arXiv subjects

Ronald Wilhelm

Publications and source records attributed to Ronald Wilhelm.

At least 19 recordsLinked to original sources

arXiv2024

Caught in the Act: Observations of the Double-mode RR Lyrae V338 Boo During the Disappearance of a Pulsation Mode

New results on the behavior of the double-mode RR Lyrae V338 Boo are presented. The Transiting Exoplanet Survey Satellite (TESS) observed this star again in 2022, and an observing campaign of the American Association of Variable Star Observers (AAVSO) was completed after the TESS observations as a follow-up. We find that the first overtone pulsation mode in this star completely disappears during the TESS observing window. This mode reappears at the end of the TESS observations, and the AAVSO observing campaign shows that in the months that followed, the first overtone mode was not only present, but was the dominant mode of pulsation. This star, and potentially others like it, could hold the key to finally solving the mystery of the Blazhko effect in RR Lyrae.

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arXiv2023

rrlfe: Software for Generating and Applying Metallicity Calibrations for RR Lyrae Variable Stars Across a Wide Range of Phases and Temperatures

RR Lyrae stars play a central role in tracing phase-space structures within the Milky Way because they are easy to identify, are relatively luminous, and are found in large numbers in the Galactic bulge, disk, and halo. In this work, we present a new set of spectroscopic metallicity calibrations that use the equivalent widths of the Ca II K and Balmer H-gamma and H-delta lines to calculate metallicity values from low-resolution spectra. This builds on an earlier calibration from Layden by extending the range of equivalent widths which map between Ca II K and the Balmer lines. We have developed the software rrlfe to apply this calibration to spectra in a consistent, reproducible, and extensible manner. This software is open-source and available to the community. The calibration can be updated with additional datasets in the future.

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arXiv2023

Modulation of the Blazhko Cycle in LS Her

We present analysis of the RR Lyrae star, LS Her and confirm the previously reported modulation to its Blazhko cycles. We performed Fourier analysis on two sectors (Sector 24 & 25) of data from the Transiting Exoplanet Survey Satellite (TESS) spanning 53 days. We find LS Her to have a primary pulsation period of 0.2308 d and a Blazhko period of 12.7 d in keeping with previously reported results. We also identified side-band frequencies around the Blazhko multiplets suggesting the Blazhko cycle is modulated on a time scale of 112 days. Analysis of the Blazhko effect using the TESS data clearly shows a changing amplitude and phase throughout the four Blazhko cycles. We compared our modeled results, which were based on our TESS frequency analysis, to TESS data (Sector 51) taken ~700 days later and found our modulation model was not a good representation of the data. We then coupled our TESS analysis with the modulation frequency results from Wils et al. (MNRAS 387 (2008) 783-787) and found excellent agreement with the Sector 51 data. To further test this result we obtained ground-based, V-magnitude observations of LS Her in the summer of 2022. This data also showed excellent agreement with our coupled modulation model. We have verified that LS Her is a Blazhko star with a modulated Blazhko period of 109 days, stability over the 862 days of observations, and possible stability lasting over 15 years. We discuss the ramifications of the modulation for other Blazhko stars that show Blazhko effect changes over time.

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arXiv2022

SDSS-IV MaStar: Stellar Parameter Determination with Continuum-Supplemented Full-Spectrum Fitting

We present a stellar parameter catalog built to accompany the MaStar Stellar Library, which is a comprehensive collection of empirical, medium-resolution stellar spectra. We constructed this parameter catalog by using a multicomponent $\chi^{2}$ fitting approach to match MaStar spectra to models generated by interpolating the ATLAS9-based BOSZ model spectra. The total $\chi^{2}$ for a given model is defined as the sum of components constructed to characterize narrow-band features of observed spectra (e.g., absorption lines) and the broadband continuum shape separately. Extinction and systematics due to flux calibration were taken into account in the fitting. The $\chi^{2}$ distribution for a given region of model space was sampled using a Markov Chain Monte Carlo (MCMC) algorithm, the data from which were then used to extract atmospheric parameter estimates ($T_{eff}$, log g, [Fe/H], and [$\alpha$/Fe]), their corresponding uncertainties, and direct extinction measurements. Two methods were used to extract parameters and uncertainties: one that accepts the MCMC's prescribed minimum-$\chi^{2}$ result, and one that uses Bayesian inference to compute a likelihood-weighted mean from the $\chi^{2}$ distribution sampled by the MCMC. Results were evaluated for internal consistency using repeat observations where available and by comparing them with external data sets (e.g., APOGEE-2 and Gaia DR2). Our spectral-fitting exercise reveals possible deficiencies in current theoretical model spectra, illustrating the potential power of MaStar spectra for helping to improve the models. This paper represents an update to the parameters that were originally presented with SDSS-IV DR17. The MaStar parameter catalog containing our BestFit results is available on the SDSS-IV DR17 website as part of version 2 of the MaStar stellar parameter value-added catalog.

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arXiv2021

The Changing Lightcurve of the Double-Mode RR Lyrae Variable Star V338 Boo

We present an analysis of the lightcurve extracted from Transiting Exoplanet Survey Satellite Full Frame Images of the double-mode RR Lyrae V338 Boo. We find that the fundamental mode pulsation is changing in amplitude across the 54 days of observations. The first overtone mode pulsation also changes, but on a much smaller scale. Harmonics and combinations of the primary pulsation modes also exhibit unusual behavior. Possible connections with other changes in RR Lyrae pulsations are discussed, but a full understanding of the cause of the changes seen in V338 Boo should shed light on some of the most difficult and unanswered questions in stellar pulsation theory, and astrophysics more generally.

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arXiv2021

Dynamically Tagged Groups of Very Metal-poor Halo Stars from the HK and Hamburg/ESO Surveys

We analyze the dynamical properties of $\sim$1500 very metal-poor (VMP; [Fe/H] $\lesssim -2.0$) halo stars, based primarily on medium-resolution spectroscopic data from the HK and Hamburg/ESO surveys. These data, collected over the past thirty years, are supplemented by a number of calibration stars and other small samples, along with astrometric information from $Gaia$ DR2. We apply a clustering algorithm to the 4-D energy-action space of the sample, and identify a set of 38 Dynamically Tagged Groups (DTGs), containing between 5 and 30 member stars. Many of these DTGs can be associated with previously known prominent substructures such as $Gaia$-Sausage/Enceladus (GSE), Sequoia, the Helmi Stream (HStr), and Thamnos. Others are associated with previously identified smaller dynamical groups of stars and streams. We identify 10 new DTGs as well, many of which have strongly retrograde orbits. We also investigate possible connections between our DTGs and $\sim$300 individual $r$-process-enhanced (RPE) stars from a recent literature compilation. We find that several of these objects have similar dynamical properties to GSE (5), the HStr (4), Sequoia (1), and Rg5 (1), indicating that their progenitors might have been important sources of RPE stars in the Galaxy. Additionally, a number of our newly identified DTGs are shown to be associated with at least two RPE stars each (DTG-2: 3, DTG-7: 2; DTG-27: 2). Taken as a whole, these results are consistent with ultra-faint and/or dwarf spheroidal galaxies as birth environments in which $r$-process nucleosynthesis took place, and then were disrupted by the Milky Way.

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arXiv2019

SDSS-IV MaStar -- A Large and Comprehensive Empirical Stellar Spectral Library: First Release

We present the first release of the MaNGA Stellar Library (MaStar), which is a large, well-calibrated, high-quality empirical library covering the wavelength range of 3,622-10,354A at a resolving power of R~1800. The spectra were obtained using the same instrument as used by the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) project, by piggybacking on the SDSS-IV/APOGEE-2N observations. Compared to previous empirical libraries, the MaStar library will have a higher number of stars and a more comprehensive stellar-parameter coverage, especially of cool dwarfs, low-metallicity stars, and stars with different [alpha/Fe], achieved by a sophisticated target selection strategy that takes advantage of stellar-parameter catalogs from the literature. This empirical library will provide a new basis for stellar population synthesis, and is particularly well-suited for stellar-population analysis of MaNGA galaxies. The first version of the library contains 8646 high-quality per-visit spectra for 3321 unique stars. Compared to photometry, the relative flux calibration of the library is accurate to 3.9% in g-r, 2.7% in r-i, and 2.2% in i-z. The data are released as part of Sloan Digital Sky Survey Data Release 15. We expect the final release of the library to contain more than 10,000 stars.

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arXiv2018

Young Blue Straggler Stars in the Galactic Field

In this study we present an analysis of a sample of field blue straggler (BS) stars that show high ultra violet emission in their spectral energy distributions (SED): indication of a hot white dwarf (WD) companion to BS. Using photometry available in the Sloan Digital Sky Survey (SDSS) and Galaxy Evolution Explorer (GALEX ) surveys we identified 80 stars with UV excess. To determine the parameter distributions (mass, temperature and age) of the WD companions, we developed a fitting routine that could fit binary model SEDs to the observed SED. Results from this fit indicate the need for a hot WD companion to provide the excess UV flux. The WD mass distribution peaks at $\sim$$0.4\ M_{\odot}$, suggesting the primary formation channel of field BSs is case B mass transfer, i.e. when the donor star is in red giant phase of its evolution. Based on stellar evolutionary models, we estimate the lower limit of the binary mass transfer efficiency to be $β\sim 0.5$.

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arXiv2014

Characterizing the AB Doradus Moving Group Via High Resolution Spectroscopy and Kinematic Traceback

We present a detailed analysis of 10 proposed F and G members of the nearby, young moving group AB Doradus (ABD). Our sample was obtained using the 2.7m telescope at the McDonald Observatory with the coude echelle spectrograph, achieving R $\sim$ 60,000 and S/N $\sim$ 200. We derive spectroscopic T$_{eff}$, log(g), [Fe/H], and microturbulance (v$_{t}$) using a bootstrap method of the TGVIT software resulting in typical errors of 33K in T$_{eff}$, 0.08 dex in log(g), 0.03 dex in [Fe/H], and 0.13 km s$^{-1}$ in v$_{t}$. Characterization of the ABD sample is performed in three ways: (1) Chemical homogeneity, (2) Kinematic Traceback, and (3) Isochrone fitting. We find the average metal abundance is [M/H] = -0.03 $\pm$ 0.06 with a traceback age of 125 Myrs. Our stars were fit to 3 different evolutionary models (Siess et al. 2000, Baraffe et al. 1998, and YREC) and we found the best match to our ABD sample is the YREC [M/H] = -0.1 model. In our sample of 10 stars, we identify 1 star which is a probable non-member, 3 enigmatic stars, and 6 stars with confirmed membership. We also present a list of chemically coherent stars from this study and the Barenfeld et al. (2013) study.

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arXiv2013

Panning for RRL Nuggets in the SDSS-DR9, Single Epoch Spectra

The SDSS has been a gold mine for understanding properties of the Milky Way. Below, in the watershed, there remain small nuggets to be found flowing from the deepest recesses of the mine. The SDSS-DR9 included the release of the flux- and wavelength-calibrated, individual spectra which were subsequently combined to form the composite spectra found in previous SDSS data releases. These single-epoch spectra (SES) can be analyzed to find flux and spectral line variability, and to probe aspects of phase variations for objects such as RRL stars. For $\sim 45%$ of the spectra in the RRL color range, SES were taken on separate nights, sometimes weeks apart. The remaining dataset have a time baseline of 0.75 to 1.5 hours and consist of 4-7 separate exposures. In my talk I will present details of our project to detect variability and to constrain the pulsation phase at the time of observation. I will discuss our auto-detection technique that uses division of all SES for a given star to search for variations in the flux, the hydrogen Balmer lines and the CaII K line. This procedure is being tested against known variables and non-variable stars within Stripe 82 to determine the identification effectiveness as a function of signal-to-noise, RRL type, and time baseline length. I will also discuss the use of empirical standard star templates to predict pulsation phase from the combination of hydrogen line strengths and radial velocity variations. Our ultimate goal is to combine the SES analysis with our new metallicity calibration in order to increase the number of RRL stars that have reliable metallicity determinations and which can then be used to probe the structure of the Milky Way halo.

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arXiv2012

Red Clump Stars in the Sagittarius Tidal Streams

We have probed a section (l ~ 150, b ~ -60) of the trailing tidal arm of the Sagittarius dwarf spheroidal galaxy by identifying a sample of Red Clump stream stars. Red Clump stars are not generally found in the halo field, but are found in significant numbers in both the Sagittarius galaxy and its tidal streams, making them excellent probes of stream characteristics. Our target sample was selected using photometric data from the Sloan Digital Sky Survey, Data Release 6, which was constrained in color to match the Sagittarius Red Clump stars. Spectroscopic observations of the target stars were conducted at Kitt Peak National Observatory using the WIYN telescope. The resulting spectroscopic sample is magnitude limited and contains both main sequence disk stars and evolved Red Clump stars. We have developed a method to systematically separate these two stellar classes using kinematic information and a Bayesian approach for surface gravity determination. The resulting Red Clump sample allows us to determine an absolute stellar density of ρ = 2.7 +/- 0.5 RC stars kpc-3 at this location in the stream. Future measurements of stellar densities for a variety of populations and at various locations along the streams will lead to a much improved understanding of the original nature of the Sagittarius galaxy and the physical processes controlling its disruption and subsequent stream generation.

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arXiv2011

The Case for the Dual Halo of the Milky Way

Carollo et al. have recently resolved the stellar population of the Milky Way halo into at least two distinct components, an inner halo and an outer halo. This result has been criticized by Schoenrich et al., who claim that the retrograde signature associated with the outer halo is due to the adoption of faulty distances. We refute this claim, and demonstrate that the Schoenrich et al. photometric distances are themselves flawed because they adopted an incorrect main-sequence absolute magnitude relationship from the work of Ivezić et al. When compared to the recommended relation from Ivezić et al., which is tied to a Milky Way globular cluster distance scale and accounts for age and metallicity effects, the relation adopted by Schoenrich et al. yields up to 18% shorter distances for stars near the main-sequence turnoff (TO). Use of the correct relationship yields agreement between the distances assigned by Carollo et al. and Ivezić et al. for low-metallicity dwarfs to within 6-10%. Schoenrich et al. also point out that intermediate-gravity stars (3.5 <= log g <= 4.0) with colors redder than the TO region are likely misclassified, with which we concur. We implement a new procedure to reassign luminosity classifications for the TO stars that require it. New derivations of the rotational behavior demonstrate that the retrograde signature and high velocity dispersion of the outer-halo population remains. We summarize additional lines of evidence for a dual halo, including a test of the retrograde signature based on proper motions alone, and conclude that the preponderance of evidence strongly rejects the single-halo interpretation.

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arXiv2009

Proper Motions in Kapteyn Selected Area 103: A Preliminary Orbit for the Virgo Stellar Stream

We present absolute proper motions in Kapteyn Selected Area (SA) 103. This field is located 7 degrees west of the center of the Virgo Stellar Stream (VSS, Duffau et al. 2006), and has a well-defined main sequence representing the stream. In SA 103 we identify one RR Lyrae star as a member of the VSS according to its metallicity, radial velocity and distance. VSS candidate turnoff stars and subgiant stars have proper motions consistent with that of the RR Lyrae star. The 3D velocity data imply an orbit with a pericenter of 11 kpc and an apocenter of ~90 kpc. Thus, the VSS comprises tidal debris found near the pericenter of a highly destructive orbit. Examining the six globular clusters at distances larger than 50 kpc from the Galactic center, and the proposed orbit of the VSS, we find one tentative association, NGC 2419. We speculate that NGC 2419 is possibly the nucleus of a disrupted system of which the VSS is a part.

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arXiv2008

New Findings for the Blue Plume Stars in the Canis Major Over-Density

We obtained new UBV photometry and spectroscopy of Blue Plume (BP) stars in fields near the center of the Canis Major Over-Density (CMa). We combined analysis of the color-color diagrams with a new comparison of the hydrogen Balmer-line profile to the reddening-free Q parameter to improve the reddening and extinction estimates for this low-latitude, differentially reddened, area of the sky. Results of our stellar parameter analysis for B/A spectral type stars associated with the BP show that the majority of the stars have main-sequence surface gravities placing them at an average heliocentric distance of = 6.0 +/- 2.7 kpc. This distance is more consistent with membership in the intervening Perseus spiral arm and strongly suggests that the BP stars are not associated with the other stellar populations previously reported to make up the CMa. This result casts serious doubt on the proposed dwarf galaxy origin for the CMa.

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arXiv2007

Identification of RR Lyrae Variables in SDSS from Single-Epoch Photometric and Spectroscopic Observations

We describe a new RR Lyrae identification technique based on out-of-phase single-epoch photometric and spectroscopic observations contained in SDSS Data Release 6 (DR-6). This technique detects variability by exploiting the large disparity between the g-r color and the strength of the hydrogen Balmer lines when the two observations are made at random phases. Comparison with a large sample of known variables in the SDSS equatorial stripe (Stripe 82) shows that the discovery efficiency for our technique is ~85%. Analysis of stars with multiple spectroscopic observations suggests a similar efficiency throughout the entire DR-6 sample. We also develop a technique to estimate the average g apparent magnitude (over the pulsation cycle) for individual RR Lyrae stars, using the for the entire sample and measured colors for each star. The resulting distances are found to have precisions of ~14%. Finally, we explore the properties of our DR-6 sample of N = 1087 variables, and recover portions of the Sagittarius Northern and Southern Stream. Analysis of the distance and velocity for the Southern Stream are consistent with previously published data for blue horizontal-branch stars. In a sample near the North Galactic Polar Cap, we find evidence for the descending leading Northern arm, and a possible detection of the trailing arm.

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arXiv2007

The Century Survey Galactic Halo Project III: A Complete 4300 deg^2 Survey of Blue Horizontal Branch Stars in the Metal-Weak Thick Disk and Inner Halo

We present a complete spectroscopic survey of 2414 2MASS-selected blue horizontal branch (BHB) candidates selected over 4300 deg^2 of the sky. We identify 655 BHB stars in this non-kinematically selected sample. We calculate the luminosity function of field BHB stars and find evidence for very few hot BHB stars in the field. The BHB stars located at a distance from the Galactic plane |Z|<4 kpc trace what is clearly a metal-weak thick disk population, with a mean metallicity of [Fe/H]= -1.7, a rotation velocity gradient of dv_{rot}/d|Z|= -28+-3.4 km/s in the region |Z|<6 kpc, and a density scale height of h_Z= 1.26+-0.1 kpc. The BHB stars located at 5<|Z|<9 kpc are a predominantly inner-halo population, with a mean metallicity of [Fe/H]= -2.0 and a mean Galactic rotation of -4+-31 km/s. We infer the density of halo and thick disk BHB stars is 104+-37 kpc^-3 near the Sun, and the relative normalization of halo to thick-disk BHB stars is 4+-1% near the Sun.

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arXiv2006

A Catalog of Candidate Field Horizontal-Branch and A-Type Stars. III. A 2MASS-Cleaned Version

We present coordinates and available photometric information (either from previous or recent broadband UBV observations, and near-infrared photometry from the 2MASS Point Source Catalog) for 12056 stars (11516 of which are unique) identified in the HK Survey of Beers and colleagues as candidate field horizontal-branch or A-type stars. These stars, in the apparent magnitude range 10 <= B <= 16.0, were selected using an objective-prism/interference-filter survey technique. The availability of 2MASS information permits assembly of a cleaned version of this catalog, comprising likely blue horizontal-branch (BHB) stars or blue stragglers in the color interval -0.2 <= (B-V)o <= +0.2, which are of particular interest for investigation of the structure, kinematics, and dynamics of the thick disk and inner halo of the Milky Way, the total mass and mass profile of the Galaxy, and as potential foreground/background objects in efforts to bracket distances to high velocity clouds of H I. A comparison of the stars classified as high-likelihood BHB candidates with previous classifications based on UBV photometry and medium-resolution spectroscopy indicates that this class contains 78% correct identifications.

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arXiv2006

Broadband UBVRI Photometry of Horizontal-Branch and Metal-Poor Candidates from the HK and Hamburg/ESO Surveys. I

We report broadband UBV and/or BVRI CCD photometry for a total of 1857 stars in the thick-disk and halo populations of the Galaxy. The majority of our targets were selected as candidate field horizontal-branch or other A-type stars (FHB/A, N = 576), or candidate low-metallicity stars (N = 1221), from the HK and Hamburg/ESO objective-prism surveys. Similar data for a small number of additional stars from other samples are also reported. These data are being used for several purposes. In the case of the FHB/A candidates they are used to accurately separate the lower-gravity FHB stars from various higher-gravity A-type stars, a subsample that includes the so-called Blue Metal Poor stars, halo and thick-disk blue stragglers, main-sequence A-type dwarfs, and Am and Ap stars. These data are also being used to derive photometric distance estimates to high-velocity hydrogen clouds in the Galaxy and for improved measurements of the mass of the Galaxy. Photometric data for the metal-poor candidates are being used to refine estimates of stellar metallicity for objects with available medium-resolution spectroscopy, to obtain distance estimates for kinematic analyses, and to establish initial estimates of effective temperature for analysis of high-resolution spectroscopy of the stars for which this information now exists.

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