47 research outputs found
Seismic signature of envelope penetrative convection: the CoRoT star HD 52265
Aims: We aim at characterizing the inward transition from convective to
radiative energy transport at the base of the convective envelope of the
solar-like oscillator HD 52265 recently observed by the CoRoT satellite.
Methods: We investigated the origin of one specific feature found in the HD
52265 frequency spectrum. We modelled the star to derive the internal structure
and the oscillation frequencies that best match the observations and used a
seismic indicator sensitive to the properties of the base of the envelope
convection zone. Results: The seismic indicators clearly reveal that to best
represent the observed properties of HD 52265, models must include penetrative
convection below the outer convective envelope. The penetrative distance is
estimated to be , which corresponds to an extent over a distance
representing 6.0 per cents of the total stellar radius, significantly larger
than what is found for the Sun. The inner boundary of the extra-mixing region
is found at where is the stellar radius.
Conclusions: These results contribute to the tachocline characterization in
stars other than the Sun.Comment: 4 pages, 4 figures, accepted for publication in Astronomy &
Astrophysics Letter
Refining the associations of the Fermi Large Area Telescope Source Catalogs
The Fermi-Large Area Telescope (LAT) First Source Catalog (1FGL) was released
in February 2010 and the Fermi-LAT 2-Year Source Catalog (2FGL) appeared in
April 2012, based on data from 24 months of operation. Since their releases,
many follow up observations of unidentified gamma-ray sources (UGSs) were
performed and new procedures to associate gamma-ray sources with potential
counterparts at other wavelengths were developed. Here we review and
characterize all the associations as published in the 1FGL and 2FGL catalog on
the basis of multifrequency archival observations. In particular we located 177
spectra for the low-energy counterparts that were not listed in the previous
Fermi catalogs, and in addition we present new spectroscopic observations of 8
gamma-ray blazar candidates. Based on our investigations, we introduce a new
counterpart category of "candidate associations" and propose a refined
classification for the candidate low-energy counterparts of the Fermi sources.
We compare the 1FGL-assigned counterparts with those listed in the 2FGL to
determine which unassociated sources became associated in later releases of the
Fermi catalogs. We also search for potential counterparts to all the remaining
unassociated Fermi sources. Finally, we prepare a refined and merged list of
all the associations of the 1FGL plus 2FGL catalogs that includes 2219 unique
Fermi objects. This is the most comprehensive and systematic study of all the
associations collected for the gamma-ray sources available to date. We conclude
that 80% of the Fermi sources have at least one known plausible gamma-ray
emitter within their positional uncertainty regions.Comment: 26 pages, 24 figures, 7 tables, ApJS accepted for publication
(pre-proof version uploaded
Engulfing a radio pulsar: the case of PSR J1023+0038
The binary millisecond radio pulsar PSR J1023+0038 has been recently the
subject of multiwavelength monitoring campaigns which revealed that an
accretion disc has formed around the neutron star (since 2013 June). We present
here the results of X-ray and UV observations carried out by the Swift
satellite between 2013 October and 2014 May, and of optical and NIR
observations performed with the REM telescope, the Liverpool Telescope, the
2.1-m telescope at the San Pedro M\'artir Observatory and the 1.52-m telescope
at the Loiano observing station. The X-ray spectrum is well described by an
absorbed power law, which is softer than the previous quiescent epoch (up to
2013 June). The strong correlation between the X-ray and the UV emissions
indicates that the same mechanism should be responsible for part of the
emission in these bands. Optical and infrared photometric observations show
that the companion star is strongly irradiated. Double-peaked emission lines in
the optical spectra provide compelling evidence for the presence of an outer
accretion disc too. The spectral energy distribution from IR to X-rays is well
modelled if the contributions from the companion, the disc and the intra-binary
shock emission are all considered. Our extensive data set can be interpreted in
terms of an engulfed radio pulsar: the radio pulsar is still active, but
undetectable in the radio band due to a large amount of ionized material
surrounding the compact object. X-rays and gamma-rays are produced in an
intra-binary shock front between the relativistic pulsar wind and matter from
the companion and an outer accretion disc. The intense spin-down power
irradiates the disc and the companion star, accounting for the UV and optical
emissions.Comment: 11 pages, 8 figures, 5 tables; accepted for publication on MNRA
The Lyman alpha Reference Sample: Extended Lyman alpha Halos Produced at Low Dust Content
We report on new imaging observations of the Lyman alpha emission line (Lya),
performed with the Hubble Space Telescope, that comprise the backbone of the
Lyman alpha Reference Sample (LARS). We present images of 14 starburst galaxies
at redshifts 0.028 < z < 0.18 in continuum-subtracted Lya, Halpha, and the far
ultraviolet continuum. We show that Lya is emitted on scales that
systematically exceed those of the massive stellar population and recombination
nebulae: as measured by the Petrosian 20 percent radius, RP20, Lya radii are
larger than those of Halpha by factors ranging from 1 to 3.6, with an average
of 2.4. The average ratio of Lya-to-FUV radii is 2.9. This suggests that much
of the Lya light is pushed to large radii by resonance scattering. Defining the
"Relative Petrosian Extension" of Lya compared to Halpha, \xi_ext = RP20_Lya /
RP20_Ha, we find \xi_ext to be uncorrelated with total Lya luminosity. However
\xi_ext is strongly correlated with quantities that scale with dust content, in
the sense that a low dust abundance is a necessary requirement (although not
the only one) in order to spread Lya photons throughout the interstellar medium
and drive a large extended Lya halo.Comment: Published in ApJ Letters ~~ 6 pages using emulateapj, 4 figures ~~
Higher-resolution, larger, nicer jpeg versions of Figures 1 and 2 can be
found here: http://xayes.org/pub/press_lars.htm
The Lyman Alpha Reference Sample: V. The impact of neutral ISM kinematics and geometry on Lyman Alpha escape
We present high-resolution far-UV spectroscopy of the 14 galaxies of the
Lyman Alpha Reference Sample; a sample of strongly star-forming galaxies at low
redshifts (). We compare the derived properties to global
properties derived from multi band imaging and 21 cm HI interferometry and
single dish observations, as well as archival optical SDSS spectra. Besides the
Lyman line, the spectra contain a number of metal absorption features
allowing us to probe the kinematics of the neutral ISM and evaluate the optical
depth and and covering fraction of the neutral medium as a function of
line-of-sight velocity. Furthermore, we show how this, in combination with
precise determination of systemic velocity and good Ly spectra, can be
used to distinguish a model in which separate clumps together fully cover the
background source, from the "picket fence" model named by Heckman et al.
(2011). We find that no one single effect dominates in governing Ly
radiative transfer and escape. Ly escape in our sample coincides with a
maximum velocity-binned covering fraction of and bulk outflow
velocities of km s, although a number of galaxies show
these characteristics and yet little or no Ly escape. We find that
Ly peak velocities, where available, are not consistent with a strong
backscattered component, but rather with a simpler model of an intrinsic
emission line overlaid by a blueshifted absorption profile from the outflowing
wind. Finally, we find a strong anticorrelation between H equivalent
width and maximum velocity-binned covering factor, and propose a heuristic
explanatory model.Comment: 28 pages, 19 figures, 5 table
The Lyman Alpha Reference Sample: III. Properties of the Neutral ISM from GBT and VLA Observations
We present new H I imaging and spectroscopy of the 14 UV-selected
star-forming galaxies in the Lyman Alpha Reference Sample (LARS), aimed for a
detailed study of the processes governing the production, propagation, and
escape of Ly photons. New H I spectroscopy, obtained with the 100m
Green Bank Telescope (GBT), robustly detects the H I spectral line in 11 of the
14 observed LARS galaxies (although the profiles of two of the galaxies are
likely confused by other sources within the GBT beam); the three highest
redshift galaxies are not detected at our current sensitivity limits. The GBT
profiles are used to derive fundamental H I line properties of the LARS
galaxies. We also present new pilot H I spectral line imaging of 5 of the LARS
galaxies obtained with the Karl G. Jansky Very Large Array (VLA). This imaging
localizes the H I gas and provides a measurement of the total H I mass in each
galaxy. In one system, LARS 03 (UGC 8335 or Arp 238), VLA observations reveal
an enormous tidal structure that extends over 160 kpc from the main interacting
systems and that contains 10 M of H I. We compare various H I
properties with global Ly quantities derived from HST measurements. The
measurements of the Ly escape fraction are coupled with the new direct
measurements of H I mass and significantly disturbed H I velocities. Our
robustly detected sample reveals that both total H I mass and linewidth are
tentatively correlated with key Ly tracers. Further, on global scales,
these data support a complex coupling between Ly propagation and the H
I properties of the surrounding medium.Comment: Preprint form, 16 figures, accepted in Ap
The Lyman alpha reference sample. VII. Spatially resolved H kinematics
We present integral field spectroscopic observations with the Potsdam Multi
Aperture Spectrophotometer of all 14 galaxies in the Lyman Alpha
Reference Sample (LARS). We produce 2D line of sight velocity maps and velocity
dispersion maps from the Balmer (H) emission in our data
cubes. These maps trace the spectral and spatial properties of the LARS
galaxies' intrinsic Ly radiation field. We show our kinematic maps
spatially registered onto the Hubble Space Telescope H and Lyman
(Ly) images. Only for individual galaxies a causal connection
between spatially resolved H kinematics and Ly photometry can
be conjectured. However, no general trend can be established for the whole
sample. Furthermore, we compute non-parametric global kinematical statistics --
intrinsic velocity dispersion , shearing velocity ,
and the ratio -- from our kinematic maps. In
general LARS galaxies are characterised by high intrinsic velocity dispersions
(54\,km\,s median) and low shearing velocities (65\,km\,s
median). values range from 0.5 to 3.2 with an
average of 1.5. Noteworthy, five galaxies of the sample are dispersion
dominated systems with and are thus
kinematically similar to turbulent star forming galaxies seen at high redshift.
When linking our kinematical statistics to the global LARS Ly
properties, we find that dispersion dominated systems show higher Ly
equivalent widths and higher Ly escape fractions than systems with
. Our result indicates that turbulence in
actively star-forming systems is causally connected to interstellar medium
conditions that favour an escape of Ly radiation.Comment: 26 pages, 15 figures, accepted for publication in A&
Estimating stellar mean density through seismic inversions
Determining the mass of stars is crucial both to improving stellar evolution
theory and to characterising exoplanetary systems. Asteroseismology offers a
promising way to estimate stellar mean density. When combined with accurate
radii determinations, such as is expected from GAIA, this yields accurate
stellar masses. The main difficulty is finding the best way to extract the mean
density from a set of observed frequencies.
We seek to establish a new method for estimating stellar mean density, which
combines the simplicity of a scaling law while providing the accuracy of an
inversion technique.
We provide a framework in which to construct and evaluate kernel-based linear
inversions which yield directly the mean density of a star. We then describe
three different inversion techniques (SOLA and two scaling laws) and apply them
to the sun, several test cases and three stars.
The SOLA approach and the scaling law based on the surface correcting
technique described by Kjeldsen et al. (2008) yield comparable results which
can reach an accuracy of 0.5 % and are better than scaling the large frequency
separation. The reason for this is that the averaging kernels from the two
first methods are comparable in quality and are better than what is obtained
with the large frequency separation. It is also shown that scaling the large
frequency separation is more sensitive to near-surface effects, but is much
less affected by an incorrect mode identification. As a result, one can
identify pulsation modes by looking for an l and n assignment which provides
the best agreement between the results from the large frequency separation and
those from one of the two other methods. Non-linear effects are also discussed
as is the effects of mixed modes. In particular, it is shown that mixed modes
bring little improvement as a result of their poorly adapted kernels.Comment: Accepted for publication in A&A, 20 pages, 19 figure