Mid-IR high angular resolution observations of Herbig Ae/Be stars with CAMIRAS

P.O. Lagage (CEA, DSM/DAPNIA/SAp, (URA 2052 associee au CNRS), CE Saclay; lagage@sapvxa.saclay.cea.fr), S. Cabrit (Obs. de Paris; sylvie.cabrit@obspm.fr) and A. Zavagno (Obs. de Marseilles; Zavagno@obmara.cnrs-mrs.fr)

Despite considerable recent efforts, the circumstellar environment of Herbig Ae/Be stars (i.e optically visible, young emission line stars of intermediate mass, (Herbig G.: 1960, ApJSS, 4, 337)) remains much less well understood than the environment of their low-mass analogues, the T Tauri stars. When it is now generally accepted that T Tauri stars are frequently surrounded by a circumstellar accretion disk (i.e. Bertout C.: 1989, ARA&A, 27, 351), there is a hot debate as to whether Herbig Ae/Be stars are surrounding by disks or envelopes (e.g. Hillebrand L., Strom S., Vrba F. & Keene J.: 1992, ApJ, 397, 613, versus Hartmann L., Kenyon S. & Calvet N.: 1993, ApJ, 407, 219). Mid-IR imaging is one of the key probes to assess the question. Indeed Ae/Be stars are bright enough to heat radiatively their surrounding dust, which, if it consists in a large envelope of small grains transiently heated, could be spatially resolved at 10 microns (Natta A. & Krugel E.: 1995, A&A, 302, 849).

We have used the Saclay mid-IR camera, CAMIRAS (Lagage P.-O. et al.: 1991, 42nd ESO conference, ed. M.H. Ulrich, p. 601), to image several Herbig Ae/Be stars from CFHT. This camera features one of the largest arrays presently available for ground-based high flux mid-IR observations: a 192×128 Si:Ga/DVR array made by the LETI/LIR, Centre d'Etudes Nucleaire de Grenoble, France. The angular resolution achievable at 10 microns is limited by the diffraction: 0.6" on a 3.6m telescope (/D). In this note, we present the results about the environment of two objects of particularly interest: LkH 234 and AFGL 4029.

LkH 234 is a B5e-B7e type star of bolometric luminosity of about 1200 Lsun (Strom S.E. et al.: 1972, ApJ, 173, 353) located in the well studied NGC 7129 star forming region, 1 kpc away from us (Racine R.: 1968, AJ, 73, 233). We first observed this object on August 1991. We used a broad-band N filter encompassing the whole 10 microns atmospheric window and a pixel field of view of 0.4". The usual chopping and nodding technique was used to subtract the huge background generated by the telescope and the atmosphere. As can be seen on Figure 3a, a faint companion, about 10% of the LkH 234 flux, is detected 2.7" north-west from LkH 234. Additional observations of this object were made in July 1996 in the 20 microns atmospheric window. The observations were done again with CAMIRAS, but optimised for observations at 20 microns, (CdTe lenses and entrance window, instead of Germanium). The 20 microns atmospheric window is a poor window and observing from such an excellent site as Mauna Kea is definitely an advantage. The companion now appears as bright as LkH 234 (Figure 3b). A spectrum rising so steeply with wavelength is characteristic of a deeply embedded young stellar object. The companion is spatially associated with shock-excited H2 emission knots, a radio continuum source, and water masers. Hence it is probably responsible for a large fraction of the collimated ejection previously attributed to LkH 234. Such a result suggests that present estimates of the bipolar outflow activity in Herbig Ae/Be stars would need to be re-examined, as the dominant contribution might originate from optically invisible objects. The results shown here are part of a paper by Cabrit, Lagage, McCaughrean and Olofsson in press in A&A.

AFGL 4029 is another interesting Herbig Be stars of high bolometric luminosity (20 000 Lsun; Snell R., Huang Y.-L., Dickman R. & Claussen M.J.: 1988, ApJ, 325, 853) located 2.2 kpc from us (Becker W. & Fenkart R.: 1971, A&AS, 90, 225). We observed this object in July 1996 with 4 narrow-band filters: 8-9 microns, 10.45-10.85 microns, 11-11.45 microns, 11.77-12.22 microns and with a pixel field of view of 0.3". The observations were done partly during day time (thanks John!). At 10 microns, there is no significant difference between the background intensity during day or night. The only problem with day-time observations is guiding. However in the case of AFGL 4029, the object is bright so that we can correct tracking errors, a posteriori, by recentering the individual exposures. As can be seen on Figure 4, there is an extended nebulosity West of AFGL 4029. The emission from this nebulosity appears to be more intense in the 11.3 and 8.6 microns bands, which indicates that the emitters are made of small carboneous material (e.g. Pah's). A careful analysis of the images shows that AFGL 4029 itself is resolved (Zavagno, Lagage & Cabrit in preparation); the spectrum in the resolved circumstellar envelope features 11.3 and 8.6 microns bands, but the band to continuum contrast is lower than in the western nebulosity. As a bonus, thanks to the large field of view which is now available with the 192×128 LETI/LIR array, the HII region 23" to the east of AFGL 4029 (Deharveng L., Zavagno A. et al.: 1996, A&A in press) was also observed. It is clearly resolved (extension more than 8") and also shows the 8.6 and 11.3 micron emission features.

The two examples presented here show how high resolution mid-IR observations can help in understanding the complex environment of Herbig Ae/Be stars. Indeed cold or embedded companions can be revealed. Ambient cloud nebulosity of small carboneous dust or molecules can be detected. Even circumstellar envelopes can be resolved.

Acknowledgements: It is a great pleasure to thanks R. Jouan and P. Masse for perfect technical assistance during the CAMIRAS observations. We also thank E. Pantin for helping us with is filtering and deconvolution technique.

1 une grosse = douze douzaines

Copyright © 1997, Canada-France-Hawaii Telescope Corp. All rights reserved.