20 research outputs found
A data base of synthetic photometry in the GALEX ultraviolet bands for the stellar sources observed with the International Ultraviolet Explorer
The Galaxy Evolution Explorer (GALEX) has produced the largest photometric
catalogue of ultraviolet (UV) sources. As such, it has defined the new standard
bands for UV photometry: the near UV band (NUV) and the far UV band (FUV).
However, due to brightness limits, the GALEX mission has avoided the Galactic
plane which is crucial for astrophysical research and future space missions.
The International Ultraviolet Explorer (IUE) satellite obtained 63,755 spectra
in the low dispersion mode during its 18 years lifetime. We have derived the
photometry in the GALEX bands for the stellar sources in the IUE Archive to
extend the GALEX data base with observations including the Galactic plane.Good
quality spectra have been selected for all IUE classes of stellar sources. The
GALEX FUV and NUV magnitudes have been computed using the GALEX transmission
curves, as well as the conversion equations between flux and magnitudes
provided by the mission (galexgi.gsfc.nasa.gov). Consistency between GALEX and
IUE synthetic photometries has been tested using White Dwarfs (WD) contained in
both samples. The non-linear response performance of GALEX inferred from this
data agrees with the results from GALEX calibration. The photometric data base
is made available to the community through the services of the Centre de
Donn\'ees Stellaires at Strasbourg (CDS). The catalogue contains FUV magnitudes
for 1,631 sources, ranging from FUV=1.81 to FUV=18.65 mag. In the NUV band, the
catalogue includes observations for 1,005 stars ranging from NUV = 3.08 to NUV=
17.74 mag . UV photometry for 1,493 not included in the GALEX AIS GR5 catalogue
is provided; most of them are hot (O-A spectral type) stars. The sources in the
catalogue are distributed over the full sky, including the Galactic plane.Comment: Submitted to Astronomy & Astrophysic
Theoretical modelling of the adsorption of neutral and charged sulphur-bearing species on to olivine nanoclusters
Sulphur depletion in the interstellar medium (ISM) is a long-standing issue,
as only 1% of its cosmic abundance is detected in dense molecular clouds (MCs),
while it does not appear to be depleted in other environments. In addition to
gas phase species, MCs also contain interstellar dust grains, which are
irregular, micron-sized, solid aggregates of carbonaceous materials and/or
silicates. Grains provide a surface where species can meet, accrete, and react.
Although freeze-out of sulphur onto dust grains could explain its depletion,
only OCS and, tentatively, SO were observed on their surfaces. Therefore,
it is our aim to investigate the interaction between sulphur-containing species
and the exposed mineral core of the grains at a stage prior to when sulphur
depletion is observed. Here, the grain core is represented by olivine
nanoclusters, one of the most abundant minerals in the ISM, with composition
MgSiO and MgFeSiO. We performed a series of quantum
mechanical calculations to characterize the adsorption of 9 S-bearing species,
both neutral and charged, onto the nanoclusters. Our calculations reveal that
the Fe-S interaction is preferred to Mg-S, causing sometimes the chemisorption
of the adsorbate. These species are more strongly adsorbed on the bare dust
grain silicate cores than on water ice mantles, and hence therefore likely
sticking on the surface of grains forming part of the grain core. This
demonstrates that the interaction of bare grains with sulphur species in cloud
envelopes can determine the S-depletion observed in dense molecular clouds.Comment: 9 pages, 4 figures, Accepted for publication in Monthly Notices of
the Royal Astronomical Societ
Polarized microwave emission from space particles in the upper atmosphere of the Earth
Tons of space particles enter the Earth atmosphere every year, being detected
when they produce fireballs, meteor showers, or when they impact the Earth
surface. Particle detection in the showers could also be attempted from space
using satellites in low Earth orbit. Measuring the polarization would provide
extra crucial information on the dominant alignment mechanisms and the
properties of the meteor families. In this article, we evaluate the expected
signal to aid in the design of space probes for this purpose. We have used the
RADMC-3D code to simulate the polarized microwave emission of aligned dust
particles with different compositions: silicates, carbonates and irons. We have
assumed a constant spatial particle density distribution of 0.22 cm,
based on particle density measurements carried during meteor showers. Four
different grain size distributions with power indices ranging from to
and dust particles with radius ranging from 0.01 m to 1 cm
have been considered for the simulations. Silicates and carbonates align their
minor axis with the direction of the solar radiation field; during the flight
time into the Earth atmosphere, iron grains get oriented with the Earth's
magnetic field depending on their size. Alignment direction is reflected in the
-Stokes parameter and in the polarization variation along the orbit.
Polarization depends on the composition and on the size distribution of the
particles. The simulations show that some specific particle populations might
be detectable even with a small probe equipped with high sensitivity,
photon-counting microwave detectors operating in low Earth orbit
Formation of dust filaments in the diffuse envelopes of molecular clouds
The path to understanding star formation processes begins with the study of the formation of molecular clouds. The outskirts of these clouds are characterized by low column densities that allow the penetration of ultraviolet radiation, resulting in a non-negligible ionization fraction and the charging of the small dust grains that are mixed with the gas; this diffuse phase is then coupled to the ambient magnetic field. Despite the general assumption that dust and gas are tightly correlated, several observational and theoretical studies have reported variations in the dust-to-gas ratio toward diffuse and cold clouds. In this work, we present the implementation of a new charged particles module for analyzing the dust dynamics in molecular cloud envelopes. We study the evolution of a single population of small charged grains (0.05 µm) in the turbulent, magnetized molecular cloud envelope using this module. We show that variations in the dust-to-gas ratio arise due to the coupling of the grains with the magnetic field, forming elongated dust structures decoupled from the gas. This emphasizes the importance of considering the dynamics of charged dust when simulating the different phases of the interstellar medium, especially for star formation studies
Uncovering a new group of T Tauri stars in the Taurus-Auriga molecular complex from Gaia and GALEX data
In this work, we examine the list of 63 candidates to T Tauri star (TTS) in
the TAMC identified by their ultraviolet (UV) and infrared colours (IR)
measured from data obtained by the Galaxy Evolution Explorer all sky survey
(GALEX-AIS) and the Two Microns All Sky Survey (2MASS), respectively. The
objective of this work is twofold: evaluate whether they are pre-main sequence
(PMS) stars and evaluate the goodness of the UV-IR colour-colour diagram to
detect PMS stars in wide-fields.
The astrometric properties of these sources have been retrieved from the Gaia
DR3 catalogue and used to evaluate their membership probability. Several
classification algorithms have been tested to search for the kinematical groups
but the final classification has been made with k-means++ algorithms.
Membership probability has been evaluated by applying Logistic Regression. In
addition, spectroscopic information available in the archive of the Large Sky
Area Multi Object Fiber Spectroscopic Telescope has been used to ascertain
their PMS nature when available.
About 20% of the candidates share the kinematics of the TAMC members. Among
them, HD 281691 is a G8-type field star located in front of the cloud and HO
Aur is likely a halo star given the very low metallicity provided by Gaia. The
rest are three known PMS stars (HD 30171, V600 Aur and J04590305+3003004), two
previously unknown accreting M-type stars (J04510713+1708468 and
J05240794+2542438) and, five additional sources, which are very likely PMS
stars. Most of these new sources are concentrated at low galactic latitudes
over the Auriga-Perseus region.Comment: accepted by A&
Gas phase Elemental abundances in Molecular cloudS (GEMS). IX. Deuterated compounds of H2S in starless cores
H2S is thought to be the main sulphur reservoir in the ice, being therefore a
key molecule to understand sulphur chemistry in the star formation process and
to solve the missing sulphur problem. The H2S deuterium fraction can be used to
constrain its formation pathways. We investigate for the first time the H2S
deuteration in a large sample of starless cores (SC). We use observations of
the GEMS IRAM 30m Large Program and complementary IRAM 30m observations. We
consider a sample of 19 SC in Taurus, Perseus, and Orion, detecting HDS in 10
and D2S in five. The H2S single and double deuterium fractions are analysed
with regard to their relation with the cloud physical parameters, their
comparison with other interstellar sources, and their comparison with deuterium
fractions in early stage star-forming sources of c-C3H2, H2CS, H2O, H2CO, and
CH3OH. We obtain a range of X(HDS)/X(H2S)~0.025-0.2 and X(D2S)/X(HDS)~0.05-0.3.
H2S single deuteration shows an inverse relation with the cloud kinetic
temperature. H2S deuteration values in SC are similar to those observed in
Class 0. Comparison with other molecules in other sources reveals a general
trend of decreasing deuteration with increasing temperature. In SC and Class 0
objects H2CS and H2CO present higher deuteration fractions than c-C3H2, H2S,
H2O, and CH3OH. H2O shows single and double deuteration values one order of
magnitude lower than those of H2S and CH3OH. Differences between c-C3H2, H2CS
and H2CO deuterium fractions and those of H2S, H2O, and CH3OH are related to
deuteration processes produced in gas or solid phases, respectively. We
interpret the differences between H2S and CH3OH deuterations and that of H2O as
a consequence of differences on the formation routes in the solid phase,
particularly in terms of the different occurrence of the D-H and H-D
substitution reactions in the ice, together with the chemical desorption
processes.Comment: Accepted for publication in A&
Gas phase Elemental abundances in Molecular cloudS (GEMS) VIII. Unlocking the CS chemistry: the CH + S CS + H and C + S CS + C reactions
We revise the rates of reactions CH + S -> CS + H and C_2 + S -> CS + C,
important CS formation routes in dark and diffuse warm gas. We performed ab
initio calculations to characterize the main features of all the electronic
states correlating to the open shell reactants. For CH+S we have calculated the
full potential energy surfaces for the lowest doublet states and the reaction
rate constant with a quasi-classical method. For C_2+S, the reaction can only
take place through the three lower triplet states, which all present deep
insertion wells. A detailed study of the long-range interactions for these
triplet states allowed to apply a statistic adiabatic method to determine the
rate constants. This study of the CH + S reaction shows that its rate is nearly
independent on the temperature in a range of 10-500 K with an almost constant
value of 5.5 10^{-11} cm^3/s at temperatures above 100~K. This is a factor \sim
2-3 lower than the value obtained with the capture model. The rate of the
reaction C_2 + S depends on the temperature taking values close to 2.0 10^{-10}
cm^3/s at low temperatures and increasing to 5. 10^{-10} cm^3/s for
temperatures higher than 200~K. Our modeling provides a rate higher than the
one currently used by factor of \sim 2. These reactions were selected for
involving open-shell species with many degenerate electronic states, and the
results obtained in the present detailed calculations provide values which
differ a factor of \sim 2-3 from the simpler classical capture method. We have
updated the sulphur network with these new rates and compare our results in the
prototypical case of TMC1 (CP). We find a reasonable agreement between model
predictions and observations with a sulphur depletion factor of 20 relative to
the sulphur cosmic abundance, but it is not possible to fit all sulphur-bearing
molecules better than a factor of 10 at the same chemical time.Comment: 13 pages, 10 figure