The form of the initial mass function in an H II complex in NGC 6946

Evidence is beginning to accumulate that the initial mass function (IMF) is not the same everywhere but suffers at least spatial variations. A simple model aimed at determining the slope of the IMF in a cluster of young stars is constructed. Narrow-band photometry of H II complex C in the Sc I galaxy NGC 6946 is presented, and the observations are compared with the results of the model. The regions of the complex are very young and the observations fall in an ambiguous area of the model results; thus a precise value for the IMF slope is indeterminable. The star formation rate and eficiency of massive star formation are calculated using a Salpeter IMF and are compared to the average values over the entire galaxy

The range of V - R colors for a cluster of E and S0 galaxies as a function of redshift

The expected (V - R) color distribution for a centrally condensed, relaxed cluster of E and S0 galaxies has been calculated as a function of redshift. Because of the differences in the ultraviolet spectra of E and S0 galaxies, which are correlated with absolute magnitude, the spread in (V - R) colors for such a cluster becomes increasingly wide for increasing redshift. This effect becomes pronounced for redshifts of 0.4 and beyond. Thus evidence for the color evolution of cluster galaxies will be seen as an additional broadening of the color distribution

IRAS observations and the stellar content of H II regions in the Large Magellanic Cloud

IRAS far-infrared fluxes are presented for six H II regions in the LMC. These regions are the sites of recent, massive star formation where the radiative heating source is young stars rather than the general interstellar radiation field. As such, the observations become building blocks for the interpretation of far-infrared emission from spatially unresolved galaxies. In addition, the multiwavelength luminosities of the region then probe the young stellar upper initial mass function. With the long-range goal of determining the stellar content of unresolved star-forming regions, we present the results of a simple model which combines the infrared data with existing H-alpha/H-beta and thermal radio data. For the two OB associations where the mass function has been determined directly, the model correctly 'predicts' a solar neighborhood upper mass function within the uncertainties. No evidence is found for upper mass functions significantly different from that in the solar neighborhood

The Progenitor Masses of Wolf-Rayet Stars and Luminous Blue Variables Determined from Cluster Turn-offs. II. Results from 12 Galactic Clusters and OB Associations

Here we examine 12 Galactic clusters and OB associations containing Wolf-Rayet stars (W-Rs) and/or Luminous Blue Variables (LBVs), in order to assess the progentor masses of these evolved massive stars. We find that in the Milky Way, early-type WN stars evolve from stars with masses as low as 20Mo. Some early-type WN stars are found in clusters with very high turn-off masses, supporting our earlier results that the "WNE" class of W-Rs is a common stage in the evolution of stars with a wide range of mases. In contrast, the late-type WNs (particularly those of WN7 class) and the LBVs are only found in clusters with the very highest turn-off masses, suggesting that only the most massive stars evolve to these objects. An examination of the environment of the archetype LBV Eta Car reveals that this star is coeval with the rest of the Tr14/16 association, suggesting that its LBV properties is a normal consequence of evolution, and has little to do with the possibility that it is a binary.Comment: Scheduled for the Feb 2001 issue of the Astronomical Journal. Postscript version including figs from ftp://ftp.lowell.edu/pub/massey/galwr.ps.g

Empirical bounds for the ionizing fluxes of Wolf-Rayet stars.

Hα photometry and spectroscopic data were obtained for 10 Wolf-Rayet nebula, representing a wide variety of WN spectral types. The authors use these data to constrain the ionizing flux of the exciting Wolf-Rayet star, calcg. lower bounds for the Lyman continuum flux (Q0) and for the He0- and He+- ionizing fluxes (Q1 and Q2). Q0 appears independent of WN spectral type, and lower bound ests. tend to cluster around 48 dex. Finally, the authors discuss the effects of potential shock excitation and d. bounding on these nebula and compare the authors' results to recent models. The authors' results are consistent with the predictions of line-blanketed ISA-wind models and nonblanketed CMFGEN models but are consistent with only some of the line-blanketed CMFGEN models. [on SciFinder(R)

The star formation history of Trumpler 14 and Trumpler 16

H-R diagrams are presented for the very young galactic clusters Trumpler 14 and Trumpler 16, which are the two most populous clusters in the region of vigorous star formation surrounding η Carinae. Point spread function photometry of UBV CCD images is presented down to V~19 for over 560 stars in Tr 16 and 290 stars in Tr 14. We have also obtained similar data for a local background field. After determining cluster membership through proper motions from a previous work, we find that the reddening of cluster members is significantly lower than that of the local background stars. Thus, we are able to use individual reddenings to identify likely members at far deeper levels than possible with proper motions. This work has revealed a significant population of pre-main-sequence (PMS) stars in both clusters. The location of the PMS stars in the H-R diagram indicates that the theoretical ``stellar birthline'' of Palla & Stahler follows the locus of stars far better than that of Beech & Mitalas. Comparison with both pre- and post-main-sequence isochrones also reveals that although intermediate-mass stars have been forming continuously over the last 10 Myr, the high-mass stars formed within the last 3 Myr. There is no evidence that the formation of the intermediate-mass stars was truncated by the formation of the high-mass stars

The Physical Properties and Effective Temperature Scale of O-type Stars as a Function of Metallicity. III. More Results from the Magellanic Clouds

In order to better determine the physical properties of hot, massive stars as a function of metallicity, we obtained very high SNR optical spectra of 26 O and early B stars in the Magellanic Clouds. These allow accurate modeling even in cases where the He I 4471 line has an equivalent width of only a few tens of mA. The spectra were modeled with FASTWIND, with good fits obtained for 18 stars; the remainder show signatures of being binaries. We include stars in common to recent studies to investigate possible systematic differences. The "automatic" FASTWIND modeling method of Mokiem and collaborators produced temperatures 1100 K hotter on the average, presumably due to the different emphasis given to various temperature-sensitive lines. More significant, however, is that the automatic method always produced some "best" answer, even for stars we identify as composite (binaries). The temperatures found by the TLUSTY/CMFGEN modeling of Bouret, Heap, and collaborators yielded temperatures 1000 K cooler than ours, on average. Significant outliers were due either to real differences in the data (some of the Bouret/Heap data were contaminated by moonlight continua) or the fact we could detect the HeI line needed to better constrain the temperature. Our new data agrees well with the effective temperature scale we presented previously. We confirm that the "Of" emission-lines do not track luminosity classes in the exact same manner as in Milky Way stars. We revisit the the issue of the "mass discrepancy", finding that some of the stars in our sample do have spectroscopic masses that are significantly smaller than those derived from stellar evolutionary models. We do not find that the size of the mass discrepancy is simply related to either effective temperature or surface gravity.Comment: ApJ, in pres

The initial mass function and massive star evolution in the Ob associations of the northern Milky-Way

We investigate the massive star content of Milky Way clusters and OB associations in order to answer three questions: (1) How coeval is star formation? (2) How constant is the initial mass function (IMF)? (3) What is the progenitor mass of Wolf-Rayet stars? Our sample includes NGC 6823/Vul OB1, NGC 6871/Cyg OB3, Berkeley 86/Cyg OB1, NGC 6983/Cyg OB1, NGC 7235, NGC 7380/Cep OB1, Cep OB5, IC 1805/Gas OB6, NGC 1893/Aug OB2, and NGC 2244/Mon OB2. Large-field CCD imaging and multiobject, fiber spectroscopy has resulted in UBV photometry for >14000 stars and new spectral types for approximate to 200 stars. These data are used to redetermine distances and reddenings for these regions and to help exclude probable nonmembers in constructing the H-R diagrams. We reanalyze comparable data previously published on Cyg OB2, Tr 14/16, and NGC 6611 and use all of these to paint a picture of star formation and to measure the IMFs. We find the following: (1) Most of the massive stars are born during a period Delta tau 7 M. A comparison with similarly studied OB associations in the Magellanic Clouds reveals no difference in IMF slope, and hence we conclude that starformation of massive stars in clusters proceeds independently of metallicity, at least between z = 0.02 and z = 0.002. The masses of the highest mass stars are approximately equal in the Milky Way, LMC, and SMC associations, contrary to the expectation that this value should vary by a factor of 3 over this metallicity range. We conclude that radiation pressure on grains must not limit the mass of the highest mass star that can form, in accord with the suggestion of Wolfire & Cassinelli that the mere existence of massive stars suggests that shocks or other mechanisms have disrupted grains in star-forming events. (3) The four Wolf-Rayet stars in our sample have come from stars more massive than 40 M.; one WC star and one late-type WN star each appear to have come from very massive (approximate to 100 M.) progenitors

A correlation between infrared excess and period for Mira variables

The [8.7]-[11.4] infrared color is found to increase monotonically with increasing period for a sample of 41 Mira variables. We conclude that mass loss is not a stochastic process and that for any Mira variable the rate of mass loss is directly linked to the parameters which dictate the period of that variable

Proper motions, membership, and photometry of open clusters near Eta Carinae

Proper motions and photographic photometry have been derived for nearly 600 stars with 7.5 < V< 15.5 in the region of the very young open clusters Tr 14, Tr 16, and Cr 232 based on 26 plates dating from 1893 to 1990. Cluster membership probabilities have been derived from the proper motions and color-magnitude diagrams of probable members of each cluster are presented. In contrast to a few of the previous studies we find all three clusters to lie at the same distance