arXiv subjects

K. Sellgren

Publications and source records attributed to K. Sellgren.

35 records · Page 2Linked to original sources

arXiv1999

The Frequency of Barred Spiral Galaxies in the Near-IR

We have determined the fraction of barred galaxies in the H-band for a statistically well-defined sample of 186 spirals drawn from the Ohio State University Bright Spiral Galaxy survey. We find 56% of our sample to be strongly barred at H, while another 16% is weakly barred. Only 27% of our sample is unbarred in the near-infrared. The RC3 and the Carnegie Atlas of Galaxies both classify only about 30% of our sample as strongly barred. Thus strong bars are nearly twice as prevalent in the near-infrared as in the optical. The frequency of genuine optically hidden bars is significant, but lower than many claims in the literature: 40% of the galaxies in our sample that are classified as unbarred in the RC3 show evidence for a bar in the H-band, while for the Carnegie Atlas this fraction is 66%. Our data reveal no significant trend in bar fraction as a function of morphology in either the optical or H-band. Optical surveys of high redshift galaxies may be strongly biased against finding bars, as bars are increasingly difficult to detect at bluer rest wavelengths.

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arXiv1999

ISO Mid-Infrared Spectra of Reflection Nebulae

We present 5-15 micron imaging spectroscopy of the reflection nebulae vdB 17 (NGC 1333), vdB 59 (NGC 2068), vdB 101, vdB 111, vdB 133 and vdB 135, obtained with the infrared camera and circular variable filter wheel on the Infrared Space Observatory (ISO). These nebulae are illuminated by stars with Teff = 3,600-19,000 K, implying ultraviolet to total stellar flux ratios of F(< 400 nm)/F(tot) = 0.01-0.87. We detect the infrared emission features (IEFs) at 6.2, 7.7, 8.6, 11.3, and 12.7 microns, broad emission features at 6-9 microns and 11-13 microns, and 5-15 micron continuum emission, from the interstellar medium in vdB 17, vdB 59, and vdB 133 (F(< 400 nm)/F(tot) = 0.22-0.87), and place upper limits on the emission from the interstellar medium in vdB 101, vdB 111, and vdB 135 (F(< 400 nm)/F(tot) = 0.01-0.20). We observe no spectroscopic differences with varying Teff. We analyze the spectra of vdB 17 and vdB 59 as a function of distance from the star to see how the spectra depend on Go within each source. The only quantitative difference we find is a broadening of the 7.7 micron IEF at Go = 20-60 within vdB 17. We observe only a 40% change in the 6-10 micron to 10-14 micron flux ratio over Go = 20 to 60,000.

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arXiv1999

The First Stellar Abundance Measurements in the Galactic Center: the M Supergiant IRS 7

The first measurement of the photospheric abundances in a star at the Galactic Center are presented. A detailed abundance analysis of the Galactic Center M2 supergiant IRS 7 was carried out using high-resolution near-infrared echelle spectra. The Fe abundance for IRS 7 was found to be close to solar, [Fe/H] = -0.02 +/- 0.13, and nearly identical to the Fe abundances we obtained for the nearby M supergiants alpha Ori and VV Cep. Analysis of the first and second overtone lines of CO was used to derive an effective temperature of 3600 +/- 230 K, a microturbulent velocity of 3.0 +/- 0.3 km/s, and a carbon abundance log epsilon(C) = 7.78 +/- 0.13, or [C/H] = -0.77. In addition, we find a high depletion of 0.74 +/- 0.32 dex in O and an enhancement of 0.92 +/- 0.18 dex in N. These abundances are consistent with the dredge-up of CNO-cycle products but require deep mixing in excess of that predicted by standard models for red supergiants. In light of our measured solar Fe abundance for IRS 7, we discuss other indicators of metallicity at the Galactic Center, the interpretation of low-resolution near-infrared spectra of late-type giants and supergiants, including the need for caution in using such spectra as measures of metallicity, and the evolution of massive young stars at the Galactic Center. We suggest the possibility that rapid stellar rotation is common for stars formed under conditions in the Galactic Center, and that extra internal mixing induced by high rotation rates, rather than evolution at high metallicity, is the explanation for many of the unusual properties of the hot emission-line stars in the Galactic Center.

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arXiv1999

Upper limit on C60 and C60+ features in the ISO-SWS spectrum of the reflection nebula NGC 7023

We present here the 7.0-8.7 micron spectrum of the bright reflection nebula NGC 7023. Our observations are made with the Short Wavelength Spectrometer (SWS) on the European satellite Infrared Space Observatory (ISO). The vibrational bands of the ionized fullerene C60+ are expected at 7.11 and 7.51 microns, while those of the neutral fullerene C60 are expected at 7.0 and 8.45 microns. We estimate an upper limit in NGC 7023 for the C60+ abundance of <0.26% of the interstellar abundance of carbon, while C60 contains <0.27% of interstellar carbon.

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arXiv1999

Near-Infrared Spectroscopy of Molecular Hydrogen Emission in Four Reflection Nebulae: NGC 1333, NGC 2023, NGC 2068, and NGC 7023

We present near-infrared spectroscopy of fluorescent molecular hydrogen (H_2) emission from NGC 1333, NGC 2023, NGC 2068, and NGC 7023 and derive the physical properties of the molecular material in these reflection nebulae. Our observations of NGC 2023 and NGC 7023 and the physical parameters we derive for these nebulae are in good agreement with previous studies. Both NGC 1333 and NGC 2068 have no previously-published analysis of near-infrared spectra. Our study reveals that the rotational-vibrational states of molecular hydrogen in NGC 1333 are populated quite differently from NGC 2023 and NGC 7023. We determine that the relatively weak UV field illuminating NGC 1333 is the primary cause of the difference. Further, we find that the density of the emitting material in NGC 1333 is of much lower density, with n ~ 10^2 - 10^4 cm^-3. NGC 2068 has molecular hydrogen line ratios more similar to those of NGC 7023 and NGC 2023. Our model fits to this nebula show that the bright, H_2-emitting material may have a density as high as n ~ 10^5 cm^-3, similar to what we find for NGC 2023 and NGC 7023. Our spectra of NGC 2023 and NGC 7023 show significant changes in both the near-infrared continuum and H_2 intensity along the slit and offsets between the peaks of the H_2 and continuum emission. We find that these brightness changes may correspond to real changes in the density and temperatures of the emitting region, although uncertainties in the total column of emitting material along a given line of sight complicates the interpretation. The spatial difference in the peak of the H_2 and near-infrared continuum peaks in NGC 2023 and NGC 7023 shows that the near-infrared continuum is due to a material which can survive closer to the star than H_2 can.

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arXiv1999

New Three Micron Spectra of Young Stellar Objects with H_2O Ice Bands

We present new groundbased 3 $μ$m spectra of 14 young stellar objects with H_2O ice absorption bands. The broad absorption feature at 3.47 $μ$m was detected toward all objects and its optical depth is correlated with the optical depth of H_2O ice, strengthening an earlier finding. The broad absorption feature at 3.25 $μ$m was detected toward two more sources and an upper limit is given for a third source. The optical depths of the 3.25 $μ$m feature obtained to date are better correlated with the optical depth of the refractory silicate dust than with that of H_2O ice. If this trend is confirmed, this would support our proposed identification of the feature as the C--H stretch of aromatic hydrocarbons at low temperature. An absorption feature at 3.53 $μ$m due to solid methanol was detected for the first time toward MonR2/IRS2, as well as toward W33A and GL 2136. The wavelengths of the CH_3OH features toward W33A, GL 2136, and NGC7538/IRS9 can be fit by CH_3OH-rich ices, while the wavelength of the feature toward MonR2/IRS2 suggests an H_2O-rich ice environment. Solid methanol abundances toward GL 2136, NGC7538/IRS9, and MonR2/IRS2 are 3-5 % relative to H_2O ice. There is an additional narrow absorption feature near 3.47 $μ$m toward W33A. For the object W51/IRS2, spatially resolved spectra from 2 to 4 $μ$m indicate that the H_2O ice is located predominantly in front of the eastern component and that the H_2O ice extinction is much deeper than previously estimated. For the object RNO 91, spectra from 2 to 4 $μ$m reveal stellar (or circumstellar) CO gas absorption and deeper H_2O ice extinction than previously estimated.

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arXiv1997

New rotation periods in the Pleiades: Interpreting activity indicators

We present results of photometric monitoring campaigns of G, K and M dwarfs in the Pleiades carried out in 1994, 1995 and 1996. We have determined rotation periods for 18 stars in this cluster. In this paper, we examine the validity of using observables such as X-ray activity and amplitude of photometric variations as indicators of angular momentum loss. We report the discovery of cool, slow rotators with high amplitudes of variation. This contradicts previous conclusions about the use of amplitudes as an alternate diagnostic of the saturation of angular momentum loss. We show that the X-ray data can be used as observational indicators of mass-dependent saturation in the angular momentum loss proposed on theoretical grounds.

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arXiv1997

Do the Infrared Emission Features Need Ultraviolet Excitation

We present the results of imaging spectroscopy of the reflection nebula vdB 133, obtained with the infrared camera and circular variable filter wheel on the Infrared Space Observatory (ISO). Our observations reveal the infrared emission features (IEFs), at 6.2, 7.7, 8.6, 11.3, and 12.7 micron, and associated 5 -- 15 micron continuum emission. The stellar system illuminating vdB 133 has the lowest ratio of ultraviolet (shortward of 0.4 micron) to total flux of any stars demonstrated to date to excite the IEFs and associated continuum emission from adjacent interstellar dust, as opposed to circumstellar dust. The low fraction of UV flux from this system poses a problem for existing models for the emission mechanism and emitting material, which all require substantial UV radiation for the excitation of the IEFs and associated continuum.

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arXiv1997

The Extinction Law in an Occulting Galaxy

We measure the extinction law in a galaxy's spiral arm and interarm regions using a visual and infrared (BVRJHK) imaging study of the interacting galaxies NGC 2207 and IC 2163. This is an overlapping spiral galaxy pair in which NGC 2207 partially occults IC 2163. This geometry enables us to directly measure the extinction of light from the background galaxy as it passes through the disk of the foreground galaxy. We measure the extinction as a function of wavelength, and find that there is less extinction in the optical bands than expected from a normal Galactic extinction law. This deviation is significantly larger in the interarm region than in the spiral arm. The extinction curve in the spiral arm resembles a Milky Way $R_V=5.0$ dust model and the interarm extinction curve is flatter (``greyer'') still. We examine the effect of scattering of background galaxy light into the line of sight and find that it is negligible. We also examine the effect of an unresolved patchy dust distribution using a simple two-component dust model as well as the clumpy dust model of Witt & Gordon (1996). Both models clearly demonstrate that an unresolved patchy dust distribution can flatten the extinction curve significantly. When fit to the data, both models suggest that the observed difference between the arm and interarm extinction curves is caused by the interarm region of NGC 2207 having a higher degree of dust patchiness (density ratio between high-density and low-density phases) than the spiral arm region. We note that an unresolved patchy dust distribution will cause us to underestimate the average column depth of gas in a galaxy if based solely on the visual extinction. It is much better to use the infrared extinction for this purpose.

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arXiv1997

H-Band Spectroscopic Classification of OB Stars

We present a new spectroscopic classification for OB stars based on H-band (1.5 micron to 1.8 micron) observations of a sample of stars with optical spectral types. Our initial sample of nine stars demonstrates that the combination of He I 1.7002 micron and H Brackett series absorption can be used to determine spectral types for stars between about O4 and B7 (to within about +/- 2 sub-types). We find that the Brackett series exhibits luminosity effects similar to the Balmer series for the B stars. This classification scheme will be useful in studies of optically obscured high mass star forming regions. In addition, we present spectra for the OB stars near 1.1 micron and 1.3 micron which may be of use in analyzing their atmospheres and winds.

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arXiv1997

Near-Infrared Spectroscopy of Molecular Filaments in the Reflection Nebula NGC 7023

We present near-infrared spectroscopy of fluorescent molecular hydrogen (H_2) emission from molecular filaments in the reflection nebula NGC 7023. We derive the relative column densities of H_2 rotational-vibrational states from the measured line emission and compare these results with several model photodissociation regions covering a range of densities, incident UV-fields, and excitation mechanisms. Our best-fit models for one filament suggest, but do not require, either a combination of different densities, suggesting clumps of 10^6 cm^{-3} in a 10^4 - 10^5 cm^{-3} filament, or a combination of fluorescent excitation and thermally-excited gas, perhaps due to a shock from a bipolar outflow. We derive densities and UV fields for these molecular filaments that are in agreement with previous determinations.

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arXiv1996

Really Cool Stars at the Galactic Center

New and existing K-band spectra for 19 Galactic center late-type stars have been analyzed along with representative spectra of disk and bulge M giants and supergiants. Absorption strengths for strong atomic and molecular features have been measured. The Galactic center stars generally exhibit stronger absorption features centered near Na I (2.206 mic) and Ca I (2.264 mic) than representative disk M stars at the same CO absorption strength. Based on the absolute K-band magnitudes and CO and H2O absorption strengths for the Galactic center stars and known M supergiants and asymptotic giant branch (AGB) stars, we conclude that only IRS 7 must be a supergiant. Two other bright stars in our Galactic center sample are likely supergiants as well. The remaining bright, cool stars in the Galactic center that we have observed are most consistent with being intermediate mass/age AGB stars. We identify four of the Galactic center stars as long period variables based on their K-band spectral properties and associated photometric variability. Estimates of initial masses and ages for the GC stars suggest multiple epochs of star formation have occurred in the Galactic center over the last 7-100 Myr.

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arXiv1996

JHKL Photometry and the K-band Luminosity Function at the Galactic Center

J, H, K, and L photometry for the stars in the central 2' (5 pc) of the Galaxy are presented. Using the observed J-H, H-K, and K-L colors and assumed intrinsic colors, we determine the interstellar extinction at 2.2 mic (A_K) for approximately 1100 individual stars. The mean A_K (= 3.3 mag) is similar to previous results, but we find that the reddening is highly variable and some stars are likely to be seen through A_K > 6 mag. The de-reddened K-band luminosity function points to a significantly brighter component to the stellar population (> 1.5 mag at K) than found in the stellar population in Baade's window, confirming previous work done at lower spatial resolution. The observed flux of all Galactic center stars with estimated Ko (de-reddened magnitude) </= 7.0 mag is approx 25 % of the total in the 2' X 2' field. Our observations confirm the recent finding that several bright M stars in the Galactic center are variable. Our photometry also establishes the near-infrared variability of the M1-2 supergiant, IRS 7.

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arXiv1995

Medium Resolution Near Infrared (2.15micron--2.35micron) Spectroscopy of Late-type Main Sequence stars

We present an atlas of moderate resolution ($λ/Δλ\sim$1380) 2.15~\micron\ to 2.39~\micron\ spectra of 33 luminosity class V stars of spectral type F3 to M6 in the MK classification system. We find that the equivalent widths of several spectral features vary significantly with temperature and typically allow effective temperature determination accurate to approximately $\pm$350~K.

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arXiv1995

A Survey of Near Infrared Emission in Visual Reflection Nebulae

We present a survey for extended 2.2 $μ$m emission in 20 new visual reflection nebulae, illuminated by stars with temperatures of 3,600 --- 33,000 K. We detect extended 2.2 $μ$m emission in 13 new nebulae, illuminated by stars with temperatures of 6,800 -- 33,000 K. For most of these 13 nebulae we have measured $J-K$, $H-K$, and $K-L'$, as well as obtaining surface brightness measurements at the wavelength of the 3.3 $μ$m emission feature. All of the reflection nebulae with extended near infrared emission in excess over scattered starlight have very similar near infrared colors and show the 3.3 $μ$m feature in emission with similar feature-to-continuum ratios. The 3.3 $μ$m feature-to-continuum ratio ranges from $\sim$3 to $\sim$9, both within individual nebulae and from nebula to nebula, which suggests that the 3.3 $μ$m feature and its underlying continuum arise from different materials, or from different ranges of sizes within a size distribution of particles. No dependence on the temperature of the illuminating star is seen in the near infrared colors or 3.3 $μ$m feature-to-continuum ratio, over a factor of two in stellar temperature. This is similar to our previous IRAS results, in which we found no dependence of the ratio of 12 $μ$m to 100 $μ$m surface brightnesses in reflection nebulae illuminated by stars with temperatures of 5,000--33,000 K.

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arXiv1995

A Study of Absorption Features in the Three Micron Spectra of Molecular Cloud Sources with H$_2$O Ice Bands

New 3.3--3.6~$μ$m spectra were obtained of nine young stellar objects embedded in molecular clouds. An absorption feature at $\sim$3.47~$μ$m (2880~cm$^{-1}$) with FWHM$\sim$0.09 $μ$m (80 cm$^{-1}$), first identified by Allamandola et al. (1992), was definitively detected toward seven objects, and marginally in the other two. The feature is better correlated with H$_2$O ice than with the silicate dust optical depth in the data obtained to date. Assuming the feature is due to a C--H stretch absorption, the abundance of the C--H bonds averaged along the lines of sight is closely related to that of H$_2$O ice. We interpret the correlation with H$_2$O ice as indicating that the C--H bonds form together with H$_2$O ice on grain surfaces in the molecular clouds, though other formation mechanisms are not ruled out. A second absorption feature at 3.25 $μ$m (3080 cm$^{-1}$) was detected toward NGC7538/IRS 1 and S140/IRS 1; this feature was first detected in spectra of MonR2/IRS 3 (Sellgren, Smith, \& Brooke 1994; Sellgren et al. 1995). There is as yet insufficient data to tell whether this feature is better correlated with H$_2$O ice or silicates.

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arXiv1995

A New 3.25 Micron Absorption Feature toward Mon R2/IRS-3

A new 3.2--3.5~$μ$m spectrum of the protostar Mon~R2/IRS-3 confirms our previous tentative detection of a new absorption feature near 3.25 $μ$m. The feature in our new spectrum has a central wavelength of 3.256 $μ$m (3071 cm$^{-1}$) and has a full-width at half maximum of 0.079 $μ$m (75 cm$^{-1}$). We explore a possible identification with aromatic hydrocarbons at low temperatures, which absorb at a similar wavelength. If the feature is due to aromatics, the derived column density of C--H bonds is $\sim$1.8 $\times$ $10^{18}$ cm$^{-2}$. If the absorbing aromatic molecules are of roughly the same size as those responsible for aromatic emission features in the interstellar medium, then we estimate that $\sim$9\% of the cosmic abundance of carbon along this line of sight would be in aromatic hydrocarbons, in agreement with abundance estimates from emission features.

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