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The Evolution of Stars seen with ISOCAM - the Need for NICMOS and VLT Follow Up.

R. Siebenmorgen ISO Science Operations Centre, Astrophysics Division of ESA, Villafranca del Castillo, P.O. Box 50727, E-28080 Madrid

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.

 

 

Abstract:

Three examples are presented of how ISOCAM¹ sees the process of stellar evolution. i) Standard staring and coronographic observations with ISOCAM towards BD+31 643 are presented. The morphology of the detected emission is quite similar to a proto-planetary disk. ii) The class 0 object HH108MMS is detected at 15 mic. in absorption against the diffuse interstellar background. This detection confirms its protostellar nature. iii) A ring of organic matter is detected around the pre-main-sequence Herbig AeBe star HD97300. The images show extended emission, an elliptical ring structure of size about 0.045x0.03 pc as well as two peaks of emission, separated by about 3" (240 AU). One of the two peaks coincides with the position of HD97300, while the other may be an embedded companion. The data show that the emission in this region is dominated by the infrared emission bands centered at 6.2, 7.7, 8.7, 11.3 and 12.5$\mu m$, with a very small contribution from continuum emission at longer wavelengths. The spectra are fit with a three component dust model which includes organic molecules such as polycyclic aromatic hydrocarbons, very small graphite and very small silicates, as well as large grains. The fit to the ISOCAM data is very good if one applies a classical oscillator model for the infrared emission bands. The new fitting procedure allows to estimate the total mass of the ring, which is  0.03M$_{\odot }$.

star formation, IS dust, spectroscopy, continuum emission

Introduction

The Infrared Space Observatory (ISO) completed its observations at 14:00 CET on 16th of May 1998. The targets of ISO had been all over in the Universe: in the Solar System, towards Comets and the Zodiacal Light, Giant Molecular Clouds, Protoplanetary Disks, the Galaxy and still further away to Interacting Systems and QSO in distant Cluster. The observatory performed a total of 40000 IR observations and successfully executed as of end March 1998 a total of 26220 science observations. In this contribution I present three observations performed with the infrared camera ISOCAM (Cesarsky et al., 1996) on board of ISO (Kessler et al., 1996). Those observations are dedicated to the process of stellar evolution. Together with L. Metcalfe and K. Okumura we observed in a coronographic mode a potential proto planetary disk system. This experiment is somehow unique since no real coronograph was mounted on ISO. The second experiment, performed with E. Krügel and R. Chini, shows the detection of a protostar in absorption against the diffuse background. I close discussing the nature of a ring detected around a Herbig Ae/Be star (Siebenmorgen, Natta, Krügel, Prusti, 1998).

A Proto-Planetary Disk around BD+31 643 ?

Recently Kalas & Jewitt (1997) discovered a circumbinary disc in scattered light around BD+31 643 (B5 V) in the young cluster IC 348. The disk has quite a similar morphology as the scattering disk of $\beta$ Pictoris. Although the disk around BD+31 643 is much larger.

Together with L. Metcalfe and K. Okumura we pointed with ISOCAM towards this target and detected at 15 $\mu m$ 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 $\mu m$. 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).

The Protostellar Candidate HH108MMS

Although the definition of a protostar is debatable, the detection of such an object is likened to finding the holy grail. No wonder that numerous detections of protostars have been claimed during the last two decades. However, with improving technical equipment, all sources turned out to be more or less evolved young stellar objects.

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).

  

Figure: Profile at 15 $\mu m$ along the major axis of the absorbing disk like structure centered at the position of HH108MMS is shown as histogram in the upper panel. The background flux is indicated as dashed line. The optical depth at 15 $\mu m$ shown in the lower panel is derived by taking the minimum source flux as zero point.

Although there is blending with zodiacal light, one can derive from the absorption feature an optical depth at 15$\mu m$ 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).

A Ring of Organic Molecules around HD 97300.

Imaging spectroscopy of the pre-main-sequence star HD 97300, was obtained with ISOCAM. Those data are from a program which aimed at investigating the nature of the mid-infrared emission associated to Herbig AeBe stars. They are discussed in more detail by Siebenmorgen, Natta, Krügel, Prusti (1998) and some other ``first'' results are given by Siebenmorgen et al., 1997.

The young star HD 97300 is located in the Chamalion I cloud (Whittet et al. 1997), at distance $D\sim$ 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 $\mu$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).

Results

Fig. 2 shows the images of HD 97300 obtained in the four CAM narrow-band filters centered at 6.0 $\mu$m (lw4), 6.8 $\mu$m (lw5), 11.3 $\mu$m (lw8) and 14.9 $\mu$m (lw9).

  

Figure: Logarithmic grey scale images of HD 97300 of the four narrow-band filters. The images are re-sampled to 0.5$\arcsec$ pixel scale. The numbers in the lw5 image indicate the positions of the CVF spectra shown in Fig. 3.

The morphology of the object is very similar in the four filters: we can see extended emission centered on the star and an elliptical ring of size $\sim$50$\times$36$\arcsec$ around it. This same morphology is also seen in the CVF images at all frequencies, with very similar characteristics.

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$\arcsec$ (240 AU) north of the star.

Fig. 3 shows CVF spectra between 5.8 and 13.8 $\mu$m in 8 positions roughly aligned along P.A. = 142.4^o.

  

Figure: CVF spectra of HD 97300 in the 8 positions shown in Fig. 2. Position 4 is closest to the star; Position 6 coincides roughly with the minimum of emission seen in the images; position 7 with the ring. The background emission has been measured in the upper-left corner of the imaged area. Each spectrum has been measured over a single pixel, i.e., over an area of $3\arcsec \times 3\arcsec$. The uncertainties are typically smaller than a few %. Data are shown by diamonds and the best-fit models by the full lines.

This line intersects the star, as well as the emission minimum and the ring seen in the SE direction. The location of the 8 positions is indicated in Fig. 2. In all positions, we detect strong IEBs over a weak continuum.

The IEBs Carriers

To acquire a better understanding the spectra of Fig. 3 are fit using the three component dust model of Siebenmorgen and Krügel (1992). This model computes the emission per unit mass of dust heated by radiation of known intensity and spectral distribution. We use T$_\star$=10700 K (Rydgren 1980) and luminosity L$_\star$=35 L$_{\odot }$(van den Ancker et al., 1997). The computed spectra are smoothed to the CVF spectral resolution. We adjust the PAH parameters and vary the gas column density $N_{\rm H}$ until a satisfactory fit is obtained.

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 ( $\sigma_\nu$; Schutte et al. (1993)) of such a driven damped oscillator is a Lorentzian profile

$$\sigma_\nu \ = \ {\sigma\over 2\pi} \cdot {\gamma\over (\omega-\omega_0)^2 + (\gamma/2)^2 } \ ,$$ (19.1)

where $\omega = 2\pi\nu$.

Note that an excited level in an atom has an average lifetime $t_{\rm L} = A^{-1}$, 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 $\Delta E\cdot\Delta t \ge \hbar$, the energy of the upper level is then only defined to an accuracy $\Delta E = A\hbar$. This leads naturally to a Lorentzian emission profile, and A may be identified with the damping constant, $\gamma = A$. In this picture, the line width is determined by the timescale $\Delta t$. For PAH resonances with a width of $\simeq 0.1\mu$m, the characteristic time $\Delta t$ would be 10^-12s. This would imply immense values for A (10¹²s^-1) as well as for the associated dipole moment $\mu\sim 10^5$Debye because $A \propto \mu^2\omega^3$. A way out of the dilemma would be to assume that the IEBs arise from a superposition of many narrow lines.

Mass of Circumstellar Material

An interesting possibility opened with our model is to derive the mass of circumstellar material from the observed intensity of the PAH features.

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 14$\mu$m 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 $N_{\rm C}$ derived by fitting the feature intensities into a hydrogen column density $N_{\rm H}$. The uncertainty on $N_{\rm H}$ 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 $N_{\rm H}$ 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$\arcsec$) of about 0.07M$_{\odot }$ of which 0.03M$_{\odot }$ is in the elongated ring structure.

The Origin of the Ring

The presence of PAHs in the ring indicates that it is made of interstellar matter, rather than of matter ejected from the star. Its morphology suggests that the ring structure has been created by the interaction of the star with the surrounding matter.

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 LkH$\alpha$198 (Leinert et al. 1991). In that case also the ring has an elliptical shape and the exciting star is shifted from its center.

Summary

I presented three examples of the evolution of stars as seen with ISOCAM. All those cases are again a good demonstration of the high sensitivity of ISO.

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.