Abstract
Deep near-IR images from the VISTA Variables in the Vía Láctea (VVV) Survey were used to search for RR Lyrae stars within 100 arcmin from the Galactic Center. A large sample of 960 RR Lyrae ...of type ab (RRab) stars were discovered. A catalog is presented featuring the positions, magnitudes, colors, periods, and amplitudes for the sample, in addition to estimated reddenings, distances, and metallicities, and measured individual relative proper motions. We use the reddening-corrected Wesenheit magnitudes, defined as
, in order to isolate bona fide RRL belonging to the Galaxy Center, finding that 30 RRab are foreground/background objects. We measure a range of extinctions from
to 1.75 mag for the RRab in this region, finding that large extinction is the main cause of the sample incompleteness. The mean period is
P
= 0.5446 ± 0.0025 days, yielding a mean metallicity of Fe/H = −1.30 ± 0.01 (
σ
= 0.33) dex for the RRab sample in the Galactic Center region. The median distance for the sample is
D
= 8.05 ± 0.02 kpc. We measure the RRab surface density using the less reddened region sampled here, finding a density of 1000 RRab/sq deg at a projected Galactocentric distance
R
G
= 1.6 deg. Under simple assumptions, this implies a large total mass (
M
> 10
9
M
⊙
) for the old and metal-poor population contained inside
R
G
. We also measure accurate relative proper motions, from which we derive tangential velocity dispersions of
σV
l
= 125.0 and
σV
b
= 124.1 km s
−1
along the Galactic longitude and latitude coordinates, respectively. The fact that these quantities are similar indicate that the bulk rotation of the RRab population is negligible, and implies that this population is supported by velocity dispersion. In summary, there are two main conclusions of this study. First, the population as a whole is no different from the outer bulge RRab, predominantly a metal-poor component that is shifted with respect to the Oosterhoff type I population defined by the globular clusters in the halo. Second, the RRab sample, as representative of the old and metal-poor stellar population in the region, has high velocity dispersions and zero rotation, suggesting a formation via dissipational collapse.
Abstract Phylogenetic methods have long been used in biology and more recently have been extended to other fields—for example, linguistics and technology—to study evolutionary histories. Galaxies ...also have an evolutionary history and fall within this broad phylogenetic framework. Under the hypothesis that chemical abundances can be used as a proxy for the interstellar medium’s DNA, phylogenetic methods allow us to reconstruct hierarchical similarities and differences among stars—essentially, a tree of evolutionary relationships and thus history. In this work, we apply phylogenetic methods to a simulated disk galaxy obtained with a chemodynamical code to test the approach. We found that at least 100 stellar particles are required to reliably portray the evolutionary history of a selected stellar population in this simulation, and that the overall evolutionary history is reliably preserved when the typical uncertainties in the chemical abundances are smaller than 0.08 dex. The results show that the shapes of the trees are strongly affected by the age–metallicity relation, as well as the star formation history of the galaxy. We found that regions with low star formation rates produce shorter trees than regions with high star formation rates. Our analysis demonstrates that phylogenetic methods can shed light on the process of galaxy evolution.
Abstract
We use deep, multi-epoch near-IR images of the VISTA Variables in the Vía Láctea (VVV) Survey to measure proper motions (PMs) of stars in the Milky Way globular cluster (GC) ...FSR1716 = VVV-GC05. The color-magnitude diagram of this object, made by using PM-selected members, shows an extended horizontal branch, nine confirmed RR Lyrae (RRL) members in the instability strip, and possibly several hotter stars extending to the blue. Based on the fundamental-mode (ab-type) RRL stars that move coherently with the cluster, we confirmed that FSR1716 is an Oosterhoff I GC with a mean period
= 0.574 days. Intriguingly, we detect tidal extensions to both sides of this cluster in the spatial distribution of PM-selected member stars. Also, one of the confirmed RRabs is located ∼11 arcmin in projection from the cluster center, suggesting that FSR1716 may be losing stars due to the gravitational interaction with the Galaxy. We also measure radial velocities (RVs) for five cluster red giants selected using the PMs. The combination of RVs and PMs allow us to compute for the first time the orbit of this GC, using an updated Galactic potential. The orbit results to be confined within
kpc, and has eccentricity 0.4 <
e
< 0.6, with perigalactic distance
, and apogalactic distance
. We conclude that, in agreement with its relatively low metallicity (Fe/H = −1.4 dex), this is an inner-halo GC plunging into the disk of the Galaxy. As such, this is a unique object with which to test the dynamical processes that contribute to the disruption of Galactic GCs.
ABSTRACT
We use data of ∼13 000 stars from the Sloan Digital Sky Survey/Apache Point Observatory Galactic Evolution Experiment survey to study the shape of the bulge metallicity distribution function ...(MDF) within the region |ℓ| ≤ 11° and |b| ≤ 13°, and spatially constrained to RGC ≤ 3.5 kpc. We apply Gaussian mixture modelling and non-negative matrix factorization decomposition techniques to identify the optimal number and the properties of MDF components. We find that the shape and spatial variations of the MDF (at Fe/H ≥ −1 dex) are well represented as a smoothly varying contribution of three overlapping components located at Fe/H = +0.32, −0.17, and −0.66 dex. The bimodal MDF found in previous studies is in agreement with our trimodal assessment once the limitations in sample size and individual measurement errors are taken into account. The shape of the MDF and its correlations with kinematics reveal different spatial distributions and kinematical structure for the three components co-existing in the bulge region. We confirm the consensus physical interpretation of metal-rich stars as associated with the secularly evolved disc into a boxy/peanut X-shape bar. On the other hand, metal-intermediate stars could be the product of in-situ formation at high redshift in a gas-rich environment characterized by violent and fast star formation. This interpretation would help us to link a present-day structure with those observed in formation in the centre of high-redshift galaxies. Finally, metal-poor stars may correspond to the metal-rich tail of the population sampled at lower metallicity from the study of RR Lyrae stars. Conversely, they could be associated with the metal-poor tail of the early thick disc.
Abstract
APOGEE is a high-resolution (
R
∼ 22,000), near-infrared, multi-epoch, spectroscopic survey of the Milky Way. The second generation of the APOGEE project, APOGEE-2, includes an expansion of ...the survey to the Southern Hemisphere called APOGEE-2S. This expansion enabled APOGEE to perform a fully panoramic mapping of all of the main regions of the Milky Way; in particular, by operating in the
H
band, APOGEE is uniquely able to probe the dust-hidden inner regions of the Milky Way that are best accessed from the Southern Hemisphere. In this paper we present the targeting strategy of APOGEE-2S, with special attention to documenting modifications to the original, previously published plan. The motivation for these changes is explained as well as an assessment of their effectiveness in achieving their intended scientific objective. In anticipation of this being the last paper detailing APOGEE targeting, we present an accounting of all such information complete through the end of the APOGEE-2S project; this includes several main survey programs dedicated to exploration of major stellar populations and regions of the Milky Way, as well as a full list of programs contributing to the APOGEE database through allocations of observing time by the Chilean National Time Allocation Committee and the Carnegie Institution for Science. This work was presented along with a companion article, Beaton et al. (2021), presenting the final target selection strategy adopted for APOGEE-2 in the Northern Hemisphere.
Abstract
The Apache Point Observatory Galactic Evolution Experiment 2 (APOGEE-2) is a dual-hemisphere, near-infrared (NIR), spectroscopic survey with the goal of producing a chemodynamical mapping of ...the Milky Way. The targeting for APOGEE-2 is complex and has evolved with time. In this paper, we present the updates and additions to the initial targeting strategy for APOGEE-2N presented in Zasowski et al. (2017). These modifications come in two implementation modes: (i) “Ancillary Science Programs” competitively awarded to Sloan Digital Sky Survey IV PIs through proposal calls in 2015 and 2017 for the pursuit of new scientific avenues outside the main survey, and (ii) an effective 1.5 yr expansion of the survey, known as the Bright Time Extension (BTX), made possible through accrued efficiency gains over the first years of the APOGEE-2N project. For the 23 distinct ancillary programs, we provide descriptions of the scientific aims, target selection, and how to identify these targets within the APOGEE-2 sample. The BTX permitted changes to the main survey strategy, the inclusion of new programs in response to scientific discoveries or to exploit major new data sets not available at the outset of the survey design, and expansions of existing programs to enhance their scientific success and reach. After describing the motivations, implementation, and assessment of these programs, we also leave a summary of lessons learned from nearly a decade of APOGEE-1 and APOGEE-2 survey operations. A companion paper, F. Santana et al. (submitted; AAS29036), provides a complementary presentation of targeting modifications relevant to APOGEE-2 operations in the Southern Hemisphere.
ABSTRACT
Stellar chemical abundances have proved themselves a key source of information for understanding the evolution of the Milky Way, and the scale of major stellar surveys such as GALAH have ...massively increased the amount of chemical data available. However, progress is hampered by the level of precision in chemical abundance data as well as the visualization methods for comparing the multidimensional outputs of chemical evolution models to stellar abundance data. Machine learning methods have greatly improved the former; while the application of tree-building or phylogenetic methods borrowed from biology are beginning to show promise with the latter. Here, we analyse a sample of GALAH solar twins to address these issues. We apply The Cannon algorithm to generate a catalogue of about 40 000 solar twins with 14 high precision abundances which we use to perform a phylogenetic analysis on a selection of stars that have two different ranges of eccentricities. From our analyses, we are able to find a group with mostly stars on circular orbits and some old stars with eccentric orbits whose age–Y/Mg relation agrees remarkably well with the chemical clocks published by previous high precision abundance studies. Our results show the power of combining survey data with machine learning and phylogenetics to reconstruct the history of the Milky Way.
Deep near-IR images from the VISTA Variables in the Vía Láctea (VVV) Survey were used to search for RR Lyrae stars within 100 arcmin from the Galactic Center. A large sample of 960 RR Lyrae of type ...ab (RRab) stars were discovered. A catalog is presented featuring the positions, magnitudes, colors, periods, and amplitudes for the sample, in addition to estimated reddenings, distances, and metallicities, and measured individual relative proper motions. We use the reddening-corrected Wesenheit magnitudes, defined as , in order to isolate bona fide RRL belonging to the Galaxy Center, finding that 30 RRab are foreground/background objects. We measure a range of extinctions from to 1.75 mag for the RRab in this region, finding that large extinction is the main cause of the sample incompleteness. The mean period is P = 0.5446 0.0025 days, yielding a mean metallicity of Fe/H = −1.30 0.01 ( = 0.33) dex for the RRab sample in the Galactic Center region. The median distance for the sample is D = 8.05 0.02 kpc. We measure the RRab surface density using the less reddened region sampled here, finding a density of 1000 RRab/sq deg at a projected Galactocentric distance RG = 1.6 deg. Under simple assumptions, this implies a large total mass (M > 109 M ) for the old and metal-poor population contained inside RG. We also measure accurate relative proper motions, from which we derive tangential velocity dispersions of Vl = 125.0 and Vb = 124.1 km s−1 along the Galactic longitude and latitude coordinates, respectively. The fact that these quantities are similar indicate that the bulk rotation of the RRab population is negligible, and implies that this population is supported by velocity dispersion. In summary, there are two main conclusions of this study. First, the population as a whole is no different from the outer bulge RRab, predominantly a metal-poor component that is shifted with respect to the Oosterhoff type I population defined by the globular clusters in the halo. Second, the RRab sample, as representative of the old and metal-poor stellar population in the region, has high velocity dispersions and zero rotation, suggesting a formation via dissipational collapse.
We use deep, multi-epoch near-IR images of the VISTA Variables in the Vía Láctea (VVV) Survey to measure proper motions (PMs) of stars in the Milky Way globular cluster (GC) FSR1716 = VVV-GC05. The ...color-magnitude diagram of this object, made by using PM-selected members, shows an extended horizontal branch, nine confirmed RR Lyrae (RRL) members in the instability strip, and possibly several hotter stars extending to the blue. Based on the fundamental-mode (ab-type) RRL stars that move coherently with the cluster, we confirmed that FSR1716 is an Oosterhoff I GC with a mean period = 0.574 days. Intriguingly, we detect tidal extensions to both sides of this cluster in the spatial distribution of PM-selected member stars. Also, one of the confirmed RRabs is located ∼11 arcmin in projection from the cluster center, suggesting that FSR1716 may be losing stars due to the gravitational interaction with the Galaxy. We also measure radial velocities (RVs) for five cluster red giants selected using the PMs. The combination of RVs and PMs allow us to compute for the first time the orbit of this GC, using an updated Galactic potential. The orbit results to be confined within kpc, and has eccentricity 0.4 < e < 0.6, with perigalactic distance , and apogalactic distance . We conclude that, in agreement with its relatively low metallicity (Fe/H = −1.4 dex), this is an inner-halo GC plunging into the disk of the Galaxy. As such, this is a unique object with which to test the dynamical processes that contribute to the disruption of Galactic GCs.
Stellar chemical abundances have proved themselves a key source of information for understanding the evolution of the Milky Way, and the scale of major stellar surveys such as GALAH have massively ...increased the amount of chemical data available. However, progress is hampered by the level of precision in chemical abundance data as well as the visualization methods for comparing the multidimensional outputs of chemical evolution models to stellar abundance data. Machine learning methods have greatly improved the former; while the application of tree-building or phylogenetic methods borrowed from biology are beginning to show promise with the latter. Here we analyse a sample of GALAH solar twins to address these issues. We apply The Cannon algorithm to generate a catalogue of about 40,000 solar twins with 14 high precision abundances which we use to perform a phylogenetic analysis on a selection of stars that have two different ranges of eccentricities. From our analyses we are able to find a group with mostly stars on circular orbits and some old stars with eccentric orbits whose age-Y/Mg relation agrees remarkably well with the chemical clocks published by previous high precision abundance studies. Our results show the power of combining survey data with machine learning and phylogenetics to reconstruct the history of the Milky Way.
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