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

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

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 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

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

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 stellar content of two OB associations in the LMC - LH 117 (NGC 2122) and LH 118

Photometry for stars in two adjacent OB associations in the LMC, numbers 117 and 118 in the list of Lucke and Hodge (1970), is presented. Each association contains about 50 stars with mass larger than 10 solar, including two newly confirmed red supergiants. Most of the stars in these associations are in the first half of their lives on the main sequence. However, the two red supergiants and a B2 I stars are of lower mass than several of the unevolved, more massive members of the associations, and must have formed six to 10 million yr earlier than most of the association members. The initial mass function of the associations has a slope of - 1.8, similar to the one observed for massive stars near the sun

Massive Stars in the Field and Associations of the Magellanic Clouds: the Upper Mass Limit, the Initial Mass Function, and a Critical Test of Main-Sequence Stellar Evolutionary Theory

We investigate the massive star population of the Magellanic Clouds with an emphasis on the field population, which we define as stars located further from any OB association than massive stars are likely to travel during their short lifetimes. The field stars must have been born as part of more modest star-forming events than those that have populated the large OB associations found throughout the Clouds. We use new and existing data to answer the following questions: Does the field produce stars as massive as those found in associations? Is the initial mass function (IMF) of these field massive stars the same as those of large OB complexes? How well do the Geneva low-metallicity evolutionary models reproduce what is seen in the field population, with its mixed ages? To address these issues we begin by updating existing catalogs of LMC and SMC members with our own new spectral types and derive H-R diagrams (HRDs) of 1584 LMC and 512 SMC stars. We use new photometry and spectroscopy of selected regions in order to determine the incompleteness corrections of the catalogs as a function of mass and find that we can reliably correct the number of stars in our HRDs down to 25 M.. Using these data, we derive distance moduli for the Clouds via spectroscopic parallax, finding values of 18.4 +/- 0.1 and 19.1 +/- 0.3 for the LMC and SMC. The average reddening of the field stars is small: E(B - V) = 0.13 (LMC) and 0.09 (SMC), with little spread. We find that the field does produce stars as massive as any found in associations, with stars as massive as 85 M. present in the HRD even when safeguards against the inclusion of runaway stars are included. However, such massive stars are much less likely to be produced in the field (relative to lower mass stars) than in large OB complexes: the slope of the IMF of the field stars is very steep, GAMMA = -4.1 +/- 0.2 (LMC) and GAMMA = -3.7 +/- 0.5 (SMC). These may be compared with GAMMA = -1.3 +/- 0.3, which we rederive for the Magellanic Cloud associations. (We compare our association IMFs with the somewhat different results recently derived by Hill et al. and demonstrate that the latter suffer from systematic effects due to the lack of spectroscopy.) Our reanalysis of the Garmany et al. data reveals that the Galactic field population has a similarly steep slope, with GAMMA = -3.4 +/- 1.3, compared to GAMMA = -1.5 +/- 0.2 for the entire Galactic sample. We do not see any difference in the IMFs of associations in the Milky Way, LMC, and SMC. We find that the low metallicity evolutionary tracks and isochrones do an excellent job of reproducing the distribution of stars in the HRD at higher masses, and in particular match the width of the main-sequence well. There may or may not be an absence of massive stars with ages less than 2 Myr in the Magellanic Clouds, as others have found for Galactic stars; our reddening data renders unlikely the suggestion that such an absence (if real) would be due to the length of time it takes for a massive star to emerge. There is an increasing discrepancy between the theoretical ZAMS and the blue edge of the main-sequence at lower luminosities; this may reflect a metallicity dependence for the intrinsic colors of stars of early B and later beyond that predicted by model atmospheres, or it may be that the low metallicity ZAMS is misplaced to higher temperatures. Finally, we use the relative number of field main-sequence and Wolf-Rayet stars to provide a selection-free determination of what mass progenitors become WR stars in the Magellanic Clouds. Our data suggest that stars with initial masses > 30 M. evolve to a WR phase in the LMC; while the statistics are considerably less certain for the SMC, they are consistent with this limit being modestly higher there, possibly 50 M., in qualitative agreement with modern evolutionary calculations

The Search for Low-mass Companions of B Stars in the Carina Nebula Cluster Trumpler 16

We have developed lists of likely B3--A0 stars (called "late B" stars) in the young cluster Trumpler 16. The following criteria were used: location within 3' of Eta Car, an appropriate V and B-V combination, and proper motion (where available). Color and magnitude cuts have been made assuming an E(B-V) =0.55 mag +/- 0.1, which is a good approximation close to the center of Trumpler 16. These lists have been cross-correlated with X-ray sources found in the Chandra Carina Complex Project (CCCP). Previous studies have shown that only very rarely (if at all) do late main sequence B stars produce X-rays. We present evidence that the X-ray detected sources are binaries with low-mass companions, since stars less massive than 1.4 Msun are strong X-ray sources at the age of the cluster. Both the median X-ray energies and X-ray luminosities of these sources are in good agreement with values for typical low-mass coronal X-ray sources. We find that 39% of the late B stars based on a list with proper motions have low-mass companions. Similarly, 32% of a sample without proper motions have low-mass companions. We discuss the X-ray detection completeness. These results on low-mass companions of intermediate mass stars are complementary to spectroscopic and interferometric results, and probe new parameter space of low mass companions at all separations. They do not support a steeply rising distribution of mass ratios to low masses for intermediate-mass (5 Msun) primaries, such as would be found by random pairing from the Initial Mass Function.Comment: Accepted for the ApJS Special Issue on the Chandra Carina Complex Project (CCCP), scheduled for publication in May 2011. All 16 CCCP Special Issue papers are available at http://cochise.astro.psu.edu/Carina_public/special_issue.html through 2011 at leas

The Distance and Metallicity of the Newly Discovered, Nearby Irregular Galaxy HIZSS3

HIZSS3 is an H I source in the Zone of Avoidance. Its radio characteristics are consistent with it being nearby (~ 1.8 Mpc) low-mass dwarf irregular (dIm) galaxy. It contains a modest H II region, but its stellar population has not been resolved by optical observations. MMT spectra of the H II region are used to derive the line-of-sight extinction (E(B-V) = 1.41 +/- 0.04) and gas metallicity (log O/H + 12 ~ 7.8). ESO VLT near-IR images clearly reveal a resolved stellar population. Narrow-band Pbeta images of the H II region are used in combination with previously published Halpha data to obtain an independent line-of-sight extinction estimate: E(B-V) = 1.32 +/- 0.04. The adopted foreground extinction is E(B-V) = 1.36 +/- 0.06. Based on the K-band luminosity function and K,J-K color-magnitude diagram, the apparent magnitude and color of the tip of the red-giant branch (TRGB) are derived. In turn, these parameters are combined with the adopted foreground extinction to estimate the distance (1.69 +/- 0.07 Mpc) and mean red giant branch metallicity ([Fe/H] = -0.5 +/- 0.1). As an ensemble, these new observations significantly strengthen the conclusion that HIZSS3 is a newly discovered low-mass dIm lurking behind the Milky Way in the outskirts of the Local Group.Comment: Accepted, scheduled for ApJ, 10 Apr 05, V623 ; 14 page