D I S C U S S I O N S E S S I O N O N S T A R F O R M A T I O N ,

M O L E C U L A R C L O U D S

A N D T H E I N T E R S T E L L A R M E D I U M

KAREN M. STROM Astronomy Program, UniverJity of MassaehuJetta, Amherat, MA 01003

LENNART NORDH Stockholm Observatory, S-13336 Saltsjoebaden, Sweden

and

ELI DWEK NASA/GSFC, Code 697, LEP, Greenbelt, MD 20771

March 21, 1994

A b s t r a c t . In this panel discussion contributions were made by K. Strom, L. Nordh and H. Zinnecker on the contributions of surveys to the study of star formation regions, by B. Burton on a survey of galactic H I and by E. Dwek on the detection of galactic supernovae by infrared surveys. The contributions ofK. Strom, L. Nordh and E. Dwek are summarized here.

K e y words : stars:formation, stars:pre-main sequence, circumstellar matter, infrared:stars

1. I n i t i a l S u r v e y R e s u l t s a n d s o m e K e y Q u e s t i o n s - K. S t r o m

Large scale maps in the near infrared of nearby Giant Molecular Clouds reveal 3 distinct populations (Strom, Strom & Merrill 1993): 1) stellar clus- ters with more than 50 members within a region of ,-~1 pc size; 2) stellar aggregates of 10 ~ 50 stars within regions of - 1 pc size; 3) a large distribut- ed populat ion throughout the cloud•

It also appears tha t high mass star formation may be restr icted to dense stellar groups while low mass stars may form in all environments.

These results motivate the following questions: - What is the initial mass function characterizing an individual star for-

mat ion site, and does this vary from site to site? - What conditions favor the production of high or low mass stars?

• initial core conditions ( M, p, Av, J /M, ...) or • environmental conditions (evidence of compression, etc.).

- Can we unders tand the origin of the t ime and space averaged IMF? In order to address these questions we must 1) locate individual stars,

aggregates and clusters whose age is small enough (t < 1 Myr) so that they have not moved far (_< 0.5 pc) from their birthplaces (the dense molecular

Astrophysics and Space Science 217: 227-230, 1994. © 1994 Kluwer Academic Publishers. Printed in Belgium.

228 K.M. STROM ET AL.

cores); 2) characterize the physical properties of the molecular cores associ- ated with a) isolated young stars of differing mass, b) young aggregates and 3) young clusters; and 3) determine whether certain types of stars/clusters form in cores produced only under certain conditions ( e.g. compression by an external "trigger").

Following this strategy requires 1) locating the distributed, aggregate and cluster populations in molecular clouds (2MASS, DENIS, other special- ized surveys), 2) determining masses and ages of the embedded population of YSOs by combining infrared photometric and optical and infrared spec- troscopic observations, and 3) using molecular cloud maps to characterize molecular core properties.

The characterization of younger, more deeply embedded stellar groups depends, for the moment , entirely on photometric surveys. Three bands are necessary to provide a measure of the extinction and the presence of an active accretion disk for each star. An example of such a very young star forming group is Mon R2 (Dougados, Carpenter, Meyer and Strom 1993). Analysis of the photometry for this group reveals 1) a dense cluster of age t < 1 Myr defining an episode of recent star formation within a complex which has been forming stars for > 10 Myr; 2) This cluster has an extraordinary fraction (>90%) of stars surrounded by envelopes and active accretion disks; and 3) a high proportion of apparently higher mass stars near the cluster center (although the high luminosities could be indicative of elevated accretion rates).

2. ISOCAM Survey Sensitivity for Young Very Low M a s s S t a r s - L. N o r d h

The ISOCAM Survey will be conducted in two filters, 5 - 8.5#m and 12 - 17#m, with a pixel FOV of 6" and an integration time per frame of 40 sec. Using conservative sensitivity parameters, we can compute the luminosity limits that can be reached by the survey for assumed spectral energy dis- tributions and compare these to predicted luminosities of these presumably low-mass stars as a function of stellar age. We will do the comparison for the long wavelength filter (12 - 17#m) and for 2 possible energy distributions, blackbodies and power laws: uFL, ---- u -a.

In Table 1 we show our results for an assumed cloud distance of 150 pc; in col. I is the type of spectral energy distribution; in col. 2 is the luminosity radiated by the object from 2.2 - 100#m; in col. 3 is the fraction, f, of that luminosity emit ted at wavelengths spanned by the survey filters (5 - 17#m); in col. 4 is the K magni tude of the object assuming no extinction.

It is obvious from Table 1 that the ISOCAM survey will be capable of detecting very low luminosity objects in nearby, star-forming molecular clouds and that these objects are detectable at K as well, even if large

DISCUSSION SESSION 229

telescopes and long in t eg ra t i on t imes will be needed in some cases.

I n F igure 1, t aken f r o m Nelson, R a p p a p o r t & Joss(1986) , is p lo t t ed the

bo lome t r i c l uminos i ty as a func t ion of t ime for a selection of masses for very

low mass stars . A compar i son wi th the luminosi t ies of table 1 d e m o n s t r a t e s t h a t a significant f r ac t ion of the substel lar objec ts younger t h a n 106-7 years

bu t still a s soc ia ted wi th the observed clouds will be de tec ted by the I S O C A M

survey. However , add i t iona l p h o t o m e t r i c observat ions at b o t h shor ter and longer wavelengths will be needed to accura t e ly e s t ima te the bo lome t r i c

l uminos i t y and an empir ica l stellar age ind ica to r m u s t be es tabl ished to

al low a p rope r c o m p a r i s o n wi th any mode l predict ions .

° I

Table 1 Luminosity Limits (10~)

a/BB L(2.2-100#m) f K(Av=0)

0 1.6 × I0 -a 0.32 15.2 1 2.7 × I0 -a 0.12 17.4 2 9.0 × 10 -S 0.03 19.6 3 4 .0 × 10 -2 <0.01 21.6

3000K BB 9.6 × 10 -2 0.03 10.4 2000K BB 3.1 × 10 -u 0.08 11.1 1000K BB 5 .0 x 10 - 3 0 . 3 8 13.4 500K BB 2.0 x 10 -a 0.71 19.1

' I I ~ I ~ I l I

( ~ BOLOMETRIC LUMINOSITY

5"2 - 8

I I i I I I I I I I 5 6 7 8 9 I 0

1o9 t ( y r )

Fig. 1. Evolution of bolometric luminosity as a function of age for very low mass stars. Each track is labeled with the corresponding stellar mass in units of solar masses. [courtesy of L. Nelson]

230 K.M. STROM ET AL.

3. D E T E C T I N G A G A L A C T I C S U P E R N O V A W I T H T H E 2 M A S S / D E N I S P R O J E C T S - E . D w e k

The proposed near-infrared (NItt) sky surveys 2MASS and DENIS offer a unique opportuni ty for the detection of a galactic supernova (SN) exploding in a molecular cloud. The UV-visual output of an exploding star is a few × 1049 ergs released with an e-folding t ime of typically ~30 d. Peak luminosi- ties are about 101°L®. Inside an optically-thick molecular cloud this energy output will remain undetected at visual wavelengths, but will emerge as a prominent source at NIR wavelengths. The SN outburst will destroy any ambient dust that is heated up to temperatures in excess of the evaporation temperature. Since graphite grains have a higher condensation temperature than silicates (~2000 K compared to ~1500 K for silicates) the dust environ- ment around the SN will be characterized by the presence of three regions: the first is a region extending out to a radius K1 = 0.02 pc (24 light days) which will be devoid of any dust; a second region extending from K1 to a radius of R2 = 0.10 pc (120 light days) in which graphite grains survive the outburst, but devoid of any silicate grains; and a third region beyond R2 in which both graphite and silicates survive the outburst. The radiative output of the SN will be absorbed by the surrounding dust and reradiated at NII~ wavelengths. The emission from graphite or silicate grains radiating at their respective evaporation temperatures will peak around 2pm. Due to time delay effects the reradiated IR emission from the inner graphite shell boundary will be spread over a period of 48 d, and the silicate contribution to the emission will be spread over a period of about 240 d. Adopting a dust shell radius of 100 days, the total NII~ luminosity will be equal to about a few ×107L®. At the distance of the galactic center (D = 8.5 kpc) the SN will appear as a bright source with a K magni tude of -3.6. Accounting for extinction, the SN will appear as a K = 0 mag. object. The magni tude of the source will make it easily distinguishable from other objects. Further- more, because of its complex dust environment the SN K-band light curve will show a t ime variability of the order of months, providing an addit ional means of verifying the nature of the radiat ing object.

References

Dougados,C, Carpenter, J, Meyer, M. & Strom, S. E.: 1994~ Ap.J. , In preparation Dwek, E.: 1985, Ap.J. 29Y, 719 Nelson, L.A., Rappaport, S.A., & Joss, P.C.: 1986, Ap.J. 311,226 Strom, K. M., Strom, S. E., & Merrill, K. M.: 1993, Ap.J. 412,233