arXiv subjects

Kim A. Venn

Publications and source records attributed to Kim A. Venn.

82 records · Page 5Linked to original sources

arXiv2009

Stellar Metallicities and Kinematics in a Gas-Rich Dwarf Galaxy: First Calcium Triplet Spectroscopy of RGB Stars in WLM

We present the first determination of the radial velocities and metallicities of 78 red giant stars in the isolated dwarf irregular galaxy WLM. Observations of the calcium II triplet in these stars were made with FORS2 at the VLT-UT2 in two separated fields of view in WLM, and the [Fe/H] values were conformed to the Carretta & Gratton (1997) metallicity scale. The mean metallicity is <[Fe/H]> = -1.27 +/- 0.04 dex, with a standard deviation of 0.37. We find that the stars in the inner field are more metal rich by [Fe/H] =0.30 +/- 0.06 dex. These results are in agreement with previous photometric studies that found a radial population gradient, as well as the expectation of higher metallicities in the central star forming regions. Age estimates using Victoria-Regina stellar models show that the youngest stars in the sample (< 6 Gyr) are more metal rich by [Fe/H] = 0.32 +/- 0.08 dex. These stars also show a lower velocity dispersion at all elliptical radii compared to the metal-poor stars. Kinematics for the whole red giant sample suggest a velocity gradient approximately half that of the gas rotation curve, with the stellar component occupying a thicker disk decoupled from the HI rotation plane. Taken together, the kinematics, metallicities, and ages in our sample suggest a young metal-rich, and kinematically cold stellar population in the central gas-rich regions of WLM, surrounded by a separate dynamically hot halo of older, metal poor stars.

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arXiv2007

Ram pressure stripping of an isolated Local Group dwarf galaxy: evidence for an intra-group medium

We compare the stellar structure of the isolated, Local Group dwarf galaxy Pegasus (DDO216) with low resolution HI maps from Young et al. (2003). Our comparison reveals that Pegasus displays the characteristic morphology of ram pressure stripping; in particular, the HI has a ``cometary'' appearance which is not reflected in the regular, elliptical distribution of the stars. This is the first time this phenomenon has been observed in an isolated Local Group galaxy. The density of the medium required to ram pressure strip Pegasus is at least $10^{-5} - 10^{-6}$, cm$^{-3}$. We conclude that this is strong evidence for an inter-galactic medium associated with the Local Group.

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arXiv2006

The Araucaria Project. VLT spectra of blue supergiants in WLM: classification and first abundances

As part of the Araucaria Project, we present the first spectral catalog of supergiant stars in the Local Group dwarf irregular galaxy WLM. In assigning a spectral classification to these stars we accounted for the low metal content of WLM relative to the galactic standards used in the MK process, by using classification criteria developed for B and A supergiants contained in the Small Magellanic Cloud. Our spectral catalog shows that our higher S/N spectroscopic sample of 19 objects contains at least 6 early-B (B0-B5) supergiants and 6 late-B and early-A (B8-A2) stars of luminosity class between Ia and II, as well as an O7~V star and an O9.7 Ia star. The spectra of several of these stars is of sufficiently high quality for a determination of the stellar parameters and abundances. We have acquired also a second set of lower S/N spectra for mostly BA stars, however their quality does not allow a further analysis. We have carried out a quantitative analysis for three early-B supergiants. The mean oxygen abundance we derive is 12+log(O/H)=7.83 +/- 0.12. This value agrees very well with the measurement that is obtained from HII regions. We therefore find no additional evidence for the discrepancy between stellar and nebular oxygen abundances measured for a single A-type supergiant by Venn et al. The analysis of B- and A-type supergiants yields compatible results for nitrogen, silicon and magnesium. We show that the photometric variability of the blue supergiants included in our spectroscopic sample is negligible for the use of these stars as distance indicators.

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arXiv2005

The Spatial Homogeneity of Nebular and Stellar Oxygen Abundances in the Local Group Dwarf Irregular Galaxy NGC 6822

To test the existence of a possible radial gradient in oxygen abundances within the Local Group dwarf irregular galaxy NGC 6822, we have obtained optical spectra of 19 nebulae with the EFOSC2 spectrograph on the 3.6-m telescope at ESO La Silla. The extent of the measured nebulae spans galactocentric radii in the range between 0.05 kpc and 2 kpc (over four exponential scale lengths). In five H II regions (Hubble I, Hubble V, Kalpha, Kbeta, KD28e), the temperature-sensitive [O III] 4363 emission line was detected, and direct oxygen abundances were derived. Oxygen abundances for the remaining H II regions were derived using bright-line methods. The oxygen abundances for three A-type supergiant stars are slightly higher than nebular values at comparable radii. Linear least-square fits to various subsets of abundance data were obtained. When all of the measured nebulae are included, no clear signature is found for an abundance gradient. A fit to only newly observed H II regions with [O III] 4363 detections yields an oxygen abundance gradient of -0.14 +/- 0.07 dex/kpc. The gradient becomes slightly more significant (-0.16 +/- 0.05 dex/kpc) when three additional H II regions with [O III] 4363 measurements from the literature are added. Assuming no abundance gradient, we derive a mean nebular oxygen abundance 12+log(O/H) = 8.11 +/- 0.10 from [O III] 4363 detections in the five H II regions from our present data; this mean value corresponds to [O/H] = -0.55.

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arXiv2005

A Comparison of Elemental Abundance Ratios in Globular Clusters, Field Stars, and Dwarf Spheroidal Galaxies

We have compiled a sample of globular clusters with high quality stellar abundances from the literature to compare to the chemistries of stars in the Galaxy and those in dwarf spheroidal galaxies. Of the 45 globular clusters examined, 29 also have kinematic information. Most of the globular clusters belong to the Galactic halo, however a signficant number have disk kinematics or belong to the bulge. Focusing on the [alpha/Fe] and light r-process element ratios, we find that most globular cluster stars mimic those of the field stars of similar metallicities, and neither clearly resembles the presently available stellar abundances in the dwarf galaxies (including the globular clusters in the Large Magellanic Cloud). The exceptions to these general elemental ratio comparisons are already known in the literature, e.g., omega Centauri, Palomar 12, and Terzan 7 associated with the Sagittarius remnant, and Ruprecht 106 which has a high radial velocity and low [alpha/Fe] ratio. A few other globular clusters show more marginal peculiarities. The most notable one being the halo cluster M68 which has a high Galactocentric rotational velocity, a slightly younger age, and a unique [Si/Ti] ratio. The [Si/Ti] ratios decrease with increasing [Fe/H] at intermediate metallicities, which is consistent with very massive stars playing a larger role in the early chemical evolution of the Galaxy. The chemical similarities between globular clusters and field stars with [Fe/H]<-1.0 suggests a shared chemical history in a well mixed early Galaxy. The differences to the published chemistries of stars in the dwarf spheroidal galaxies suggests that neither the globular clusters, halo stars, nor thick disk stars had their origins in small isolated systems like the present-day Milky Way dwarf satellites.

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arXiv2004

Investigating The Possible Anomaly Between Nebular and Stellar Oxygen Abundances in the Dwarf Irregular Galaxy WLM

We obtained new optical spectra of 13 H II regions in WLM with EFOSC2; oxygen abundances are derived for nine H II regions. The temperature-sensitive [O III] 4363 emission line was measured in two bright H II regions HM7 and HM9. The direct oxygen abundances for HM7 and HM9 are 12+log(O/H) = 7.72 +/- 0.04 and 7.91 +/- 0.04, respectively. We adopt a mean oxygen abundance of 12+log(O/H) = 7.83 +/- 0.06. This corresponds to [O/H] = -0.83 dex, or 15% of the solar value. In H II regions where [O III] 4363 was not measured, oxygen abundances derived with bright-line methods are in general agreement with direct values of the oxygen abundance to an accuracy of about 0.2 dex. In general, the present measurements show that the H II region oxygen abundances agree with previous values in the literature. The nebular oxygen abundances are marginally consistent with the mean stellar magnesium abundance ([Mg/H] = -0.62). However, there is still a 0.62 dex discrepancy in oxygen abundance between the nebular result and the A-type supergiant star WLM15 ([O/H] = -0.21). Non-zero reddening values derived from Balmer line ratios were found in H II regions near a second H I peak. There may be a connection between the location of the second H I peak, regions of higher extinction, and the position of WLM15 on the eastern side of the galaxy.

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arXiv2004

First Stellar Abundances in the Dwarf Irregular Galaxy Sextans A

We present the abundance analyses of three isolated A-type supergiant stars in the dwarf irregular galaxy Sextans A from high-resolution spectra the UVES spectrograph at the VLT. Detailed model atmosphere analyses have been used to determine the stellar atmospheric parameters and the elemental abundances of the stars. The mean iron group abundance was determined from these three stars to be [(FeII,CrII)/H]=-0.99+/-0.04+/-0.06. This is the first determination of the present-day iron group abundances in Sextans A. These three stars now represent the most metal-poor massive stars for which detailed abundance analyses have been carried out. The mean stellar alpha element abundance was determined from the alpha element magnesium as [alpha(MgI)/H]=-1.09+/-0.02+/-0.19. This is in excellent agreement with the nebular alpha element abundances as determined from oxygen in the H II regions. These results are consistent from star-to-star with no significant spatial variations over a length of 0.8 kpc in Sextans A. This supports the nebular abundance studies of dwarf irregular galaxies, where homogeneous oxygen abundances are found throughout, and argues against in situ enrichment. The alpha/Fe abundance ratio is [alpha(MgI)/FeII,CrII]=-0.11+/-0.02+/-0.10, which is consistent with the solar ratio. This is consistent with the results from A-supergiant analyses in other Local Group dwarf irregular galaxies but in stark contrast with the high [alpha/Fe] results from metal-poor stars in the Galaxy, and is most clearly seen from these three stars in Sextans A because of their lower metallicities. The low [alpha/Fe] ratios are consistent with the slow chemical evolution expected for dwarf galaxies from analyses of their stellar populations.

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arXiv2002

New Nitrogen and Carbon in AF-supergiants

The AF-supergiants in the Galaxy and the SMC allow us to examine predictions from evolution models through their CNO abundances. In these proceedings, we recalculate the NLTE nitrogen abundances in 22 Galactic and 9 SMC A-supergiants using improved atomic data and model atmospheres to compare with new evolution models. The new abundances are higher than previously published values, and suggest that most of these stars have undergone substantial mixing with CN-cycled gas. While there is no clear relationship with mass, there is an apparent relation with metallicity since the SMC stars (including B-stars) have larger nitrogen enrichments. We suggest that rotational mixing is indicated from the main-sequence throughout the supergiant range, with more substantial rotational mixing in the SMC stars. In addition, the SMC AF-supergiants appear to have undergone the first dredge-up during a previous red giant phase, and possibly the Galactic AF-supergiants have as well. All abundances are compared to the new solar abundances from M. Asplund (this conference).

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arXiv2000

Analysis of Four A-F Supergiants in M31 from Keck HIRES Spectroscopy

The first stellar abundances in M31 are presented, based on Keck I HIRES spectroscopy and model atmospheres analyses of three A-F supergiants, 41-2368, 41-3712, and A-207. We also present the preliminary analysis of a fourth star, 41-3654. We find that the stellar oxygen abundances are in good agreement with those determined from nebular studies, even though the stars do {\it not} show a clear radial gradient in oxygen. The uncertainties in the stellar abundances are smaller than the range in the nebular results, making these stars ideal objects for further studies of the distribution of oxygen in M31. We show that the stars can be used to study the abundance distributions of other elements as well, including iron-group and heavier elements. The A-F supergiants also provide direct information on the metallicity and reddening of nearby Cepheid stars. We have examined the metallicity and reddening assumptions used for Cepheids within 1' of our targets and noted the differences from values used in the literature.

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arXiv1995

The Abundance of Boron in Evolved A- and B-Type Stars

Boron abundances in A- and B-type stars may be a successful way to track evolutionary effects in these hot stars. The light elements -- Li, Be, and B -- are tracers of exposure to temperatures more moderate than those in which the H-burning CN-cycle operates. Thus, any exposure of surface stellar layers to deeper layers will affect these light element abundances. Li and Be are used in this role in investigations of evolutionary processes in cool stars, but are not observable in hotter stars. An investigation of boron, however, is possible through the B II 1362=C5 resonance line. We have gathered high resolution spectra from the IUE database of A- and B-type stars near 10~M$_\odot$ for which nitrogen abundances have been determined (by Gies & Lambert, 1992, and Venn 1995). The B II 1362=C5 line is blended throughout the temperature range of this program, requiring spectrum syntheses to recover the boron abundances. For no star could we synthesize the 1362=C5 region using the meteoritic/solar boron abundance of log(B) =3D 2.88 (Anders & Grevesse 1989); a lower boron abundance was necessary which may reflect evolutionary effects (e.g., mass loss or mixing near the main-sequence), the natal composition of the star forming regions, or a systematic error in the analyses (e.g., non-LTE effects). Regardless of the initial boron abundance, and despite the possibility of non-LTE effects, it seems clear that boron is severely depleted in some stars. It may be that the nitrogen and boron abundances are anticorrelated, as would be expected from mixing between the H-burning and outer stellar layers. If, as we suspect, a residue of boron is present in the A-type supergiants, we may exclude a scenario in which mixing occurs continuously between the surface and the deep

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