2,547 research outputs found
Chemical compositions of thin-disk, high-metallicity red horizontal-branch field stars
We present a detailed abundance analysis and atmospheric parameters of 76
stars from a survey to identify field Galactic red horizontal-branch (RHB)
stars. High-resolution echelle spectra (R\simeq60,000, S/N>=100) were obtained
with 2.7 m Smith Telescope at McDonald Observatory. The target stars were
selected only by color and parallax information. Overall metallicities and
relative abundances of proton-capture elements (C I, N I, O I, Li I),
alpha-elements (Ca I and Si I), and neutron-capture elements (Eu II and La II)
were determined by either equivalent width or synthetic spectrum analyses. We
used CN features at 7995-8040 {\AA} region in order to determine 12^C/13^C
ratios of our targets. Investigation of the evolutionary stages, using
spectroscopic T_eff and log g values along with derived 12^C/13^C ratios,
revealed the presence of 18 probable RHB stars in our sample. We also derived
kinematics of the stars with available distance information. Taking into
account both the kinematics and probable evolutionary stages, we conclude that
our sample contains five thick disk and 13 thin disk RHB stars. Up until now,
RHB stars have been considered as members of the thick disk, and were expected
to have large space velocities and sub-solar metallicities. However, our sample
is dominated by low velocity solar-metallicity RHB stars; their existence
cannot be easily explained with standard stellar evolution.Comment: manuscript format, 65 pages, 18 figures, 8 tables, accepted for
publication in A
The Spectral Energy Distribution of Powerful Starburst Galaxies I : Modelling the Radio Continuum
This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society. © 2018 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.We have acquired radio-continuum data between 70MHz and 48 GHz for a sample of 19 southern starburst galaxies at moderate redshifts (0.067 < z < 0.227) with the aim of separating synchrotron and free-free emission components. Using a Bayesian framework, we find the radio continuum is rarely characterized well by a single power law, instead often exhibiting lowfrequency turnovers below 500 MHz, steepening at mid to high frequencies, and a flattening at high frequencies where free-free emission begins to dominate over the synchrotron emission. These higher order curvature components may be attributed to free-free absorption across multiple regions of star formation with varying optical depths. The decomposed synchrotron and free-free emission components in our sample of galaxies form strong correlations with the total-infrared bolometric luminosities. Finally, we find that without accounting for free-free absorption with turnovers between 90 and 500MHz the radio continuum at low frequency (v < 200 MHz) could be overestimated by upwards of a factor of 12 if a simple power-law extrapolation is used from higher frequencies. The mean synchrotron spectral index of our sample is constrained to be α = -1.06, which is steeper than the canonical value of -0.8 for normal galaxies. We suggest this may be caused by an intrinsically steeper cosmic ray distribution.Peer reviewe
Murchison Widefield Array and XMM-Newton observations of the Galactic supernova remnant G5.9+3.1
In this paper we discuss the radio continuum and X-ray properties of the
so-far poorly studied Galactic supernova remnant (SNR) G5.9+3.1. We present the
radio spectral energy distribution (SED) of the Galactic SNR G5.9+3.1 obtained
with the Murchison Widefield Array (MWA). Combining these new observations with
the surveys at other radio continuum frequencies, we discuss the integrated
radio continuum spectrum of this particular remnant. We have also analyzed an
archival XMM-Newton observation, which represents the first detection of X-ray
emission from this remnant. The SNR SED is very well explained by a simple
power-law relation. The synchrotron radio spectral index of G5.9+3.1, is
estimated to be 0.420.03 and the integrated flux density at 1GHz to be
around 2.7Jy. Furthermore, we propose that the identified point radio source,
located centrally inside the SNR shell, is most probably a compact remnant of
the supernova explosion. The shell-like X-ray morphology of G5.9+3.1 as
revealed by XMM-Newton broadly matches the spatial distribution of the radio
emission, where the radio-bright eastern and western rims are also readily
detected in the X-ray while the radio-weak northern and southern rims are weak
or absent in the X-ray. Extracted MOS1+MOS2+PN spectra from the whole SNR as
well as the north, east, and west rims of the SNR are fit successfully with an
optically thin thermal plasma model in collisional ionization equilibrium with
a column density N_H~0.80x cm and fitted temperatures spanning
the range kT~0.14-0.23keV for all of the regions. The derived electron number
densities n_e for the whole SNR and the rims are also roughly comparable
(ranging from ~ cm to ~ cm, where f
is the volume filling factor). We also estimate the swept-up mass of the X-ray
emitting plasma associated with G5.9+3.1 to be ~.Comment: Accepted for publication in A&
WALLABY early science − V. ASKAP HI imaging of the Lyon Group of Galaxies 351
We present an HI study of the galaxy group LGG 351 using Widefield ASKAP L-band Legacy All-sky Blind Survey (WALLABY) early science data observed with the Australian Square Kilometre Array Pathfinder (ASKAP). LGG 351 resides behind the M 83 group at a velocity range (cz) of ∼3500–4800 km s−1 within the rich Hydra-Centaurus overdensity region. We detect 40 sources with the discovery of a tidally interacting galaxy pair and two new HI sources that are not presented in previous optical catalogues. 23 out of 40 sources have new redshifts derived from the new HI data. This study is the largest WALLABY sub-sample to date and also allows us to further validate the performance of ASKAP and the data reduction pipeline ASKAPSOFT. Extended HI emission is seen in six galaxies indicating interaction within the group, although no HI debris is found. We also detect HI in a known ultra-faint dwarf galaxy (dw 1328−29), which demonstrates that it is not a satellite of the M 83 group as previously thought. In conjunction with multiwavelength data, we find that our galaxies follow the atomic gas fraction and baryonic Tully–Fisher scaling relations derived from the GALEX Arecibo SDSS Survey. In addition, majority of our galaxies fall within the star formation main sequence indicating inefficiency of gas removal processes in this loose galaxy group
First Attempt at Spectroscopic Detection of Gravity Modes in a Long-Period Pulsating Subdwarf B Star -- PG 1627+017
In the first spectroscopic campaign for a PG 1716 variable (or long-period
pulsating subdwarf B star), we succeeded in detecting velocity variations due
to g-mode pulsations at a level of 1.0-1.5 km/s.The observations were obtained
during 40 nights on 2-m class telescopes in Arizona, South Africa,and
Australia. The target,PG1627+017, is one of the brightest and largest amplitude
stars in its class.It is also the visible component of a post-common envelope
binary.Our final radial velocity data set includes 84 hours of time-series
spectroscopy over a time baseline of 53 days. Our derived radial velocity
amplitude spectrum, after subtracting the orbital motion, shows three potential
pulsational modes 3-4 sigma above the mean noise level, at 7201.0s,7014.6s and
7037.3s.Only one of the features is statistically likely to be real,but all
three are tantalizingly close to, or a one day alias of, the three strongest
periodicities found in the concurrent photometric campaign. We further
attempted to detect pulsational variations in the Balmer line amplitudes. The
single detected periodicity of 7209 s, although weak, is consistent with
theoretical expectations as a function of wavelength.Furthermore, it allows us
to rule out a degree index of l= 3 or l= 5 for that mode. Given the extreme
weakness of g-mode pulsations in these stars,we conclude that anything beyond
simply detecting their presence will require larger telescopes,higher
efficiency spectral monitoring over longer time baselines,improved longitude
coverage, and increased radial velocity precision.Comment: 39 pages, 9 figures, 4 tables, ApJ accepted. See postscript for full
abtrac
WALLABY pre-pilot survey: ultra-diffuse galaxies in the Eridanus supergroup
We present a pilot study of the atomic neutral hydrogen gas (H I) content of ultra-diffuse galaxy (UDG) candidates. In this paper, we use the pre-pilot Eridanus field data from the Widefield ASKAP L-band Legacy All-sky Blind Survey to search for H I in UDG candidates found in the Systematically Measuring Ultra-diffuse Galaxies survey (SMUDGes). We narrow down to 78 SMUDGes UDG candidates within the maximum radial extents of the Eridanus subgroups for this study. Most SMUDGes UDGs candidates in this study have effective radii smaller than 1.5 kpc and thus fail to meet the defining size threshold. We only find one H I detection, which we classify as a low-surface-brightness dwarf. Six putative UDGs are H I-free. We show the overall distribution of SMUDGes UDG candidates on the size–luminosity relation and compare them with low-mass dwarfs on the atomic gas fraction versus stellar mass scaling relation. There is no correlation between gas-richness and colour indicating that colour is not the sole parameter determining their H I content. The evolutionary paths that drive galaxy morphological changes and UDG formation channels are likely the additional factors to affect the H I content of putative UDGs. The actual numbers of UDGs for the Eridanus and NGC 1332 subgroups are consistent with the predicted abundance of UDGs and the halo virial mass relation, except for the NGC 1407 subgroup, which has a smaller number of UDGs than the predicted number. Different group environments suggest that these putative UDGs are likely formed via the satellite accretion scenario
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