1,128 research outputs found
A high resolution spectroscopic atlas of M subdwarfs - Effective temperature and metallicity
Context. M subdwarfs are metal poor and cool stars. They are important probes
of the old galactic populations. However, they remain elusive due to their low
luminosity. Observational and modeling efforts are required to fully understand
their physics and to investigate the effects of metallicity in their cool
atmospheres.
Aims. We perform a detailed study of a sample of subdwarfs to determine their
stellar parameters and constrain the stat-of-the art atmospheric models.
Methods. We present UVES/VLT high resolution spectra of three late-K
subdwarfs and 18 M subdwarfs. Our atlas covers the optical region from 6400 AA
up to the near infrared at 8900 AA. We show spectral details of cool
atmospheres at very high resolution (R= 40 000) and compare with synthetic
spectra computed from the recent BT-Settl atmosphere models.
Results. Our comparison shows that molecular features (TiO, VO, CaH), and
atomic features (Fe I, Ti I, Na I, K I) are well fitted by current models. We
produce an effective temperature versus spectral type relation all over the
subdwarf spectral sequence. Thanks to the high resolution of our spectra, we
perform a detailed comparison of line profiles of individual elements such as
Fe I, Ca II, Ti I, and are able to determine accurate metallicities of these
stars. These determinations contribute to calibrate the relation between
metallicity and molecular band strength indices from low-resolution spectra.
Conclusions. This work shows that the new generation of models are able to
reproduce various spectral features of M subdwarfs. Working with these high
resolution spectra allowed us to disentangle the atmospheric parameters
(effective temperature, gravity, metallicity), which is not possible when using
low resolution spectroscopy or photometry.Comment: 15 pages, 20 figures, 2 tables, accepted for publication in Astronomy
and Astrophysic
Effect of enzymic methylation of heterogeneous ribonucleoprotein particle A1 on its nucleic-acid binding and controlled proteolysis
New filamentary remnant radio emission and duty cycle constraints in the radio galaxy NGC 6086
Radio galaxies are a subclass of active galactic nuclei in which accretion
onto the supermassive black hole releases energy via relativistic jets. The
jets are not constantly active throughout the life of the host galaxy and
alternate between active and quiescent phases. Remnant radio galaxies are
detected during a quiescent phase and define a class of unique sources to
constrain the AGN duty cycle. We present, a spatially resolved radio analysis
of the radio galaxy associated with NGC 6086 and constraints on the spectral
age of the diffuse emission to investigate the duty cycle and evolution of the
source. We use three new low-frequency, high-sensitivity observations,
performed with the Low Frequency Array at 144 MHz and with the upgraded Giant
Metrewave Radio Telescope at 400 MHz and 675 MHz. To these, we add two Very
Large Array archival observations at 1400 and 4700 MHz. In the new
observations, we detect a second pair of larger lobes and three regions with a
filamentary morphology. We analyse the spectral index trend in the inner
remnant lobes and see systematic steeper values at the lower frequencies
compared to the GHz ones. Steeper spectral indices are found in the newly
detected outer lobes (up to 2.1), as expected if they trace a previous phase of
activity of the AGN. However, the differences between the spectra suggest
different dynamical evolution within the intragroup medium during their
expansion and/or different magnetic field values. We place constraints on the
age of the inner and outer lobes and derive the duty cycle of the source. This
results in a total active time of 39%. The filamentary structures have a
steep spectral index (1) without any spectral index trend and only one of
them shows a steepening in the spectrum. Their origin is not yet clear, but
they may have formed due to the compression of the plasma or due to magnetic
field substructures
Radio continuum tails in ram pressure-stripped spiral galaxies: experimenting with a semi-empirical model in Abell 2255
Wide-field radio continuum observations of galaxy clusters are revealing an
increasing number of spiral galaxies hosting tens of kpc-long radio tails
produced by the nonthermal interstellar medium being displaced by the ram
pressure. We present a semi-empirical model for the multi-frequency radio
continuum emission from ram pressure stripped tails based on the pure
synchrotron cooling of a radio plasma moving along the stripping direction with
a uniform velocity. We combine LOFAR and uGMRT observations at 144 and 400 MHz
to study the flux density and spectral index profiles of the radio tails of 7
galaxies in Abell 2255, and use the model to reproduce the flux density and
spectral index profiles, and infer the stripped radio plasma velocity. For 5
out of 7 galaxies we observe monotonic decrease in both flux density and
spectral index up to kpc from their stellar disk. Our model reproduces
the observed trends with a radio plasma bulk projected velocity between 160 and
430 km s. This result represents the first indirect measure of the
stripped, nonthermal interstellar medium velocity. The observed spectral index
trends indicate that the synchrotron cooling is faster than the adiabatic
expansion losses, thus suggesting that the stripped radio plasma can survive
for a few tens of Myr outside of the stellar disk. This provides a lower limit
for the lifetime of the stripped ISM outside of the disk. As a proof of
concept, we use the best-fit velocities to constrain the galaxies' 3D velocity
in the cluster to be in the 300-1300 km s. We estimate the ram pressure
affecting these galaxies to be between 0.1 and 2.9 erg
cm, and measure the inclination between their stellar disk and the ram
pressure wind.Comment: 15 pages, 9 figures, 5 tables. Accepted for publication on A&A on May
31st 202
Physical insights from the spectrum of the radio halo in MACS J0717.5+3745
We present new LOFAR observations of the massive merging galaxy cluster MACS
J0717.5+3745. The cluster hosts the most powerful radio halo known to date.
These new observations, in combination with published uGMRT (300850 MHz) and
VLA (16.5 GHz) data, reveal that the halo is more extended than previously
thought, with a largest linear size of . The halo shows a
steep spectrum () and a
steepening () above 1.5 GHz.
We find a strong scattering in spectral index maps on scales of 50100 kpc.
We suggest that such a strong scattering may be a consequence of the regime
where inverse Compton dominate the energy losses of electrons. The spectral
index becomes steeper and shows an increased curvature in the outermost regions
of the halo. We combined the radio data with \textit{Chandra} observations to
investigate the connection between the thermal and non-thermal components of
the intracluster medium (ICM). Despite a significant substructure in the halo
emission, the radio brightness correlates strongly with the X-ray brightness at
all observed frequencies. The radio-versus-X-ray brightness correlation slope
steepens at a higher radio frequency (from to
) and the spectral index shows a significant
anti correlation with the X-ray brightness. Both pieces of evidence further
support a spectral steepening in the external regions. The compelling evidence
for a steep spectral index, the existence of a spectral break above 1.5 GHz,
and the dependence of radio and X-ray surface brightness correlation on
frequency are interpreted in the context of turbulent reacceleration models.
Under this scenario, our results allowed us to constrain that the turbulent
kinetic pressure of the ICM is up to 10%.Comment: 16 pages, 12 figures, accepted for publication in A&
Physical insights from the spectrum of the radio halo in MACS J0717.5+3745
We present new LOw-Frequency ARray observations of the massive merging galaxy cluster MACS J0717.5+3745, located at a redshift of 0.5458. The cluster hosts the most powerful radio halo known to date. These new observations, in combination with published uGMRT (300-850 MHz) and VLA (1-6.5 GHz) data, reveal that the halo is more extended than previously thought, with a largest linear size of ∼2.2 Mpc, making it one of the largest known halos. The halo shows a steep spectrum (α144 MHz1.5 GHz ∼-1.4) and a steepening (α1.5 GHz5.5 GHz ∼-1.9) above 1.5 GHz. We find a strong scattering in spectral index maps on scales of 50-100 kpc. We suggest that such a strong scattering may be a consequence of the regime where inverse Compton dominates the energy losses of electrons. The spectral index becomes steeper and shows an increased curvature in the outermost regions of the halo. We combined the radio data with Chandra observations to investigate the connection between the thermal and nonthermal components of the intracluster medium (ICM). Despite a significant substructure in the halo emission, the radio brightness correlates strongly with the X-ray brightness at all observed frequencies. The radio-versus-X-ray brightness correlation slope steepens at a higher radio frequency (from b144? MHz? =? 0.67? ±? 0.05 to b3.0? GHz? =? 0.98? ±? 0.09) and the spectral index shows a significant anticorrelation with the X-ray brightness. Both pieces of evidence further support a spectral steepening in the external regions. The compelling evidence for a steep spectral index, the existence of a spectral break above 1.5 GHz, and the dependence of radio and X-ray surface brightness correlation on frequency are interpreted in the context of turbulent reacceleration models. Under this scenario, our results allowed us to constrain that the turbulent kinetic pressure of the ICM is up to 10%
Deep Low-frequency Radio Observations of A2256. I. the Filamentary Radio Relic
We present deep and high-fidelity images of the merging galaxy cluster A2256 at low frequencies using the upgraded Giant Metrewave Radio Telescope (uGMRT) and LOw-Frequency ARray (LOFAR). This cluster hosts one of the most prominent known relics with a remarkably spectacular network of filamentary substructures. The new uGMRT (300-850 MHz) and LOFAR (120-169 MHz) observations, combined with the archival Karl G. Jansky Very Large Array (VLA; 1-4 GHz) data, allowed us to carry out the first spatially resolved spectral analysis of the exceptional relic emission down to 6″ resolution over a broad range of frequencies. Our new sensitive radio images confirm the presence of complex filaments of magnetized relativistic plasma also at low frequencies. We find that the integrated spectrum of the relic is consistent with a single power law, without any sign of spectral steepening, at least below 3 GHz. Unlike previous claims, the relic shows an integrated spectral index of -1.07 ± 0.02 between 144 MHz and 3 GHz, which is consistent with the (quasi)stationary shock approximation. The spatially resolved spectral analysis suggests that the relic surface very likely traces the complex shock front, with a broad distribution of Mach numbers propagating through a turbulent and dynamically active intracluster medium. Our results show that the northern part of the relic is seen edge-on and the southern part close to face-on. We suggest that the complex filaments are regions where higher Mach numbers dominate the (re)acceleration of electrons that are responsible for the observed radio emission
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The SDSS-III APOGEE Radial Velocity Survey Of M Dwarfs. I. Description Of The Survey And Science Goals
We are carrying out a large ancillary program with the Sloan Digital Sky Survey, SDSS-III, using the fiber-fed multi-object near-infrared APOGEE spectrograph, to obtain high-resolution H-band spectra of more than 1200 M dwarfs. These observations will be used to measure spectroscopic rotational velocities, radial velocities, physical stellar parameters, and variability of the target stars. Here, we describe the target selection for this survey, as well as results from the first year of scientific observations based on spectra that will be publicly available in the SDSS-III DR 10 data release. As part of this paper we present radial velocities and rotational velocities of over 200 M dwarfs, with a v sin i precision of similar to 2 km s(-1) a measurement floor at v sin i = 4 km s(-1). This survey significantly increases the number of M dwarfs studied for rotational velocities and radial velocity variability (at similar to 100-200 m s(-1)), and will inform and advance the target selection for planned radial velocity and photometric searches for low-mass exoplanets around M dwarfs, such as the Habitable Zone Planet Finder, CARMENES, and TESS. Multiple epochs of radial velocity observations enable us to identify short period binaries, and adaptive optics imaging of a subset of stars enables the detection of possible stellar companions at larger separations. The high-resolution APOGEE spectra, covering the entire H band, provide the opportunity to measure physical stellar parameters such as effective temperatures and metallicities for many of these stars. At the culmination of this survey, we will have obtained multi-epoch spectra and radial velocities for over 1400 stars spanning the spectral range M0-L0, providing the largest set of near-infrared M dwarf spectra at high resolution, and more than doubling the number of known spectroscopic a sin i values for M dwarfs. Furthermore, by modeling telluric lines to correct for small instrumental radial velocity shifts, we hope to achieve a relative velocity precision floor of 50 m s(-1) for bright M dwarfs. With three or more epochs, this precision is adequate to detect substellar companions, including giant planets with short orbital periods, and flag them for higher-cadence followup. We present preliminary, and promising, results of this telluric modeling technique in this paper.Center for Exoplanets and Habitable WorldsPennsylvania State UniversityEberly College of SciencePennsylvania Space Grant ConsortiumNSF AST 1006676, AST 1126413National Science FoundationNational Aeronautics and Space Administration NNX-08AE38A, NNX13AB03GAlfred P. Sloan FoundationU.S. Department of Energy Oce of ScienceUniversity of ArizonaBrazilian Participation GroupBrookhaven National LaboratoryUniversity of CambridgeCarnegie Mellon UniversityUniversity of FloridaFrench Participation GroupGerman Participation GroupHarvard UniversityInstituto de Astrosica de CanariasMichigan State/Notre Dame/JINA Participation GroupJohns Hopkins UniversityLawrence Berkeley National LaboratoryMax Planck Institute for AstrophysicsMax Planck Institute for Extraterrestrial PhysicsNew Mexico State UniversityNew York UniversityOhio State UniversityUniversity of PortsmouthPrinceton UniversitySpanish Participation GroupUniversity of TokyoUniversity of UtahVanderbilt UniversityUniversity of VirginiaUniversity of WashingtonYale UniversityMcDonald Observator
The SDSS-III APOGEE Radial Velocity Survey of M dwarfs I: Description of Survey and Science Goals
We are carrying out a large ancillary program with the SDSS-III, using the
fiber-fed multi-object NIR APOGEE spectrograph, to obtain high-resolution
H-band spectra of more than 1200 M dwarfs. These observations are used to
measure spectroscopic rotational velocities, radial velocities, physical
stellar parameters, and variability of the target stars. Here, we describe the
target selection for this survey and results from the first year of scientific
observations based on spectra that is publicly available in the SDSS-III DR10
data release. As part of this paper we present RVs and vsini of over 200 M
dwarfs, with a vsini precision of ~2 km/s and a measurement floor at vsini = 4
km/s. This survey significantly increases the number of M dwarfs studied for
vsini and RV variability (at ~100-200 m/s), and will advance the target
selection for planned RV and photometric searches for low mass exoplanets
around M dwarfs, such as HPF, CARMENES, and TESS. Multiple epochs of radial
velocity observations enable us to identify short period binaries, and AO
imaging of a subset of stars enables the detection of possible stellar
companions at larger separations. The high-resolution H-band APOGEE spectra
provide the opportunity to measure physical stellar parameters such as
effective temperatures and metallicities for many of these stars. At the
culmination of this survey, we will have obtained multi-epoch spectra and RVs
for over 1400 stars spanning spectral types of M0-L0, providing the largest set
of NIR M dwarf spectra at high resolution, and more than doubling the number of
known spectroscopic vsini values for M dwarfs. Furthermore, by modeling
telluric lines to correct for small instrumental radial velocity shifts, we
hope to achieve a relative velocity precision floor of 50 m/s for bright M
dwarfs. We present preliminary results of this telluric modeling technique in
this paper.Comment: Submitted to Astronomical Journa
Energy-Dependent Timing of Thermal Emission in Solar Flares
We report solar flare plasma to be multi-thermal in nature based on the
theoretical model and study of the energy-dependent timing of thermal emission
in ten M-class flares. We employ high-resolution X-ray spectra observed by the
Si detector of the "Solar X-ray Spectrometer" (SOXS). The SOXS onboard the
Indian GSAT-2 spacecraft was launched by the GSLV-D2 rocket on 8 May 2003.
Firstly we model the spectral evolution of the X-ray line and continuum
emission flux F(\epsilon) from the flare by integrating a series of isothermal
plasma flux. We find that multi-temperature integrated flux F(\epsilon) is a
power-law function of \epsilon with a spectral index (\gamma) \approx -4.65.
Next, based on spectral-temporal evolution of the flares we find that the
emission in the energy range E= 4 - 15 keV is dominated by temperatures of T=
12 - 50 MK, while the multi-thermal power-law DEM index (\gamma) varies in the
range of -4.4 and -5.7. The temporal evolution of the X-ray flux F(\epsilon,t)
assuming a multi-temperature plasma governed by thermal conduction cooling
reveals that the temperature-dependent cooling time varies between 296 and 4640
s and the electron density (n_e) varies in the range of n_e= (1.77-29.3)*10^10
cm-3. Employing temporal evolution technique in the current study as an
alternative method for separating thermal from non-thermal components in the
energy spectra, we measure the break-energy point ranging between 14 and
21\pm1.0 keV.Comment: Solar Physics, in pres
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