arXiv subjects

Andrea Kunder

Publications and source records attributed to Andrea Kunder.

At least 55 records · Page 3Linked to original sources

arXiv2016

The RAVE-on catalog of stellar atmospheric parameters and chemical abundances for chemo-dynamic studies in the Gaia era

The orbits, atmospheric parameters, chemical abundances, and ages of individual stars in the Milky Way provide the most comprehensive illustration of galaxy formation available. The Tycho-Gaia Astrometric Solution (TGAS) will deliver astrometric parameters for the largest ever sample of Milky Way stars, though its full potential cannot be realized without the addition of complementary spectroscopy. Among existing spectroscopic surveys, the RAdial Velocity Experiment (RAVE) has the largest overlap with TGAS ($\gtrsim$200,000 stars). We present a data-driven re-analysis of 520,781 RAVE spectra using The Cannon. For red giants, we build our model using high-fidelity APOGEE stellar parameters and abundances for stars that overlap with RAVE. For main-sequence and sub-giant stars, our model uses stellar parameters from the K2/EPIC. We derive and validate effective temperature $T_{\rm eff}$, surface gravity $\log{g}$, and chemical abundances of up to seven elements (O, Mg, Al, Si, Ca, Fe, Ni). We report a total of 1,685,851 elemental abundances with a typical precision of 0.07 dex, a substantial improvement over previous RAVE data releases. The synthesis of RAVE-on and TGAS is the most powerful data set for chemo-dynamic analyses of the Milky Way ever produced.

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arXiv2016

Before the Bar: Kinematic Detection of A Spheroidal Metal-Poor Bulge Component

We present 947 radial velocities of RR Lyrae variable stars in four fields located toward the Galactic bulge, observed within the data from the ongoing Bulge RR Lyrae Radial Velocity Assay (BRAVA-RR). We show that these RR Lyrae stars exhibit hot kinematics and null or negligible rotation and are therefore members of a separate population from the bar/pseudobulge that currently dominates the mass and luminosity of the inner Galaxy. Our RR Lyrae stars predate these structures, and have metallicities, kinematics, and spatial distribution that are consistent with a "classical" bulge, although we cannot yet completely rule out the possibility that they are the metal-poor tail of a more metal rich ([Fe/H] ~ -1 dex) halo-bulge population. The complete catalog of radial velocities for the BRAVA-RR stars is also published electronically.

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arXiv2016

Chemical abundances in a high velocity RR Lyrae star near the bulge

Low-mass, variable, high-velocity stars are interesting study cases for many aspects of Galactic structure and evolution. Until recently, the only known high- or hyper-velocity stars were young stars thought to originate from the Galactic centre. Wide-area surveys like APOGEE and BRAVA have found several low-mass stars in the bulge with Galactic rest-frame velocities larger than 350 km/s. In this study we present the first abundance analysis of a low-mass, RR Lyrae star, located close to the Galactic bulge, with a space motion of ~ -400 km/s. Using medium-resolution spectra, we derive abundances (including upper limits) of 11 elements. These allow us to chemically tag the star and discuss its origin, although our derived abundances and metallicity, at [Fe/H] =-0.9 dex, do not point toward one unambiguous answer. Based on the chemical tagging, we cannot exclude that it originated in the bulge. However, combining its retrograde orbit and the derived abundances suggests that the star was accelerated from the outskirts of the inner (or even outer) halo during many-body interactions. Other possible origins include the bulge itself, or the star could be stripped from a star cluster or the Sagittarius dwarf galaxy when it merged with the Milky Way.

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arXiv2015

Interstellar Extinction Curve Variations Toward the Inner Milky Way: A Challenge to Observational Cosmology

We investigate interstellar extinction curve variations toward $\sim$4 deg$^{2}$ of the inner Milky Way in $VIJK_{s}$ photometry from the OGLE-III and $VVV$ surveys, with supporting evidence from diffuse interstellar bands and $F435W,F625W$ photometry. We obtain independent measurements toward $\sim$2,000 sightlines of $A_{I}$, $E(V-I)$, $E(I-J)$, and $E(J-K_{s})$, with median precision and accuracy of 2%. We find that the variations in the extinction ratios $A_{I}/E(V-I)$, $E(I-J)/E(V-I)$ and $E(J-K_{s})/E(V-I)$ are large (exceeding 20%), significant, and positively correlated, as expected. However, both the mean values and the trends in these extinction ratios are drastically shifted from the predictions of Cardelli and Fitzpatrick, regardless of how $R_{V}$ is varied. Furthermore, we demonstrate that variations in the shape of the extinction curve has at least two degrees of freedom, and not one (e.g. $R_{V}$), which we conform with a principal component analysis. We derive a median value of $ =13.44$, which is $\sim$60% higher than the "standard" value. We show that the Wesenheit magnitude $W_{I}=I-1.61(I-J)$ is relatively impervious to extinction curve variations. Given that these extinction curves are linchpins of observational cosmology, and that it is generally assumed that $R_{V}$ variations correctly capture variations in the extinction curve, we argue that systematic errors in the distance ladder from studies of type Ia supernovae and Cepheids may have been underestimated. Moreover, the reddening maps from the Planck experiment are shown to systematically overestimate dust extinction by $\sim$100%, and lack sensitivity to extinction curve variations.

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arXiv2015

A high-velocity bulge RR Lyrae variable on a halo-like orbit

We report on the RR Lyrae variable star, MACHO 176.18833.411, located toward the Galactic bulge and observed within the data from the ongoing Bulge RR Lyrae Radial Velocity Assay (BRAVA-RR), which has the unusual radial velocity of -372 +- 8 km/s and true space velocity of -482 +- 22 km/s relative to the Galactic rest frame. Located less than 1 kpc from the Galactic center and toward a field at (l,b)=(3,-2.5), this pulsating star has properties suggesting it belongs to the bulge RR Lyrae star population yet a velocity indicating it is abnormal, at least with respect to bulge giants and red clump stars. We show that this star is most likely a halo interloper and therefore suggest that halo contamination is not insignificant when studying metal-poor stars found within the bulge area, even for stars within 1 kpc of the Galactic center. We discuss the possibility that MACHO 176.18833.411 is on the extreme edge of the bulge RR Lyrae radial velocity distribution, and also consider a more exotic scenario in which it is a runaway star moving through the Galaxy.

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arXiv2015

G2C2 - IV: A novel approach to study the radial distributions of multiple populations in Galactic globular clusters

We use the HB morphology of 48 Galactic GCs to study the radial distributions of the different stellar populations known to exist in globular clusters. Assuming that the (extremely) blue HB stars correspond to stars enriched in Helium and light elements, we compare the radial distributions of stars selected according to colour on the HB to trace the distribution of the secondary stellar populations in globular clusters. Unlike other cases, our data show that the populations are well mixed in 80% of the cases studied. This provides some constraints on the mechanisms proposed to pollute the interstellar medium in young globular clusters.

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arXiv2015

Kinematics of the X-shaped Milky Way Bulge: Expectations from a Self-consistent N-body Model

We explore the kinematics (both the radial velocity and the proper motion) of the vertical X-shaped feature in the Milky Way with an N-body bar/bulge model. From the solar perspective, the distance distribution of particles is double-peaked in fields passing through the X-shape. The separation and amplitude ratio between the two peaks qualitatively match the observed trends towards the Galactic bulge. We confirm clear signatures of cylindrical rotation in the pattern of mean radial velocity across the bar/bulge region. We also find possible imprints of coherent orbital motion inside the bar structure in the radial velocity distribution along l=0 degree, where the near and far sides of the bar/bulge show excesses of approaching and receding particles. The coherent orbital motion is also reflected in the slight displacement of the zero-velocity-line in the mean radial velocity, and the displacement of the maximum/minimum in the mean longitudinal proper motion across the bulge region. We find some degree of anisotropy in the stellar velocity within the X-shape, but the underlying orbital family of the X-shape cannot be clearly distinguished. Two potential applications of the X-shape in previous literature are tested, i.e., bulge rotation and Galactic center measurements. We find that the proper motion difference between the two sides of the X-shape can be used to estimate the mean azimuthal streaming motion of the bulge, but not the pattern speed of the bar. We also demonstrate that the Galactic center can be located with the X-shape, but the accuracy depends on the fitting scheme, the number of fields, and their latitudinal coverage.

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arXiv2015

Hydra II: a faint and compact Milky Way dwarf galaxy found in the Survey of the Magellanic Stellar History

We present the discovery of a new dwarf galaxy, Hydra II, found serendipitously within the data from the ongoing Survey of the MAgellanic Stellar History (SMASH) conducted with the Dark Energy Camera on the Blanco 4m Telescope. The new satellite is compact (r_h = 68 +/- 11 pc) and faint (M_V = -4.8 +/- 0.3), but well within the realm of dwarf galaxies. The stellar distribution of HydraII in the color-magnitude diagram is well-described by a metal-poor ([Fe/H] = -2.2) and old (13 Gyr) isochrone and shows a distinct blue horizontal branch, some possible red clump stars, and faint stars that are suggestive of blue stragglers. At a heliocentric distance of 134 +/- 10 kpc, Hydra II is located in a region of the Galactic halo that models have suggested may host material from the leading arm of the Magellanic Stream. A comparison with N-body simulations hints that the new dwarf galaxy could be or could have been a satellite of the Magellanic Clouds.

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arXiv2014

Spectroscopic Signatures of Extra-Tidal Stars Around the Globular Clusters NGC 6656 (M22), NGC 3201 and NGC 1851 from RAVE

Stellar population studies of globular clusters have suggested that the brightest clusters in the Galaxy might actually be the remnant nuclei of dwarf spheroidal galaxies. If the present Galactic globular clusters formed within larger stellar systems, they are likely surrounded by extra-tidal halos and/or tails made up of stars that were tidally stripped from their parent systems. The stellar surroundings around globular clusters are therefore one of the best places to look for the remnants of an ancient dwarf galaxy. Here an attempt is made to search for tidal debris around the supernovae enriched globular clusters M22 and NGC 1851 as well as the kinematically unique cluster NGC 3201. The stellar parameters from the Radial Velocity Experiment (RAVE) are used to identify stars with RAVE metallicities, radial velocities and elemental-abundances consistent with the abundance patterns and properties of the stars in M22, NGC 1851 and NGC 3201. The discovery of RAVE stars that may be associated with M22 and NGC 1851 are reported, some of which are at projected distances of ~10 degrees away from the core of these clusters. Numerous RAVE stars associated with NGC 3201 suggest that either the tidal radius of this cluster is underestimated, or that there are some unbound stars extending a few arc minutes from the edge of the cluster's radius. No further extra-tidal stars associated with NGC 3201 could be identified. The bright magnitudes of the RAVE stars make them easy targets for high resolution follow-up observations, allowing an eventual further chemical tagging to solidify (or exclude) stars outside the tidal radius of the cluster as tidal debris. In both our radial velocity histograms of the regions surrounding NGC 1851 and NGC 3201, a peak of stars at 230 km/s is seen, consistent with extended tidal debris from omega Centauri.

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arXiv2014

Light, Alpha, and Fe-Peak Element Abundances in the Galactic Bulge

We present radial velocities and chemical abundances of O, Na, Mg, Al, Si, Ca, Cr, Fe, Co, Ni, and Cu for a sample of 156 red giant branch stars in two Galactic bulge fields centered near (l,b)=(+5.25,-3.02) and (0,-12). The (+5.25,-3.02) field also includes observations of the bulge globular cluster NGC 6553. The results are based on high resolution (R~20,000), high signal-to-noise (S/N>70) FLAMES-GIRAFFE spectra obtained through the ESO archive. However, we only selected a subset of the original observations that included spectra with both high S/N and that did not show strong TiO absorption bands. The present work extends previous analyses of this data set beyond Fe and the alpha-elements Mg, Si, Ca, and Ti. While we find reasonable agreement with past work, the data presented here indicate that the bulge may exhibit a different chemical composition than the local thick disk, especially at [Fe/H]>-0.5. In particular, the bulge [alpha/Fe] ratios may remain enhanced to a slightly higher [Fe/H] than the thick disk and the Fe-peak elements Co, Ni, and Cu appear enhanced compared to the disk. There is also some evidence that the [Na/Fe] (but not [Al/Fe]) trends between the bulge and local disk may be different at low and high metallicity. We also find that the velocity dispersion decreases as a function of increasing [Fe/H] for both fields, and do not detect any significant cold, high velocity population. A comparison with chemical enrichment models indicates that a significant fraction of hypernovae are required to explain the bulge abundance trends, and that initial mass functions that are steep, top-heavy (and do not include strong outflow), or truncated to avoid including contributions from stars >40 solar masses are ruled out, in particular because of disagreement with the Fe-peak abundance data. [abridged]

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arXiv2014

Are high velocity peaks in the Milky Way bulge due to the bar?

Recently the commissioning APOGEE observations of the Galactic bulge reported that a significant fraction of stars ($\sim10%$) are in a cold ($σ_{\rm V} \approx 30$ km/s) high velocity peak (Galactocentric radial velocity $\approx 200$ km/s). These stars are speculated to reflect the stellar orbits in the Galactic bar. In this study, we use two $N$-body models of the Milky Way-like disk galaxy with different bar strengths to critically examine this possibility. The general trends of the Galactocentric radial velocity distribution in observations and simulations are similar, but neither our models nor the BRAVA data reveal a statistically significant cold high velocity peak. A Monte Carlo test further suggests that it is possible for a spurious high velocity peak to appear if there are only a limited number of stars observed. Thus, the reported cold high velocity peak, even if it is real, is unlikely due to stars on the bar-supporting orbits. Our models do predict an excess of stars with high radial velocity, but not in a distinct peak. In the distance--velocity diagram, the high velocity particles in different fields exist at a similar distance $\sim8.5 \pm 1$ kpc away from the Sun. This result may be explained with geometric intersections between the line-of-sight and the particle orbits; high velocity stars naturally exist approximately at the tangent point, without constituting a distinct peak. We further demonstrate that even without the presence of a bar structure, particle motions in an axisymmetric disk can also exhibit an excess of high velocity stars.

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arXiv2014

Proper motions for HST observations in three off-axis bulge fields

Aims. This is the second in a series of papers that attempt to unveil the kinematic structure of the Galactic bulge through studying radial velocities and proper motions. We report here ~15000 new proper motions for three low foreground-extinction off-axis fields of the Galactic bulge. Methods. Proper motions were derived from a combination of Hubble Space Telescope Wide Field Planetary Camera 2 (WFPC2) and Advanced Camera for Surveys (ACS) images taken 8 and 9 years apart, and ACS observations taken 9 and 10 years apart, and they reach accuracies better than 0.9 mas/yr for more than ~10000 objects with magnitudes F814W < 24. Results. The proper motion distributions in these fields are similar to those of Galactic minor axis bulge fields. We observe the rotation of main sequence stars below the turn-off within the Galactic bulge, as in the minor axis fields. Conclusions. Our stellar proper motions measurements show a significant bulge rotation for fields as far from the galactic plane as b=-8.

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arXiv2013

RR Lyrae in the LMC: Insights Into the Oosterhoff Phenomenon

Although more than eight decades have passed since P. Th. Oosterhoff drew attention to differences in the properties of RR Lyrae variables in globular clusters, the origin and significance of the Oosterhoff groups remain unclear. Nonetheless, the accumulation of extensive new observations of RR Lyrae stars in globular clusters of the Milky Way and Local Group galaxies allows a fresh look at the phenomenon. Insights come not only from surveys of variables within the original Oosterhoff groups I and II but also from recent observations of the Oosterhoff-intermediate systems found especially in smaller Local Group galaxies. We will compare properties of RR Lyrae in several systems to investigate what they reveal about system-to-system differences of transition temperature between fundamental-mode and first overtone pulsators and of horizontal branch luminosity. Both transition temperature and horizontal branch luminosity have at various times been credited as playing roles in the creation of the Oosterhoff dichotomy.

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arXiv2013

The RR Lyrae variables and Horizontal Branch of NGC6656 (M22)

The first calibrated broadband UBVI time-series photometry is presented for the RR Lyrae variable stars in NGC6656 (M22), with observations spanning a range of twenty-two years. We have also redetermined the variability types and periods for the RR Lyrae stars identified previously by photographic observations, revising the number of fundamental-mode RR Lyrae variables (RR0) to 10 and the number of first-overtone variables (RR1) to 16. The mean periods of the RR0 and RR1 variables are _{RR0}=0.66 \pm 0.02 d and _{RR1}=0.33 \pm 0.01 d, respectively, supporting an Oosterhoff II classification for the cluster. The number ratio of RR1- to all RR-type variables is N_1/N_{RR}=0.61, also consistent with an Oosterhoff II designation. Both the RR Lyrae stars' minimum light colors and the blue edge of the RR Lyrae instability strip suggest E(B-V)=0.36 \pm 0.02 mag toward M22. Regarding the HB morphology of M22, we find (B-R)/(B+V+R)=+0.97 \pm 0.1 and at least one "gap" located in an unusual part of the blue HB, in the middle of the so-called hot HB stars.

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arXiv2013

Variable Stars in Large Magellanic Cloud Globular Clusters III: Reticulum

This is the third in a series of papers studying the variable stars in old globular clusters in the Large Magellanic Cloud. The primary goal of this series is to look at how the characteristics and behavior of RR Lyrae stars in Oosterhoff-intermediate systems compare to those of their counterparts in Oosterhoff-I/II systems. In this paper we present the results of our new time-series BVI photometric study of the globular cluster Reticulum. We found a total of 32 variables stars (22 RRab, 4 RRc, and 6 RRd stars) in our field of view. We present photometric parameters and light curves for these stars. We also present physical properties, derived from Fourier analysis of light curves, for some of the RR Lyrae stars. We discuss the Oosterhoff classification of Reticulum and use our results to re-derive the distance modulus and age of the cluster.

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arXiv2013

Identification of metal-poor stars using the artificial neural network

Identification of metal-poor stars among field stars is extremely useful for studying the structure and evolution of the Galaxy and of external galaxies. We search for metal-poor stars using the artificial neural network (ANN) and extend its usage to determine absolute magnitudes. We have constructed a library of 167 medium-resolution stellar spectra (R ~ 1200) covering the stellar temperature range of 4200 to 8000 K, log g range of 0.5 to 5.0, and [Fe/H] range of -3.0 to +0.3 dex. This empirical spectral library was used to train ANNs, yielding an accuracy of 0.3 dex in [Fe/H], 200 K in temperature, and 0.3 dex in log g. We found that the independent calibrations of near-solar metallicity stars and metal-poor stars decreases the errors in T_eff and log g by nearly a factor of two. We calculated T_eff, log g, and [Fe/H] on a consistent scale for a large number of field stars and candidate metal-poor stars. We extended the application of this method to the calibration of absolute magnitudes using nearby stars with well-estimated parallaxes. A better calibration accuracy for M_V could be obtained by training separate ANNs for cool, warm, and metal-poor stars. The current accuracy of M_V calibration is (+-)0.3 mag. A list of newly identified metal-poor stars is presented. The M_V calibration procedure developed here is reddening-independent and hence may serve as a powerful tool in studying galactic structure.

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arXiv2013

SOAR Adaptive Optics Observations of the Globular Cluster NGC6496

We present high-quality BVRI photometric data in the field of globular cluster NGC 6496 obtained with the SOAR Telescope Adaptive Module (SAM). Our observations were collected as part of the ongoing SAM commissioning. The distance modulus and cluster color excess as found from the red clump is $\mMv = 15.71 \pm 0.02$\,mag and $\EVI = 0.28 \pm 0.02$\,mag. An age of $10.5 \pm 0.5$\,Gyr is determined from the difference in magnitude between the red clump and the subgiant branch. These parameters are in excellent agreement with the values derived from isochrone fitting. From the color-magnitude diagram we find a metallicity of $\feh = -0.65$\,dex and hence support a disk classification for NGC 6496. The complete $BVRI$ data set for NGC 6469 is made available in the electronic edition of the Journal.

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arXiv2013

Metallicity Distribution Functions, Radial Velocities, and Alpha Element Abundances in Three Off-Axis Bulge Fields

We present radial velocities and chemical abundance ratios of [Fe/H], [O/Fe], [Si/Fe], and [Ca/Fe] for 264 red giant branch (RGB) stars in three Galactic bulge off-axis fields located near (l,b)=(-5.5,-7), (-4,-9), and (+8.5,+9). The results are based on equivalent width and spectrum synthesis analyses of moderate resolution (R~18,000), high signal-to-noise ratio (S/N~75-300) spectra obtained with the Hydra spectrographs on the Blanco 4m and WIYN 3.5m telescopes. The targets were selected from the blue side of the giant branch to avoid cool stars that would be strongly affected by CN and TiO; however, a comparison of the color-metallicity distribution in literature samples suggests our selection of bluer targets should not present a significant bias against metal-rich stars. We find a full range in metallicity that spans [Fe/H]\approx-1.5 to +0.5, and that, in accordance with the previously observed minor-axis vertical metallicity gradient, the median [Fe/H] also declines with increasing Galactic latitude in off-axis fields. The off-axis vertical [Fe/H] gradient in the southern bulge is estimated to be ~0.4 dex/kpc. The (+8.5,+9) field exhibits a higher than expected metallicity, with a median [Fe/H]=-0.23, that might be related to a stronger presence of the X--shaped bulge structure along that line-of-sight. All fields exhibit an identical, strong decrease in velocity dispersion with increasing metallicity that is consistent with observations in similar minor-axis outer bulge fields. Additionally, the [O/Fe], [Si/Fe], and [Ca/Fe] versus [Fe/H] trends are identical among our three fields, and are in good agreement with past bulge studies. [abridged]

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