Abstracts of previous talks

Pleiades low-mass binaries: do companions affect the evolution of protoplanetary disks?

J. Bouvier, F. Rigaut, and D. Nadeau

Abstract: We have observed 144 G and K dwarf members of the Pleiades cluster to search for close multiple systems using CFHT's Adaptive Optics adaptor in the near-IR. We detected 22 binary systems and 3 triples, with a separation between 0.08 and 6.9 arcsec (11-910 AU). After correction for incompleteness, we derive a binary frequency in the orbital period range from 4.2 to 7.1 log days, of 28 +/- 4% for G and K Pleiades dwarfs, similar to that of field G-type dwarfs (27%). The distributions of both orbital periods and mass-ratios of the Pleiades systems also appear similar to those of G dwarf binaries of the field. The binary frequency in the 100 Myr-old Pleiades cluster is much lower than that observed for Myr-old pre-main sequence (PMS) stars in the Taurus-Auriga cloud. We argue that this difference does not result from the evolution of the binary systems during the pre-main sequence. Instead, we suggest that the low Pleiades binary frequency is typical of stellar populations formed in dense protoclusters, while the higher binary frequency observed among Tau-Aur PMS stars is more typical of loose T associations. The implication is that most field stars are born in dense protostellar clusters.

All 144 surveyed stars have known rotational velocities. Based on the current beliefs that i) the rotation rate of Pleiades late-type dwarfs is largely dictated by the lifetime of their pre-main sequence circumstellar disks and that ii) the evolution of the disks is affected by the presence of a close companion, we searched for a relationship between rotational velocity and binarity among Pleiades G and K dwarfs. We find no significant difference between the distribution of rotational velocities of single and binary stars. Unless current models of PMS angular momentum evolution are flawed, this indicates that the presence of a companion within a distance of 10-1000 AU does not prevent accretion from occurring onto the primary at a rate similar to that observed for single PMS stars. For the closest systems, this implies that accretion must proceed from the circumbinary disk onto the central stars. For slightly wider systems, it suggests that the truncated circumstellar disks of the primary and of the secondary are fed by an external (circumbinary) reservoir of mass.

Accepted by Astron. Astrophys.

Kevin Healy, September 16

Looking for Distributed Star Formation in L1630: A Near-infrared (J, H, K) Survey

Wenbin Li, Neal J. Evans, II and Elizabeth A. Lada

Abstract:

We have carried out a simultaneous, multi-band (J, H, K) survey over an area of 1320 arcmin^2 in the L1630 region, concentrating on the region away from the dense molecular cores and with modest visual extinctions (<= 10 mag). Previous studies found that star formation in L1630 occurs mainly in four localized clusters, which in turn are associated with the four most massive molecular cores (Lada et al. 1991; Lada 1992). The goal of this study is to look for a distributed population of pre-main-sequence stars in the outlying areas outside the known star-forming cores. More than 60% of the pre-main-sequence stars in the active star forming regions of NGC 2024 and NGC 2023 show a near-infrared excess in the color-color diagram. In the outlying areas of L1630, excluding the known star forming regions, we found that among 510 infrared sources with the near-infrared colors ((J-H) and (H-K)) determined and photometric uncertainty at K better than 0.10 mag, the fraction of the sources with a near-infrared excess is 3%--8%; the surface density of the sources with a near-infrared excess is less than half of that found in the distributed population in L1641, and 1/20 of that in the young cluster NGC 2023. This extremely low fraction and low surface density of sources with a near-infrared excess strongly indicates that recent star formation activity has been very low in the outlying region of L1630. The sources without a near-infrared excess could be either background/foreground field stars, or associated with the cloud, but formed a long time ago (more than 2 Myrs). Our results are consistent with McKee's model of photoionization-regulated star formation.

Accepted by ApJ for the Oct 10 1997 issue, Vol 488

Gravitational Infall in the Dense Cores of L1527 and L483

P. C. Myers, P. Caselli, D. Mardones, M. Tafalla, D. J. Wilner, R. Bachiller, and G. A. Fuller

Abstract:

Lines of N₂H⁺, C₃H₂ , and H₂CO show kinematic evidence of gravitational infall in the star-forming dense core L1527, and probably also in L483. Three systematic trends appear to indicate infall motions, rather than outflow and rotation: (a) at each protostar position, line peak and centroid velocities get bluer by 0.1 to 0.3 km s^-1 with lines of increasing optical depth; (b) in maps of C₃H₂ and H₂CO lines, line peak and centroid velocities get bluer by a similar amount as positions approach each protostar, and (c) C₃H₂ line widths in L1527 increase as positions approach the protostar. Also, in both sources H₂CO lines show the infall "signature" of spatially concentrated double-peaked profiles, with their blue peaks brighter than their red peaks, as seen previously in B335. Many asymmetric profiles have a single blueshifted peak with a red shoulder, rather than two peaks. Matching these profiles with an infall model appears to require a departure from front-back spatial symmetry, due perhaps to effects of the bipolar outflow. In each source, H₂CO line wings show clear evidence of collimated outflow, even where the lower-velocity portion of the profile shows infall asymmetry.

From ApJ Letters (1995) Vol. 449, pp. L65-L68.

The Stellar Initial Mass Function

John M. Scalo

Abstract: This paper has no abstract - you get to read all 278 pages to find out what happens!

From Fundamentals of Cosmic Physics (1986), Vol. 11, pp. 1-278

The Initial Stellar Mass Function from Random Sampling in a Turbulent Fractal Cloud

Bruce G. Elmegreen

Abstract:

The initial mass function (IMF) for stars is proposed to result from two distinct physical processes that determine its shape separately in two intervals of mass: random sampling of mass in a fractal cloud gives the power-law portion at intermediate to high mass, and insufficient self-gravity at the local temperature and pressure gives the cutoff at low mass. The entire function is modeled numerically, with the assumption that a star's mass is proportional to the mass of the piece of cloud in which it forms. The results typically give an IMF with the Salpeter value for the slope and a flattening at a low mass. There is little sensitivity to parameters at masses greater than the cutoff, although slightly shallower IMFs might be expected in regions with high levels of ionization and turbulence. The low-mass cutoff is essentially the thermal Jeans mass in the star-forming cloud; models with a high value of this mass produce a truncated IMF similar to that proposed for starburst galaxies. The mass of the largest star increases with total stellar mass because of the stochastic nature of the model. The star IMF is steeper than the cloud mass spectrum because of competition for mass and the density dependence of star formation.

From the Astrophysical Journal (1997) Vol. 486, p. 944.

Star Formation in R136: A Cluster of O3 Stars Revealed by Hubble Space Telescope Spectroscopy

Philip Massey and Deidre A. Hunter

Abstract:

The R136 cluster in 30 Doradus is the prototype ``super star cluster'', and the only example sufficiently close that its massive star content can be studied directly. We have used Hubble Space Telescope to obtain spectra of 65 of the bluest, most luminous stars in R136, and find that the majority of these stars are of type O3, the hottest, most luminous and massive stars known. The total number of O3 stars in this one cluster exceed the total number known elsewhere in the Milky Way or Magellanic Clouds. The highest luminosity stars found are O3 If*, O4 If+, O3 If/WN6-A, and H-rich WN stars, with masses in excess of 120 Mo, the highest masses for which appropriate evolutionary tracks are currently available. In accord with de Koter, Heap, & Hubeny, we conclude that these WN stars must be core-H burning stars whose spectra are WR-like due to high luminosity, and we find that their individual luminosities are a factor of 10 higher than normal WN stars of similar type, but like those found in the Galactic cluster NGC 3603, which they also resemble spectroscopically. Our spectroscopy does include stars as late as B0 V, and samples most stars in the core of the R136 cluster with masses > 50 Mo. The spectroscopy has been combined with HST photometry to study the star formation history and initial mass function of the R136 cluster. The young age ( < 1 - 2 Myr) for the highest mass stars, combined with what was previously known for the intermediate-mass populations, suggests that the lower mass stars began forming 4 - 5 Myr ago, and continued until the high mass stars formed, consistent with the paradigm in which the formation of massive stars shuts down further star formation in the molecular cloud. Despite the unique preponderance of the highest mass and luminosity stars ever seen, the IMF is found to be completely normal, with a slope Gamma = -1.3 to -1.4. The number of high mass stars is in good accord with that predicted by the IMF of the intermediate-mass stars, suggesting that a Salpeter-like IMF holds over the mass range 2.8 Mo to 120 Mo within the R136 cluster. The fact that the IMF slope in R136 is indistinguishable from those of Galactic and Magellanic Cloud OB associations suggests that star formation produces the same distribution of masses over a range of ~200× in stellar density, from that of sparse OB associations to that typical of globular clusters. The large number of O3 stars in R136 is then simply a consequence of its youth ( < 1 - 2 Myr) and its richness, suggesting that the upper mass ``cutoff'' to the IMF seen in OB associations may simply be the result of their sparcity.

Accepted by the Astrophysical Journal.

Star Formation in Irregular Galaxies: A Review of Several Key Questions

Deidre A. Hunter

Abstract:

Star formation takes place in a different and simpler environment in irregular galaxies compared to spirals. Because they lack spiral density waves, non-interacting irregular galaxies are interesting laboratories for all of the internal processes other than spiral density waves that are involved in normal star formation. Most irregular galaxies are currently forming stars, and some are forming stars at rates that are comparable to those for spiral galaxies. In addition, there is a larger range in star formation rates among irregular galaxies than spirals. I discuss the status of our knowledge about large-scale star formation processes in irregular galaxies in the context of four key questions: What regulates star formation on global and local scales in irregular galaxies? What feedback processes are operating? What are the consequences of differences in the interstellar medium on the star formation process? What role does the extended gas play in the evolution of irregular galaxies? Answers to these questions are important for understanding not only how star formation works in irregular galaxies, but in understanding the process by which galaxies form stars and evolve anywhere.

From Publications of the Astronomical Society of the Pacific, 109, 937 (September 1997).

Recent star-formation press releases from HST

	The Pistol Nebula and associated super-massive star;
	The Antennae and triggered star formation.

Abstract:

The press releases can be found by following the links above. Use the text links for the press release page, or click on the images to see higher-resolution versions.

Possible Stellar Metallicity Enhancements from the Accretion of Planets

Gregory Laughlin and Fred C. Adams

Abstract:

A number of recently discovered extrasolar planet candidates have surprisingly small orbits, which may indicate that considerable orbital migration takes place in protoplanetary systems. A natural consequence of orbital migration is for a series of planets to be accreted, destroyed, and then thoroughly mixed into the convective envelope of the central star. We study the ramifications of planet accretion for the final main sequence metallicity of the star. If maximum disk lifetimes are on the order of ~ 10 Myr, stars with masses near 1.0 M are predicted to have virtually no metallicity enhancement. On the other hand, early F and late A type stars with masses M 1.5-2.0 M can experience significant metallicity enhancements due to their considerably smaller convection zones during the first 10 Myr of pre-main-sequence evolution. We show that the metallicities of an aggregate of unevolved F stars are consistent with an average star accreting ~ 2 Jupiter-mass planets from a protoplanetary disk having a 10 Myr dispersal time.

To appear in ApJ Letters.

Cloning Hubble Deep Fields: A Model-Independent Measurement of Galaxy Evolution

by Rychard J. Bouwens, Tom Broadhurst, and Joseph Silk (UC Berkeley)

Abstract:

We present a model-independent method of quantifying galaxy evolution in high- resolution images, which we apply to the Hubble Deep Field (HDF). Our procedure is to k-correct the pixels belonging to the images of a complete set of bright galaxies and then to replicate each galaxy image to higher redshift by the product of its space density, 1/V_{max}, and the cosmological volume. The set of bright galaxies is itself selected from the HDF because presently the HDF provides the highest quality UV images of a redshift-complete sample of galaxies (31 galaxies with I<21.9, \bar{z}=0.5, and for which V/V_{max} is spread fairly). These galaxies are bright enough to permit accurate pixel-by-pixel k-corrections into the restframe UV (\sim 2000 A). We match the shot noise, spatial sampling and PSF smoothing of the HDF, resulting in entirely empirical and parameter free ``no-evolution'' deep fields of galaxies for direct comparison with the HDF. We obtain the following results. Faint HDF galaxies (I>24) are much smaller, more numerous, and less regular than our ``no-evolution'' extrapolation, for any relevant geometry. A higher proportion of HDF galaxies ``dropout'' in both U and B, indicating that some galaxies were brighter at higher redshifts than our ``cloned'' z\sim0.5 population. By simple image transformations we demonstrate that bolometric luminosity evolution generates galaxies which are too large and the contribution of any evolving dwarf population is uninterestingly small. A plausible fit is provided by `mass-conserving' density-evolution, consistent with hierarchical growth of small-scale structure. Finally, we show the potential for improvement using the Advanced Camera, with its superior UV and optical performance.