982 research outputs found
Near-infrared spectroscopy of EX Lupi in outburst
EX Lup is the prototype of the EXor class of young eruptive stars: objects
showing repetitive brightenings due to increased accretion from the
circumstellar disk to the star. In this paper, we report on medium-resolution
near-infrared spectroscopy of EX\,Lup taken during its extreme outburst in
2008, as well as numerical modeling with the aim of determining the physical
conditions around the star. We detect emission lines from atomic hydrogen,
helium, and metals, as well as first overtone bandhead emission from carbon
monoxide. Our results indicate that the emission lines are originating from gas
located in a dust-free region within ~ 0.2 AU of the star. The profile of the
CO bandhead indicates that the CO gas has a temperature of 2500 K, and is
located in the inner edge of the disk or in the outer parts of funnel flows.
The atomic metals are probably co-located with the CO. Some metallic lines are
fluorescently excited, suggesting direct exposure to ultraviolet photons. The
Brackett series indicates emission from hot (10000 K) and optically thin gas.
The hydrogen lines display a strong spectro-astrometric signal, suggesting that
the hydrogen emission is probably not coming from an equatorial boundary layer;
a funnel flow or disk wind origin is more likely. This picture is broadly
consistent with the standard magnetospheric accretion model usually assumed for
normally accreting T Tauri stars. Our results also set constraints on the
eruption mechanism, supporting a model where material piles up around the
corotation radius and episodically falls onto the star.Comment: 12 pages, 8 figures, 1 table, accepted for publication in Ap
Optical spectroscopy of EX Lupi during quiescence and outburst: Infall, wind, and dynamics in the accretion flow
We explore the accretion mechanisms in EX Lupi, prototype of EXor variables,
during its quiescence and outburst phases. We analyse high-resolution optical
spectra taken before, during, and after its 2008 outburst. In quiescence and
outburst, the star presents many permitted emission lines, including typical
CTTS lines and numerous neutral and ionized metallic lines. During the
outburst, the number of emission lines increases to over a thousand, with
narrow plus broad component structure (NC+BC). The BC profile is highly
variable on short timescales (24-72h). An active chromosphere can explain the
metallic lines in quiescence and the outburst NC. The dynamics of the BC line
profiles suggest an origin in a hot, dense, non-axisymmetric, and non-uniform
accretion column that suffers velocity variations along the line-of-sight on
timescales of days. Assuming Keplerian rotation, the emitting region would be
located at ~0.1-0.2 AU, consistent with the inner disk rim, but the velocity
profiles of the lines reveal a combination of rotation and infall. Line ratios
of ions and neutrals can be reproduced with a temperature of T~6500 K for
electron densities of a few times 10cm in the line-emitting
region. The data confirm that the 2008 outburst was an episode of increased
accretion, albeit much stronger than previous EX Lupi and typical EXors
outbursts. The line profiles are consistent with the infall/rotation of a
non-axisymmetric structure that could be produced by clumpy accretion during
the outburst phase. A strong inner disk wind appears in the epochs of higher
accretion. The rapid recovery of the system after the outburst and the
similarity between the pre-outburst and post-outburst states suggest that the
accretion channels are similar during the whole period, and only the accretion
rate varies, providing a superb environment for studying the accretion
processes.Comment: 15 pages plus 26 pages online material, accepted by A&
The Galaxy Population of Low-Redshift Abell Clusters
We present a study of the luminosity and color properties of galaxies
selected from a sample of 57 low-redshift Abell clusters. We utilize the
non-parametric dwarf-to-giant ratio (DGR) and the blue galaxy fraction (fb) to
investigate the clustercentric radial-dependent changes in the cluster galaxy
population. Composite cluster samples are combined by scaling the counting
radius by r200 to minimize radius selection bias. The separation of galaxies
into a red and blue population was achieved by selecting galaxies relative to
the cluster color-magnitude relation. The DGR of the red and blue galaxies is
found to be independent of cluster richness (Bgc), although the DGR is larger
for the blue population at all measured radii. A decrease in the DGR for the
red and red+blue galaxies is detected in the cluster core region, while the
blue galaxy DGR is nearly independent of radius. The fb is found not to
correlate with Bgc; however, a steady decline toward the inner-cluster region
is observed for the giant galaxies. The dwarf galaxy fb is approximately
constant with clustercentric radius except for the inner cluster core region
where fb decreases. The clustercentric radial dependence of the DGR and the
galaxy blue fraction, indicates that it is unlikely that a simple scenario
based on either pure disruption or pure fading/reddening can describe the
evolution of infalling dwarf galaxies; both outcomes are produced by the
cluster environment.Comment: 28 pages, 6 figures, accepted for publication in Ap
Quantifying the Importance of Ram Pressure Stripping in a Galaxy Group at 100 Mpc
We examine two members of the NGC 4065 group of galaxies: a bent-double
(a.k.a. wide angle tail) radio source and an HI deficient spiral galaxy. Models
of the X-ray emitting intragroup gas and the bent-double radio source, NGC
4061, are used to probe the density of intergalactic gas in this group. HI
observations reveal an asymmetric, truncated distribution of neutral gas in
spiral galaxy, UGC 07049, and the accompanying radio continuum emission reveals
strong star formation. We examine the effectiveness of ram pressure stripping
as a gas removal mechanism and find that it alone cannot account for the HI
deficiency that is observed in UGC 07049 unless this galaxy has passed through
the core of the group with a velocity of ~800 km/s. A combination of tidal and
ram pressure stripping are necessary to produce the HI deficiency and asymmetry
in this galaxy.Comment: 8 pages, 6 figures, MNRAS accepte
The 2008 outburst of EX Lup - silicate crystals in motion
EX Lup is the prototype of the EXor class of eruptive young stars. These
objects show optical outbursts which are thought to be related to runaway
accretion onto the star. In a previous study we observed in-situ crystal
formation in the disk of EX Lup during its latest outburst in 2008, making the
object an ideal laboratory to investigate circumstellar crystal formation and
transport. This outburst was monitored by a campaign of ground-based and
Spitzer Space Telescope observations. Here we modeled the spectral energy
distribution of EX Lup in the outburst from optical to millimeter wavelengths
with a 2D radiative transfer code. Our results showed that the shape of the SED
at optical wavelengths was more consistent with a single temperature blackbody
than a temperature distribution. We also found that this single temperature
component emitted 80-100 % of the total accretion luminosity. We concluded that
a thermal instability, the most widely accepted model of EXor outbursts, was
likely not the triggering mechanism of the 2008 outburst of EX Lup. Our
mid-infrared Spitzer spectra revealed that the strength of all crystalline
bands between 8 and 30 um increased right after the end of the outburst. Six
months later, however, the crystallinity in the 10 um silicate feature complex
decreased. Our modeling of the mid-infrared spectral evolution of EXLup showed
that, although vertical mixing should be stronger during the outburst than in
the quiescent phase, fast radial transport of crystals (e.g., by stellar/disk
wind) was required to reproduce the observed mid-infrared spectra.Comment: Accepted for publication in ApJ, 37 pages, 11 figures, 2 table
The XMM/2dF survey III: Comparison between optical and X-ray cluster detection methods
We directly compare X-ray and optical techniques of cluster detection by
combining SDSS photometric data with a wide-field ( deg)
XMM-{\em Newton} survey in the North Galactic Pole region. The optical cluster
detection procedure is based on merging two independent selection methods - a
smoothing+percolation technique, and a Matched Filter Algorithm. The X-ray
cluster detection is based on a wavelet-based algorithm, incorporated in the
SAS v.5.2 package. The final optical sample counts 9 candidate clusters with
richness of more than 20 galaxies, corresponding roughly to APM richness class.
Three, of our optically detected clusters are also detected in our X-ray
survey. The most probable cause of the small number of optical cluster
candidates detected in our X-ray survey is that they are relatively poor
clusters, fainter than the X-ray flux limit (for extended sources) of our
survey .Comment: 9 pages, 5 figures, MNRAS accepte
The Benchmark Ultracool Subdwarf HD 114762B: A Test of Low-Metallicity Atmospheric and Evolutionary Models
We present a near-infrared spectroscopic study of HD 114762B, the latest-type
metal-poor companion discovered to date and the only ultracool subdwarf with a
known metallicity, inferred from the primary star to be [Fe/H] = -0.7. We
obtained a medium-resolution Keck/OSIRIS J-band spectrum and a low-resolution
IRTF/SpeX 0.8-2.4 um spectrum of HD 114762B. HD 114762B exhibits spectral
features common to both late-type dwarfs and subdwarfs, and we assign it a
spectral type of d/sdM9 +/- 1. We use a Monte Carlo technique to fit
PHOENIX/GAIA synthetic spectra to the observations, accounting for the
coarsely-gridded nature of the models. Fits to the entire OSIRIS J-band and to
the metal-sensitive J-band atomic absorption features (Fe I, K I, and Al I
lines) yield model parameters that are most consistent with the metallicity of
the primary star and the high surface gravity expected of old late-type
objects. The effective temperatures and radii inferred from the model
atmosphere fitting broadly agree with those predicted by the evolutionary
models of Chabrier & Baraffe, and the model color-absolute magnitude relations
accurately predict the metallicity of HD 114762B. We conclude that current
low-mass, mildly metal-poor atmospheric and evolutionary models are mutually
consistent for spectral fits to medium-resolution J-band spectra of HD 114762B,
but are inconsistent for fits to low-resolution near-infrared spectra of mild
subdwarfs. Finally, we develop a technique for estimating distances to
ultracool subdwarfs based on a single near-infrared spectrum. We show that this
"spectroscopic parallax" method enables distance estimates accurate to < 10% of
parallactic distances for ultracool subdwarfs near the hydrogen burning minimum
mass. (abridged)Comment: Accepted by ApJ; 23 pages, 20 figure
Star-galaxy separation in the AKARI NEP Deep Field
Context: It is crucial to develop a method for classifying objects detected
in deep surveys at infrared wavelengths. We specifically need a method to
separate galaxies from stars using only the infrared information to study the
properties of galaxies, e.g., to estimate the angular correlation function,
without introducing any additional bias. Aims. We aim to separate stars and
galaxies in the data from the AKARI North Ecliptic Pole (NEP) Deep survey
collected in nine AKARI / IRC bands from 2 to 24 {\mu}m that cover the near-
and mid-infrared wavelengths (hereafter NIR and MIR). We plan to estimate the
correlation function for NIR and MIR galaxies from a sample selected according
to our criteria in future research. Methods: We used support vector machines
(SVM) to study the distribution of stars and galaxies in the AKARIs multicolor
space. We defined the training samples of these objects by calculating their
infrared stellarity parameter (sgc). We created the most efficient classifier
and then tested it on the whole sample. We confirmed the developed separation
with auxiliary optical data obtained by the Subaru telescope and by creating
Euclidean normalized number count plots. Results: We obtain a 90% accuracy in
pinpointing galaxies and 98% accuracy for stars in infrared multicolor space
with the infrared SVM classifier. The source counts and comparison with the
optical data (with a consistency of 65% for selecting stars and 96% for
galaxies) confirm that our star/galaxy separation methods are reliable.
Conclusions: The infrared classifier derived with the SVM method based on
infrared sgc- selected training samples proves to be very efficient and
accurate in selecting stars and galaxies in deep surveys at infrared
wavelengths carried out without any previous target object selection.Comment: 8 pages, 8 figure
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Purification and functional characterisation of rhiminopeptidase A, a novel aminopeptidase from the venom of Bitis gabonica rhinoceros
This study describes the discovery and characterisation of a novel aminopeptidase A from the venom of B. g. rhinoceros and highlights its potential biological importance. Similar to mammalian aminopeptidases, rhiminopeptidase A might be capable of playing roles in altering the blood pressure and brain function of victims. Furthermore, it could have additional effects on the biological functions of other host proteins by cleaving their N-terminal amino acids. This study points towards the importance of complete analysis of individual components of snake venom in order to develop effective therapies for snake bites
The impact of a major cluster merger on galaxy evolution in MACS\,J0025.4-1225
We present results of an extensive morphological, spectroscopic, and
photometric study of the galaxy population of MACS J0025.41225 (z=0.586), a
major cluster merger with clear segregation of dark and luminous matter, to
examine the impact of mergers on galaxy evolution. Based on 436 galaxy spectra
obtained with Keck DEIMOS, we identified 212 cluster members within 4 Mpc of
the cluster centre, and classified them using three spectroscopic types; we
find 111 absorption-line, 90 emission-line (including 23 e(a) and 11 e(b)), and
6 E+A galaxies. The fraction of absorption(emission)-line galaxies is a
monotonically increasing(decreasing) function of both projected galaxy density
and radial distance to the cluster center. More importantly, the 6 observed E+A
cluster members are all located between the dark-matter peaks of the cluster
and within ~0.3Mpc radius of the X-ray flux peak, unlike the E+A galaxies in
other intermediate-redshift clusters which are usually found to avoid the core
region. In addition, we use Hubble Space Telescope imaging to classify cluster
members according to morphological type. We find the global fraction of spiral
and lenticular galaxies in MACS J0025 to be among the highest observed to date
in clusters at z>0.5. The observed E+A galaxies are found to be of lenticular
type with Sersic indices of ~2, boosting the local fraction of S0 to 70 per
cent between the dark-matter peaks. Combing the results of our analysis of the
spatial distribution, morphology, and spectroscopic features of the galaxy
population, we propose that the starburst phase of these E+A galaxies was both
initiated and terminated during the first core-passage about 0.5--1Gyr ago, and
that their morphology has already been transformed into S0 due to ram pressure
and/or tidal forces near the cluster core. By contrast, ongoing starbursts are
observed predominantly in infalling galaxies, and thus appears to be unrelated
to the cluster merger.Comment: 18 pages, 17 figures, accepted by MNRA
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