arXiv subjects

Wenjin Huang

Publications and source records attributed to Wenjin Huang.

17 recordsLinked to original sources

arXiv2012

A catalogue of Paschen-line profiles in standard stars

We have assembled an atlas of line profiles of the Paschen Delta line at 10,049 angstroms for the use of stellar modelling. For a few stars we have substituted the Paschen Gamma line at 10,938 angstroms because the Paschen Delta line blends with other features. Most of the targets are standard stars of spectral types from B to M. A few metal-poor stars have been included. For many of the stars we have also observed the Hydrogen Alpha line so as to compare the profiles of lines originating from the meta-stable n=2 level with lines originating from the n=3 level. The greatest difference in line profile is found for high luminosity and cool stars where the departures from LTE in the population of the n=2 level is expected to be the greatest. For a few stars, sample line profiles have been calculated in the LTE approximation to demonstrate the usefulness of the tabulated and displayed catalogue.

astro-ph.SR↗

arXiv2012

Metal Abundance Calibration of the Ca II Triplet Lines in RR Lyrae Stars

The GAIA satellite is likely to observe thousands of RR Lyrae stars within a small spectral window, between 8470A and 8750A, at a resolution of 11,500. In order to derive the metallicity of RR Lyrae stars from Gaia, we have obtained numerous spectra of RR Lyrae stars at a resolution of 35,000 with the Apache Point Observatory 3.5 m echelle spectrograph. We have correlated the Ca II triplet line strengths with metallicity as derived from Fe II abundances, analogous to Preston's (1959) use of the Ca II K line to estimate the metallicity of RR Lyrae stars. The Ca II line at 8498A is the least blended with neighboring Paschen lines and thus provides the best correlation.

astro-ph.SR↗

arXiv2011

The Peculiar Chemical Inventory of NGC 2419 -- An Extreme Outer Halo "Globular Cluster"

NGC 2419 is a massive outer halo Galactic globular cluster whose stars have previously been shown to have somewhat peculiar abundance patterns. We have observed seven luminous giants that are members of NGC 2419 with Keck/HIRES at reasonable SNR. One of these giants is very peculiar, with an extremely low [Mg/Fe] and high [K/Fe] but normal abundances of most other elements. The abundance pattern does not match the nucleosynthetic yields of any supernova model. The other six stars show abundance ratios typical of inner halo Galactic globular clusters, represented here by a sample of giants in the nearby globular cluster M30. Although our measurements show that NGC 2419 is unusual in some respects, its bulk properties do not provide compelling evidence for a difference between inner and outer halo globular clusters.

astro-ph.GA↗

arXiv2010

A Stellar Rotation Census of B Stars: from ZAMS to TAMS

Two recent observing campaigns provide us with moderate dispersion spectra of more than 230 cluster and 370 field B stars. Combining them and the spectra of the B stars from our previous investigations ($\sim$430 cluster and $\sim$100 field B stars) yields a large, homogeneous sample for studying the rotational properties of B stars. We derive the projected rotational velocity $V\sin i$, effective temperature, gravity, mass, and critical rotation speed $V_{\rm crit}$ for each star. We find that the average $V\sin i$ is significantly lower among field stars because they are systematically more evolved and spun down than their cluster counterparts. The rotational distribution functions of $V_{\rm eq}/V_{\rm crit}$ for the least evolved B stars show that lower mass B stars are born with a larger proportion of rapid rotators than higher mass B stars. However, the upper limit of $V_{\rm eq}/V_{\rm crit}$ that may separate normal B stars from emission line Be stars (where rotation promotes mass loss into a circumstellar disk) is smaller among the higher mass B stars. We compare the evolutionary trends of rotation (measured according to the polar gravity of the star) with recent models that treat internal mixing. The spin-down rates observed in the high mass subset ($\sim 9 M_\odot$) agree with predictions, but the rates are larger for the low mass group ($\sim 3 M_\odot$). The faster spin down in the low mass B stars matches well with the predictions based on conservation of angular momentum in individual spherical shells. Our results suggest the fastest rotators (that probably correspond to the emission line Be stars) are probably formed by evolutionary spin up (for the more massive stars) and by mass transfer in binaries (for the full range of B star masses).

astro-ph.SR↗

arXiv2010

The Chemical Evolution of the Ursa Minor Dwarf Spheroidal Galaxy

We present an abundance analysis based on high resolution spectra of 10 stars selected to span the full range in metallicity in the Ursa Minor dwarf spheroidal galaxy. We find [Fe/H] for the sample stars ranges from -1.35 to -3.10 dex and establish the trends of the abundance ratios [X/Fe]. In key cases, particularly for the alpha-elements, these resemble those for stars in the outer part of the Galactic halo, especially at the lowest metallicities probed. The n-capture elements show a r-process distribution over the full range of Fe-metallicity. This suggests that the duration of star formation in the UMi dSph was shorter than in other dSph galaxies. The derived ages for a larger sample of UMi stars with more uncertain metallicities also suggest a population dominated by uniformly old (~13 Gyr) stars, with a hint of an age-metallicity relationship. In comparing our results for UMi, our earlier work in Draco, and published studies of more metal-rich dSph Galactic satellites, there appears to be a pattern of moving from a chemical inventory for dSph giants with [Fe/H] < -2 dex which is very similar to that of stars in the outer part of the Galactic halo (enhanced alpha/Fe relative to the Sun, coupled with subsolar [X/Fe] for the heavy neutron capture elements and r-process domination), switching to subsolar alpha-elements and super-solar s-process dominated neutron capture elements for the highest [Fe/H] dSph stars. The combination of low star formation rates over a varying and sometimes extended duration that produced the stellar populations in the local dSph galaxies with [Fe/H] > -1.5 dex leads to a chemical inventory wildly discrepant from that of any component of the Milky Way.

astro-ph.CO↗

arXiv2010

KP Cyg: an Unusual Metal-rich RR Lyr Type Star of Long Period

We present the results of a detailed spectroscopic study of the long period ($P=0.856$ days) RR Lyrae star, KP Cyg. We derived abundances of many chemical elements including the light species, iron-group elements and elements of the s-processes. Most RR Lyrae stars with periods longer than 0.7 days are metal-deficient objects. Surprisingly, our results show that KP Cyg is very metal rich ([Fe/H] $= +0.18\pm 0.23$). By comparison with a number of short period ($P=1\sim 6$ days), metal-rich CWB stars, we suggest that KP Cyg may be a very short period CWB star (BL Her star) rather than an RR Lyrae star. As seen in some CWB stars, KP Cyg shows strong excesses of carbon and nitrogen in its atmosphere. This indicates that the surface of KP Cyg has been polluted by material that has undergone helium burning (to enhance carbon) and proton capture (to transform carbon into nitrogen). We also note that UY CrB, whose period is 0.929 days, also shows an enhancement of C and N, and that two carbon cepheids of short period, V553 Cen and RT TrA, show similar excesses of carbon and nitrogen.

astro-ph.SR↗

arXiv2010

The Composition of RR Lyrae Stars: Start-line for the AGB

This paper sumarizes research on abundances in RR Lyrae stars that one of us (GW) has been engaged in with various astronomers. In addition we report on preliminary analysis of the abundances of C, Si, S and Fe in 24 RR Lyrae stars. Our model atmosphere analysis, including NLTE effects, are based on the spectra of resolving power 30,000 obtained at the Apache Poing Observatory.

astro-ph.SR↗

arXiv2009

A Puzzle Involving Galactic Bulge Microlensing Events

We study a sample of 16 microlensed Galactic bulge main sequence turnoff region stars for which high dispersion spectra have been obtained with detailed abundance analyses. We demonstrate that there is a very strong and highly statistically significant correlation between the maximum magnification of the microlensed bulge star and the value of the [Fe/H] deduced from the high resolution spectrum of each object. Physics demands that this correlation, assuming it to be real, be the result of some sample bias. We suggest several possible explanations, but are forced to reject them all,and are left puzzled. To obtain a reliable metallicity distribution in the Galactic bulge based on microlensed dwarf stars it will be necessary to resolve this issue through the course of additional observations.

astro-ph.GA↗

arXiv2009

HD 69686: A Mysterious High Velocity B Star

We report on the discovery of a high velocity B star, HD 69686. We estimate its space velocity, distance, surface temperature, gravity, and age. With these data, we are able to reconstruct the trajectory of the star and to trace it back to its birthplace. We use evolutionary tracks for single stars to estimate that HD 69686 was born 73 Myr ago in the outer part of our Galaxy ($r \sim 12$ kpc) at a position well below the Galactic plane ($z \sim -1.8$ kpc), a very unusual birthplace for a B star. Along the star's projected path in the sky, we also find about 12 other stars having similar proper motions, and their photometry data suggest that they are located at the same distance as HD 69686 and probably have the same age. We speculate on the origin of this group by star formation in a high velocity cloud or as a Galactic merger fragment.

astro-ph.SR↗

arXiv2009

Variability of Be Stars in Southern Open Clusters

We recently discovered a large number of highly active Be stars in the open cluster NGC 3766, making it an excellent location to study the formation mechanism of Be star disks. To explore whether similar disk appearances and/or disappearances are common among the Be stars in other open clusters, we present here multiple epochs of H-alpha spectroscopy for 296 stars in eight open clusters. We identify 12 new transient Be stars and confirm 17 additional Be stars with relatively stable disks. By comparing the H-alpha equivalent widths to the photometric y - H-alpha colors, we present a method to estimate the strength of the H-alpha emission when spectroscopy is not available. For a subset of 128 stars in four open clusters, we also use blue optical spectroscopy and available Stromgren photometry to measure their projected rotational velocities, effective temperatures, and polar surface gravities. We combine our Be star detections from these four clusters to investigate physical differences between the transient Be stars, stable Be stars, and normal B-type stars with no line emission. Both types of Be stars are faster rotating populations than normal B-type stars, and we find no significant physical differences between the transient and stable Be stars in our sample.

astro-ph.SR↗

arXiv2009

The Chemical Evolution of the Draco Dwarf Spheroidal Galaxy

We present an abundance analysis based on high resolution spectra of 8 stars selected to span the full range in metallicity in the Draco dwarf spheroidal galaxy. We find [Fe/H] for the sample stars ranges from -1.5 to -3.0 dex. Combining our sample with previously published work for a total of 14 luminous Draco giants, we show that the abundance ratios [Na/Fe], [Mg/Fe] and [Si/Fe] for the Draco giants overlap those of Galactic halo giants at the lowest [Fe/H] probed, but are significantly lower for the higher Fe-metallicity Draco stars. For the explosive alpha-elements Ca and Ti, the abundance ratios for Draco giants with [Fe/H] > -2.4 dex are approximately constant and slightly sub-solar, well below values characteristic of Galactic halo stars. The s-process contribution to the production of heavy elements begins at significantly lower Fe-metallicity than in the Galactic halo. Using a toy model we compare the behavior of the abundance ratios within the sample of Draco giants with those from the literature of Galactic globular clusters, and the Carina and Sgr dSph galaxies. The differences appear to be related to the timescale for buildup of the heavy elements, with Draco having the slowest rate. We note the presence of a Draco giant with [Fe/H] < -3.0 dex in our sample, and reaffirm that the inner Galactic halo could have been formed by early accretion of Galactic satellite galaxies and dissolution of young globular clusters, while the outer halo could have formed from those satellite galaxies accreted later.

astro-ph.GA↗

arXiv2009

Clues to the Metallicity Distribution in the Galactic Bulge: Abundances in MOA-2008-BLG310 and MOA-2008-BLG311

We present abundance analyses based on high dispersion and high signal-to-noise ratio Magellan spectra of two highly microlensed Galactic bulge stars in the region of the main sequence turnoff with Teff ~ 5650 K. We find that MOA-2008-BLG310 has [Fe/H]= +0.41 (+,- 0.09 dex) and MOA-2008-BLG311 has +0.26 (+, - 0.09 dex). The abundance ratios for the ~20 elements for which features could be detected in the spectra of each of the two stars follow the trends with [Fe/H] found among samples of bulge giants. Combining these two bulge dwarfs with the results from previous abundance analysis of four other Galactic bulge turnoff region stars, all highly magnified by microlensing, gives a mean [Fe/H] of +0.29dex. This implies that there there is an inconsistency between the Fe-metallicity distribution of the microlensed bulge dwarfs and that derived by the many previous estimates based on surveys of cool, luminous bulge giants, which have mean [Fe/H] ~ -0.1 dex. A number of possible mechanisms for producing this difference are discussed. If one ascribes this inconsistency to systematic errors in the abundance analyses, we provide statistical arguments suggesting that a substantial systematic error in the Fe-metallicity for one or both of the two cases, bulge dwarfs vs bulge giants, is required which is probably larger than can realistically be accommodated.

astro-ph.SR↗

arXiv2008

Stellar Rotation in Field and Cluster B-Stars

We present the results of a spectroscopic investigation of 108 nearby field B-stars. We derive their key stellar parameters, $V \sin i$, $T_{\rm eff}$, $\log g$, and $\log g_{\rm polar}$, using the same methods that we used in our previous cluster B-star survey. By comparing the results of the field and the cluster samples, we find that the main reason for the overall slower rotation of the field sample is that it contains a larger fraction of older stars than found in the (mainly young) cluster sample.

astro-ph↗

arXiv2008

Clues to the Metallicity Distribution in the Galactic Bulge: Abundances in OGLE-2007_BLG-349S

We present an abundance analysis based on high dispersion and high signal-to-noise ratio Keck spectra of a very highly microlensed Galactic bulge dwarf, OGLE-2007-BLG-349S, with Teff ~ 5400 K. The amplification at the time the spectra were taken ranged from 350 to 450. This bulge star is highly enhanced in metallicity with [Fe/H] = +0.51 \pm 0.09 dex. The abundance ratios for the 28 species of 26 elements for which features could be detected in the spectra are solar. In particular, there is no evidence for enhancement of any of the alpha-elements including O and Mg. We conclude that the high [Fe/H] seen in this star, when combined with the equally high [Fe/H] derived in previous detailed abundance analysis of two other Galactic bulge dwarfs, both also microlensed, implies that the median metallicity in the Galactic bulge is very high. We thus infer that many previous estimates of the metallicity distribution in the Galactic bulge have substantially underestimated the mean Fe-metallicity there due to sample bias, and suggest a candidate mechanism for such. If our conjecture proves valid, it may be necessary to update the calibrations for the algorithms used by many groups to interpret spectra and broad band photometry of the integrated light of very metal-rich old stellar populations, including luminous elliptical galaxies.

astro-ph↗

arXiv2007

The B and Be Star Population of NGC 3766

We present multiple epochs of H-alpha spectroscopy for 47 members of the open cluster NGC 3766 to investigate the long term variability of its Be stars. Sixteen of the stars in this sample are Be stars, including one new discovery. Of these, we observe an unprecedented 11 Be stars that undergo disk appearances and/or near disappearances in our H-alpha spectra, making this the most variable population of Be stars known to date. NGC 3766 is therefore an excellent location to study the formation mechanism of Be star disks. From blue optical spectra of 38 cluster members and existing Stromgren photometry of the cluster, we also measure rotational velocities, effective temperatures, and polar surface gravities to investigate the physical and evolutionary factors that may contribute to the Be phenomenon. Our analysis also provides improvements to the reddening and distance of NGC 3766, and we find E(B-V) = 0.22 +/- 0.03 and (V-M_V)_0 = 11.6 +/- 0.2, respectively. The Be stars are not associated with a particular stage of main-sequence evolution, but they are a population of rapidly rotating stars with a velocity distribution generally consistent with rotation at 70-80% of the critical velocity, although systematic effects probably underestimate the true rotational velocities so that the rotation is much closer to critical. Our measurements of the changing disk sizes are consistent with the idea that transitory, nonradial pulsations contribute to the formation of these highly variable disks.

astro-ph↗

arXiv2006

Stellar Rotation in Young Clusters. I. Evolution of Projected Rotational Velocity Distributions

Open clusters offer us the means to study stellar properties in samples with well-defined ages and initial chemical composition. Here we present a survey of projected rotational velocities for a large sample of mainly B-type stars in young clusters to study the time evolution of the rotational properties of massive stars. The survey is based upon moderate resolution spectra made with the WIYN 3.5 m and CTIO 4 m telescopes and Hydra multi-object spectrographs, and the target stars are members of 19 young open clusters with an age range of approximately 6 to 73 Myr. We made fits of the observed lines He I 4026, 4387, 4471 and Mg II 4481 using model theoretical profiles to find projected rotational velocities for a total of 496 OB stars. We find that there are fewer slow rotators among the cluster B-type stars relative to nearby B stars in the field. We present evidence consistent with the idea that the more massive B stars (M > 9 solar masses) spin down during their main sequence phase. However, we also find that the rotational velocity distribution appears to show an increase in the numbers of rapid rotators among clusters with ages of 10 Myr and higher. These rapid rotators appear to be distributed between the zero age and terminal age main sequence locations in the Hertzsprung-Russell diagram, and thus only a minority of them can be explained as the result of a spin up at the terminal age main sequence due to core contraction. We suggest instead that some of these rapid rotators may have been spun up through mass transfer in close binary systems.

astro-ph↗

arXiv2005

Binary and Multiple O-Type Stars in the Cas OB6 Association

We present the results of time-resolved spectroscopy of 13 O-type stars in the Cas OB6 stellar association. We conducted a survey for radial velocity variability in search of binary systems, which are expected to be plentiful in young OB associations. Here we report the discovery of two new single-lined binaries, and we present new orbital elements for three double-lined binaries (including one in the multiple star system HD 17505). One of the double-lined systems is the eclipsing binary system DN Cas, and we present a preliminary light curve analysis that yields the system inclination, masses, and radii. We compare the spectra of the single stars and the individual components of the binary stars with model synthetic spectra to estimate the stellar effective temperatures, gravities, and projected rotational velocities. We also make fits of the spectral energy distributions to derive E(B-V), R=A_V/E(B-V), and angular diameter. A distance of 1.9 kpc yields radii that are consistent with evolutionary models. We find that 7 of 14 systems with spectroscopic data are probable binaries, consistent with the high binary frequency found for other massive stars in clusters and associations.

astro-ph↗