Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) with the participation of ISAS and NASA.
star formation, IS dust, spectroscopy, continuum emission
Together with L. Metcalfe and K. Okumura we pointed with ISOCAM towards this target and detected at 15 emission in a disk like structure, extending at least 45'' or 15000AU at the distance of the star (300pc).
Further we employed a ``coronographic'' observing mode of ISOCAM and detected a disk like quite asymmetrical emission structure at 4.5 . The ``coronagraphic'' ISOCAM mode rejects the light of the central source by causing it to fall off the edge of the small Fabry-mirror and thus allowing for a high enough dynamic range to detect faint emission nearby a bright target (Metcalfe, Siebenmorgen, Okumura, 1998).
A promising approach of searching for true protostars is to investigate regions of ongoing star formation at mm/submm wavelengths. In a systematic investigation of all known Herbig Haro energy sources at mm/submm wavelengths which includes mapping of dark clouds at high spatial resolution with IRAM 30m, SEST, JCMT; a new class of young stellar objects (assigned to "Class 0") have been found. All of them are strong mm/submm sources and among them are the coldest and densest dust condensations ever found (for instance, Chini et al. 1993, AA272,L5). Their known characteristics are:
a) From spectral energy distributions (SED) between 350 and 1300 microns the typical dust temperatures are about 10K.
b) Associated masses, as derived from the dust emission, are comparable or greater than the Jeans mass.
c) Molecular line data indicate that the molecules are partially frozen out.
It was predicted that Class 0 are opaque enough to be detected in absorption against the diffuse background (e.g. Buss & Yorke 1990; Fig. 10 in Siebenmorgen et al. 1992)
The ISOCAM image I took in collaboration with E. Krügel and R. Chini
towards the Class 0 source HH108MMS (Chini et al. 1997) shows indeed
this source in absorption against the diffuse
background (Figure 1).
Although there is blending with zodiacal light, one can derive from the absorption feature an optical depth at 15 of about 4. This translates into a visual extinction of Av more than 80mag. The direct measurement of dust extinction at this wavelength allows to confine the dust properties in this object which are certainly modified in the cold and dense environment of the protostar (Krügel & Siebenmorgen 1994). The ISOCAM image suggests that besides the core, we see in HH108MMS an extended disk in absorption against the diffuse interstellar background which makes this source even more remarkable (Siebenmorgen, Krügel, Chini, 1998).
The young star HD 97300 is located in the Chamalion I cloud (Whittet et al. 1997), at distance 188 pc. We report the detection of an extended, ring-like structure around HD 97300, whose emission is dominated by the infrared emission bands at 6.2, 7.7, 8.7, 11.3 and 12.5 m, (hereafter IEBs), observed in our own and other galaxies wherever neutral matter is exposed to UV radiation (see the many papers presenting CVF and SWS spectra in the special issue of Astronomy and Astrophysics on ISO published in November 1996).
The ring is not symmetric around the star, but is much more extended in the SE than in the NW direction. The off-center location of the star with respect to the ring may be the effect of a density gradient in the SE-NW direction in the outer region of the Chamalion I cloud where HD 97300 lies. The position of the star in our images coincides with the emission peaks seen near the center in lw4 and lw5, and with the secondary peak of the emission seen in lw8, roughly at the same position. In the lw8 and lw9 filters we detect a second peak of emission, about 3 (240 AU) north of the star.
Fig. 3 shows CVF spectra between 5.8 and 13.8 m in
8 positions roughly aligned along P.A. = 142.4o.
The simplest picture we can construct to model the band shapes is to consider that the bands can be described by classical oscillators. The absorption coefficient ( ; Schutte et al. (1993)) of such a driven damped oscillator is a Lorentzian profile
Note that an excited level in an atom has an average lifetime , where A is the Einstein coefficient for spontaneous transition, the probability to find the atom there decays like e-A t. Because of Heisenberg's uncertainty principle , the energy of the upper level is then only defined to an accuracy . This leads naturally to a Lorentzian emission profile, and A may be identified with the damping constant, . In this picture, the line width is determined by the timescale . For PAH resonances with a width of m, the characteristic time would be 10-12s. This would imply immense values for A (1012s-1) as well as for the associated dipole moment Debye because . A way out of the dilemma would be to assume that the IEBs arise from a superposition of many narrow lines.
We know the geometry of the emitting region, i.e., the approximate distance of the grains from the star. For a given distance and stellar luminosity, the integrated 6 to 14m flux is directly proportional to the number of C atoms in PAHs. This is because the PAHs account for the total emission in this spectral region. The fraction of C atoms in PAHs is known to within a factor of three, so we can directly convert the carbon column density derived by fitting the feature intensities into a hydrogen column density . The uncertainty on is probably comparable to the uncertainty that affects its determination from sub-millimeter continuum observations (see, for example, Krügel & Siebenmorgen, 1994). The mass of gas and dust can then be computed from the values of averaged over the region of interest.
We derive a total mass of the circumstellar material in a region of about 0.03 pc radius (33) of about 0.07M of which 0.03M is in the elongated ring structure.
It is possible that the ring results from the interaction of a stellar wind with the environment. In this hypothesis, the material in the ring is swept-up gas and the ring coincides with the inner wall of a three-dimensional cavity created by the wind. A second possibility is that the ring is due to the action of the radiation pressure from the star working on the grains.
Finally, it is interesting that HD 97300 is not the only Herbig AeBe star with a ring. A similar structure (although about 3.5 times larger) is seen in scattered light in the younger and more deeply embedded star LkH198 (Leinert et al. 1991). In that case also the ring has an elliptical shape and the exciting star is shifted from its center.
However, the hypothesis of a proto-planetary disk structure of BD+31 643 must be further confirmed and we want to know if there could be planets in the outer parts of the binary disc? Nearby IRAS18331-0035 and HH108MMS there are other absorbing ``knots'' visible in the ISOCAM images. One wants to uniquely resolve them. To unambiguously measure the spectral energy distribution of the secondary peak of HD 97300 one needs a higher spatial resolving power. One is wondering if we see a deeply embedded companion?
With these kind of questions we have an immediate response to the data presented: we need higher spatial resolving power. Some higher resolution observations are certainly existing in the NICMOS data base but just in this moment becoming available with the first light of the VLT.