess is due to the star’s flux re-radiated by grains. 61 Cygni and BD+43 ‘4305 are both binary systems. The possible presence of large orbiting grains in binary systems is interesting because it is normally assumed that binary stars form from high angular momentum protostellar clouds and pre-planetary disks form from low angular momentum clouds /18/. The amount of material necessary to produce an excess at 60 pm detectable above the photospheric flux of a star depends strongly on the star’s spectral type. For a star at d — 5.0 pc with a cloud of particles at an average temperature of 40 K, the excess would not be significant at 60 and 100 pm in IRAS pointed observations unless the ratio of total cloud luminosity (X < 120 pm) to star luminosity is 2 x 10.6 for M — +1.4 (—‘Al), 3 x lO~for M — +4.8 (—‘G2), and 1 x i0~’for M — +—‘12.5 (M5), in the absence of interference from background ISM emission. Clouds around stars in the 5 pc sample with effective optical depths below these limits would not have been detected by IRAS.

1.

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2. Gillett, F. C., in Lieht on Dark Matter , ed. F. P. Israel, Reidel Co., Dordrecht 1985, p. 61. 3. Weissman, P. R., Science 224, 987 (1984) 4. Harper, D. A., Lowenstein, R. F., and Davidson, J. A., ~ 285, 808 (1984) 5. 6. 7.

Dahn, C. C., Liebert, J., and Harrungton, R. S., Astron. J. 91, 621 (1986) Lippincott, S. L., Space Sci. Rev, 22, 153 (1979) van de Kamp, P., Ann. Rev. Astr. Astrop. 9, 103 (1971)

8. Luyten, W. L., M.N.R.A.S. 139, 221 (1968) 9. Gliese, W., Catalogue of Nearby Stars , Verbfientl. Astron. Rechen-Instituts, Heidelberg, Nr. 22., 1969. 10. Gliese, W. and Jahreiss, H., Astron.Ap. 38, 423 (1979) 11. 12. 13. 14.

lanna, P. A. and Bessel, M. S., P.A.S.P. 98, 658 (1986) Aumann, H. H., P.A.S.P. 97, 885 (1985) WoIley, R., Epps, E. A., Penston, M. J., and Pocock, S. B., Royal Obs. Ann. No. 5 (1970) R. Cutri, private communication (1986)

15. Beichman, C. A., Neugebauer, G., Habung, H. 3., Clegg, P. E., and Chester, T. J., Infrared Astronomical Satellite (IRAS~Catalogs and Atlases Explanatory Supplement (1985). 16. Low, F. 3. et al., Ap. 3. Lett. 278, L19 (1984) 17. Drake, F. D., Phys. Today 14, 40 (1961) 18. Safronov, V. S., and Ruzmaikuna, T. V., in Protostars and Planets H, eds. D. C. Black and M. S. Matthews, University of Arizona Press, Tucson 1985, p. 959.