35 research outputs found
Young Massive Clusters Near the Galactic Center: Initial Mass Function and Stellar Evolution.
Ph.D. Thesis. University of Hawaiʻi at Mānoa 2018
The Arches Cluster: Extended Structure and Tidal Radius
At a projected distance of ~26 pc from Sgr A*, the Arches cluster provides
insight to star formation in the extreme Galactic Center (GC) environment.
Despite its importance, many key properties such as the cluster's internal
structure and orbital history are not well known. We present an astrometric and
photometric study of the outer region of the Arches cluster (R > 6.25") using
HST WFC3IR. Using proper motions we calculate membership probabilities for
stars down to F153M = 20 mag (~2.5 M_sun) over a 120" x 120" field of view, an
area 144 times larger than previous astrometric studies of the cluster. We
construct the radial profile of the Arches to a radius of 75" (~3 pc at 8 kpc),
which can be well described by a single power law. From this profile we place a
3-sigma lower limit of 2.8 pc on the observed tidal radius, which is larger
than the predicted tidal radius (1 - 2.5 pc). Evidence of mass segregation is
observed throughout the cluster and no tidal tail structures are apparent along
the orbital path. The absence of breaks in the profile suggests that the Arches
has not likely experienced its closest approach to the GC between ~0.2 - 1 Myr
ago. If accurate, this constraint indicates that the cluster is on a prograde
orbit and is located front of the sky plane that intersects Sgr A*. However,
further simulations of clusters in the GC potential are required to interpret
the observed profile with more confidence.Comment: 24 pages (17-page main text, 7-page appendix), 24 figures, accepted
to Ap
A Direct Stellar Metallicity Determination in the Disk of the Maser Galaxy NGC4258
We present the first direct determination of a stellar metallicity in the
spiral galaxy NGC4258 (D=7.6 Mpc) based on the quantitative analysis of a
low-resolution (~5 AE) Keck LRIS spectrum of a blue supergiant star located in
its disk. A determination of stellar metallicity in this galaxy is important
for the absolute calibration of the Cepheid Period-Luminosity relation as an
anchor for the extragalactic distance scale and for a better characterization
of its dependence as a function of abundance. We find a value 0.2 dex lower
than solar metallicity at a galactocentric distance of 8.7 kpc, in agreement
with recent HII region studies using the weak forbidden auroral oxygen line at
4363 AE. We determine the effective stellar temperature, gravity, luminosity
and line-of-sight extinction of the blue supergiant being studied. We show that
it fits well on the flux-weighted gravity--luminosity relation (FGLR),
strengthening the potential of this method as a new extragalactic distance
indicator.Comment: Accepted for publication by ApJ Letters, 5 pages, 5 figure
The Quintuplet Cluster: Extended Structure and Tidal Radius
The Quintuplet star cluster is one of only three known young ( Myr)
massive (M M) clusters within pc of the Galactic
Center. In order to explore star cluster formation and evolution in this
extreme environment, we analyze the Quintuplet's dynamical structure. Using the
HST WFC3-IR instrument, we take astrometric and photometric observations of the
Quintuplet covering a field-of-view, which is times
larger than those of previous proper motion studies of the Quintuplet. We
generate a catalog of the Quintuplet region with multi-band, near-infrared
photometry, proper motions, and cluster membership probabilities for
stars. We present the radial density profile of candidate Quintuplet
cluster members with M out to pc from the cluster
center. A lower limit of pc is placed on the tidal radius,
indicating the lack of a tidal truncation within this radius range. Only weak
evidence for mass segregation is found, in contrast to the strong mass
segregation found in the Arches cluster, a second and slightly younger massive
cluster near the Galactic Center. It is possible that tidal stripping hampers a
mass segregation signature, though we find no evidence of spatial asymmetry.
Assuming that the Arches and Quintuplet formed with comparable extent, our
measurement of the Quintuplet's comparatively large core radius of
pc provides strong empirical evidence that young massive
clusters in the Galactic Center dissolve on a several Myr timescale.Comment: 25 pages (21-page main text, 4-page appendix), 18 figures, submitted
to Ap
Quantitative Spectroscopy of Blue Supergiants in Metal-Poor Dwarf Galaxy NGC 3109
We present a quantitative analysis of the low-resolution (4.5 A) spectra of
12 late-B and early-A blue supergiants (BSGs) in the metal-poor dwarf galaxy
NGC 3109. A modified method of analysis is presented which does not require use
of the Balmer jump as an independent temperature indicator, as used in previous
studies. We determine stellar effective temperatures, gravities, metallicities,
reddening, and luminosities, and combine our sample with the early-B type BSGs
analyzed by Evans et al. (2007) to derive the distance to NGC 3109 using the
Flux-weighted Gravity-Luminosity Relation (FGLR). Using primarily Fe-group
elements, we find an average metallicity of [Z] = -0.67 +/- 0.13, and no
evidence of a metallicity gradient in the galaxy. Our metallicities are higher
than those found by Evans et al. (2007) based on the oxygen abundances of
early-B supergiants ([Z] = -0.93 +/- 0.07), suggesting a low alpha/Fe ratio for
the galaxy. We adjust the position of NGC 3109 on the BSG-determined galaxy
mass-metallicity relation accordingly and compare it to metallicity studies of
HII regions in star-forming galaxies. We derive an FGLR distance modulus of
25.55 +/- 0.09 (1.27 Mpc) that compares well with Cepheid and tip of the red
giant branch (TRGB) distances. The FGLR itself is consistent with those found
in other galaxies, demonstrating the reliability of this method as a measure of
extragalactic distances.Comment: 50 pages, 23 figures; Accepted for publication in The Astrophysical
Journa
The former companion of the hyper-velocity star S5-HVS1
The hyper-velocity star S5-HVS1, ejected 5 Myr ago from the Galactic Center at 1800 km/s, was most likely produced by tidal break-up of a tight binary by the supermassive black hole SgrA*. Taking a Monte Carlo approach, we show that the former companion of S5-HVS1 was likely a main-sequence star between 1.5 and 6M⊙ and was captured into a highly eccentric orbit with pericenter distance in the range 1-10 AU and semimajor axis about 10³ AU. We then explore the fate of the captured star. We find that the heat deposited by tidally excited stellar oscillation modes leads to runaway disruption if the pericenter distance is smaller than about 3 AU. Over the past 5 Myr, its angular momentum has been significantly modified by orbital relaxation, which may stochastically drive the pericenter inwards below 3 AU and cause tidal disruption. We find an overall survival probability in the range 5% to 50%, depending on the local relaxation time in the close environment of the captured star, and the initial pericenter at capture. The pericenter distance of the surviving star has migrated to 10-100 AU, making it potentially the most extreme member of the S-star cluster. From the ejection rate of S5-HVS1-like stars, we estimate that there may currently be a few stars in such highly eccentric orbits. They should be detectable (typically K_s ≾18.5 mag) by the GRAVITY instrument and by future Extremely Large Telescopes and hence provide an extraordinary probe of the spin of SgrA*
An Adaptive Optics Survey of Stellar Variability at the Galactic Center
We present a year adaptive optics (AO) study of stellar
variability and search for eclipsing binaries in the central pc
() of the Milky Way nuclear star cluster. We measure the photometry
of 563 stars using the Keck II NIRC2 imager (-band, ). We achieve a photometric uncertainty floor of (), comparable to the highest precision achieved
in other AO studies. Approximately half of our sample () shows
variability. of known early-type young stars and of
known late-type giants are variable. These variability fractions are higher
than those of other young, massive star populations or late-type giants in
globular clusters, and can be largely explained by two factors. First, our
experiment time baseline is sensitive to long-term intrinsic stellar
variability. Second, the proper motion of stars behind spatial inhomogeneities
in the foreground extinction screen can lead to variability. We recover the two
known Galactic center eclipsing binary systems: IRS 16SW and S4-258 (E60). We
constrain the Galactic center eclipsing binary fraction of known early-type
stars to be at least . We find no evidence of an eclipsing
binary among the young S-stars nor among the young stellar disk members. These
results are consistent with the local OB eclipsing binary fraction. We identify
a new periodic variable, S2-36, with a 39.43 day period. Further observations
are necessary to determine the nature of this source.Comment: 69 pages, 28 figures, 12 tables. Accepted for publication in The
Astrophysical Journa
Measuring the Orbits of the Arches and Quintuplet Clusters using HST and Gaia: Exploring Scenarios for Star Formation Near the Galactic Center
We present new absolute proper motion measurements for the Arches and
Quintuplet clusters, two young massive star clusters near the Galactic center.
Using multi-epoch HST observations, we construct proper motion catalogs for the
Arches (35,000 stars) and Quintuplet (40,000 stars) fields in ICRF
coordinates established using stars in common with the Gaia EDR3 catalog. The
bulk proper motions of the clusters are measured to be (,
) = (-0.80 0.032, -1.89 0.021) mas/yr for the Arches
and (, ) = (-0.96 0.032, -2.29 0.023)
mas/yr for the Quintuplet, achieving 5x higher precision than past
measurements. We place the first constraints on the properties of the cluster
orbits that incorporate the uncertainty in their current line-of-sight
distances. The clusters will not approach closer than 25 pc to SgrA*,
making it unlikely that they will inspiral into the Nuclear Star Cluster within
their lifetime. Further, the cluster orbits are not consistent with being
circular; the average value of r / r is 1.9 (equivalent
to eccentricity of 0.31) for both clusters. Lastly, we find that the
clusters do not share a common orbit, challenging one proposed formation
scenario in which the clusters formed from molecular clouds on the open stream
orbit derived by Kruijssen et al. (2015). Meanwhile, our constraints on the
birth location and velocity of the clusters offer mild support for a scenario
in which the clusters formed via collisions between gas clouds on the x1 and x2
bar orbit families.Comment: Accepted for publication in ApJ. 38 pages, 25 figures. Proper motion
catalogs included in ancillary material