Margaret Murray Hanson

The Young Massive Stellar Objects of M17

We present a multiwavelength spectroscopic survey that reveals the ionizing cluster of M17 and uncovers a population of young stellar objects (YSOs) of high mass (M similar to 5-20 M.). The masses of the stars have been determined fairly accurately through optical or near-infrared spectral classification. We find strong circumstantial evidence for disks around the massive YSOs in the following forms: near-infrared excess, optical veiling, CO band-head emission, and/or Pa delta emission. We find a direct correlation between those YSOs that show CO band-head emission at 2.3 mu m and these stars in our survey that show Pa delta emission; in three of the four Pa delta emission stars this line is double peaked (suggestive of a bipolar wind or a rotating disk or envelope). Our data suggest that circumstellar material, possibly in the form of a disk, is prevalent among very young objects of fairly high mass. Based on considerations of disk lifetimes in other young clusters, the M17 cluster appears to be very young, perhaps less than 1 Myr.We have also identified at least nine O stars and a couple of late-O/early-B stars, most behind more than 8 mag of visible extinction using either optical or near-infrared spectral types. Several stars have inferred masses in excess of 60 M., and they look to be very close to the predicted zero-age main sequence with an estimated age of about 1 Myr, consistent with the age of the massive YSOs in the cluster. We have used the O stars to determine the distance to M17, which assumes the stars to lie on the zero-age main sequence (1300(-200)(+400) pc). While we attempt to determine an initial mass function for the cluster, it is incomplete even at high masses because of regions of extremely high extinction (A(V) > 20) in the cluster. We have also used the M17 O stars to study the dust properties in the local cloud and the behavior of the diffuse interstellar bands (DIBs) along this sight line, over the extinction range of A(V) = 3-10. The DIBs over this extinction range show little change in spectral shape nor a significant increase in strength. We suggest the features are already saturated at small A(V), or the material local to M17, where the increased extinction is being traced, does not contain the carriers of the DIB feature.

A STUDY OF CYGNUS OB2: POINTING THE WAY TOWARD FINDING OUR GALAXY'S SUPER-STAR CLUSTERS

New optical MK classification spectra have been obtained for 14 OB star candidates identified by Comeron et al. and presumed to be possible members of the Cyg OB2 cluster as recently described by Knodlseder. All 14 candidate OB stars observed are indeed early-type stars, strongly suggesting the remaining 31 candidates by Comeron et al. are also early-type stars. A thorough investigation of the properties of these new candidate members compared with the properties of the Cyg OB2 cluster star has been completed, using traditional as well as newly revised effective temperature scales for O stars. The cooler O star effective temperature scale of Martins et al. gives a very close distance for the cluster (DM = 10.4). However, even using traditional effective temperature scales, Cyg OB2 appears to be slightly closer (DM = 10.8) than previous studies determined (DM = 11.2; Massey & Thompson), when the very young age of the stellar cluster (~2 × 106 yr) is taken into account in fitting the late-O and early-B dwarfs to model isochrones. Of the 14 new OB stars observed for this study, as many as half appear to be significantly older than the previously studied optical cluster, making their membership in Cyg OB2 suspect. So, while some of the newly identified OB stars may represent a more extended halo of the Cyg OB2 cluster, the survey of Comeron et al. also picked up a large fraction of nonmembers. Presently, estimates of the very high mass of this cluster (Mcl ≈ 104 M☉ and over 100 O stars) first made by Knodlseder remain higher than this study can support. Despite this, the recognition of Cyg OB2 as a more massive and extensive star cluster than previously realized using 2MASS images, along with the recently recognized candidate super-star cluster Westerlund 1 only a few kiloparsecs away (Clark & Negueruela), reminds us that we are woefully underinformed about the massive cluster population in our Galaxy. Extrapolations of the locally derived cluster luminosity function indicate that tens to perhaps a hundred of these very massive open clusters (Mcl ≈ 104 M☉, MV ≈ -11) should exist within our galaxy. Radio surveys will not detect these massive clusters if they are more than a few million years old. Our best hope for remedying this shortfall is through deep infrared searches and follow-up near-infrared spectroscopic observations, as were used by Comeron et al. to identify candidate members of the Cyg OB2 association.

NGC 1058 - Gas motions in an extended, quiescent spiral disk

The motion of gas in the galaxy NGC 1058 is examined, using VLA observations to map emission in the 21-cm line. The hypothesis that the rotation curve drops faster than Keplerian at the outer edge of the disk is confirmed. It is found that the line width decreases with optical surface brightness, but in the extended gas disk beyond the Holmberg radius, the line width is constant. The observations show no correlation between line width and spiral phase of H I surface density in the outer disk. Evidence is found for an intermediate velocity cloud population with a velocity width several times broader than that of the bulk of the H I in the optically bright disk. 29 refs.

Near-Infrared H-Band Features in Late O and B Stars

We examine the spectral characteristics of normal OB stars with high–signal-to-noise ratio (>120) H-band (1.6 μm) spectra at a resolution of 2000. We find that several atomic lines vary smoothly with stellar temperature, as first shown by Blum et al. However, we find a previously unreported, significant variation in the strength of some of these lines with stellar luminosity. B supergiant stars show stronger He I and weaker Br 11 as compared with low-luminosity B dwarf stars of the same spectral class. It is for this reason that luminosity class must also be determined to obtain an accurate spectral type for a given star using H-band spectra. We suggest a method for estimating the spectral type and luminosity of an OB star over the wavelength range from 1.66 to 1.72 μm using hydrogen Br 11 at 1.681 μm, He I at 1.700 μm, and He II at 1.693 μm. The use of the near-infrared spectral range for classification has obvious advantages over optical classification when applied to heavily reddened stars, such as in star-forming regions or deeply embedded lines of sight within the plane of the Galaxy, such as the Galactic center. Furthermore, the H band is less likely to be contaminated by infrared excess emission, which is frequently seen around massive young stellar objects beyond 2 μm.

Near-Infrared Spectroscopy of G29.96–0.02: The First Spectral Classification of the Ionizing Star of an Ultracompact H II Region

We have obtained the first classification spectrum and present the first direct spectral classification of the ionizing star of an ultracompact H II region. The ultracompact H II region is G29.96-0.02, a well-studied object with a metallicity value roughly twice that of the Sun. The near-infrared K-band spectrum of the ionizing star exhibits C IV and N III emission and He II absorption, but lines of H I and He I are obliterated by nebular emission. We determine that the star has a spectral type of O5-O7 or possibly O8. We critically evaluate limits on the properties of the star and find that it is compatible with zero-age main-sequence properties only if it is binary and if a significant fraction of the bolometric luminosity can escape from the region. G29.96-0.02 will now be an excellent test case for nebular models, as the properties of the ionizing star are independently constrained.

The Dusty Starburst Nucleus of M33

We have thoroughly characterized the ultraviolet to near-infrared (0.15-2.2 μm) spectral energy distribution (SED) of the central parsec of the M33 nucleus through new infrared photometry and optical/near-infrared spectroscopy, combined with ultraviolet/optical observations from the literature and the Hubble Space Telescope archive. The SED shows evidence for a significant level of attenuation, which we model through a Monte Carlo radiative transfer code as a shell of clumpy Milky Way-type dust (τV ~ 2 ± 1). The discovery of Milky Way-type dust (with a strong 2175 A bump) internal to the M33 nucleus is different from previous work, which has found SMC-like dust (no bump) near starburst regions. The amount by which dust can be processed may be related to the mass and age of the starburst as well as the extent to which the dust can shield itself. Our starburst models include the effects of this dust and can fit the SED if the nucleus was the site of a moderate (~108 L☉ at 10 Myr) episode of coeval star formation about 70 Myr ago. This result is quite different from previous studies, which resorted to multiple stellar populations (between two and seven) attenuated by either no or very little internal dust. The M33 nuclear starburst is remarkably similar to an older version (70 versus 10 Myr) of the ultracompact starburst in the center of the Milky Way.

Of-type stars HD 16691 and HD 190429 show WN-like spectra in infrared K band

We present 2 micrometer K-band spectra of two early-type Of stars that have infrared emission-line morphology similar to that of WN stars. Archival International Ultraviolet Explorer (IUE) spectra of these two stars indicate they appear to be Of type, rather than WN. Recently acquired optical spectra of these stars are quantitatively similar to that in the past, namely, Of attributes. We suggest that these two Of stars have stellar wind characteristics closer to WN type than other Of stars. We discuss the consequences for K-band classification of highly obscured hot stars that might not otherwise be visible in optical or UV wavelengths.

The diffuse interstellar cloud toward HD 179406 (20 Aquilae)

An analysis of the diffuse interstellar cloud complex in front of HD 179406 (20 Aql) is presented. Along this sight line, multispectral absortion- and emission-line studies have uncovered at least three distinct velocity components due to individual clouds. A dominant velocity component is seen in both the absorption and emission-line data sets at 3 +/- 1 km/s. It is argued that the cloud associated with this velocity component is responsible for most of the atomic and all of the molecular gas in front of 20 Aql. The present chemical and physical analysis of the cloud combines the diagnostic tools of radio emission-line data with those of UV and optical absorption data. Using non-LTE models to synthesize the observed absorption profiles, (C-12)O and (C-13)O column densities along this line of sight are determined. The (C-12)O/(C-13)O abundance ratio was found to be 50 +/- 15, similar to that found by Wannier et al. toward Zeta Oph. The physical conditions of the cloud have been investigated using ultraviolet absorption lines. Measurements indicate that the dominant absorption cloud has a gas pressure similar to that found in the local diffuse molecular cloud in Ophiuchus with nT = 20,000/cu cm K.

Interstellar gas and dust in the young cluster IC 348

We have completed a multiband absorption- and emission-line study of a star embedded in the young cluster IC 348, to determine the environmental effect of star formation on the interstellar medium (ISM) local to the region. The hottest and youngest star in IC 348 is BD +31 deg 643, a B5 V star which samples the inner bright nebular region. The nearby star omicron Per, which lies only 8 min to the north and is thought to lie beyond IC 348, samples the gas and dust which has not been processed by very recent star formation. We speculate that the ISM throughout the region was originally the same as that currently seen toward omicron Per, but now the constrasting environmental conditions due to the recent star formation have led to marked differences in the atomic, molecular, and dust characteristics of IC 348. These contrasts include what we have termed a composite UV extinction curve for BD +31 deg 643, evidence for enhanced density and enhanced depletions within IC 348 and very different molecular abundances in the interstellar sightline to BD +31 deg 643. Toward BD +31 deg 643, we find a higher column density of CH, but lower CN and very much higher CH(+) than measured toward omicron Per. We conclude that the physical and chemical state of the gas and dust has been altered by local processes and conditions within IC 348. The characteristics of the ISM in IC 348, via our study of the star BD +31 deg 643, closely resembles those seen toward rho Oph, another sight line passing through a bright nebular region. However, the stars are not as hot in IC 348 as in Ophiuchus, so their effect on the local ISM is not as severe.

The relative spatial distributions of high- and low-luminosity galaxies toward Coma

The relative spatial distributions of low- and high-mass galaxies which lie in a field in the direction of the Coma Supercluster are investigated. Three tests are used to compare the distributions of high-luminosity and low-luminosity galaxies in the field: correlation functions, nearest neighbor distributions, and local density environments. All three tests indicate that the low-luminosity galaxies are significantly less confined to the structure defined by the luminous galaxies than are the luminous galaxies themselves. Several galaxies in the low-luminosity subsample are within voids. These findings lend support to various models for the formation of large-scale structure that include biased galaxy formation. In particular, the ratio of the amplitudes of the correlation functions for dwarfs and giants agrees closely with the predictions of the cold dark matter models of White et al. (1987). 54 refs.