1,036 research outputs found
UV Extinction Towards a Quiescent Molecular Cloud in the SMC
Context: The mean UV extinction law for the Small Magellanic Cloud (SMC) is
usually taken as a template for low-metallicity galaxies. However, its current
derivation is based on only five stars, thus placing doubts on its
universality. An increase in the number of targets with measured extinction
laws in the SMC is necessary to determine its possible dependence on parameters
such as metallicity and star-forming activity. Aims: To measure the UV
extinction law for several stars in the quiescent molecular cloud SMC B1-1.
Methods: We obtained HST/STIS slitless UV spectroscopy of a 25"x25" field of
view and we combined it with ground-based NIR and visible photometry of the
stars in the field. The results were processed using the Bayesian photometric
package CHORIZOS to derive the visible-NIR extinction values for each star. The
unextinguished Spectral Energy Distributions (SEDs) obtained in this way were
then used to derive the UV extinction law for the four most extinguished stars.
We also recalculated the visible-NIR extinction for the five SMC stars with
preexisting UV extinction laws. Results: The UV extinction law for four SMC
B1-1 stars within several pc of each other differs significantly from star to
star. The 2175 {\AA} bump is moderately strong in one, weak in two, and absent
in the fourth.Comment: 10 pages, 5 figures, accepted for publication in Astronomy &
Astrophysic
Calibration of the photometric G passband for Gaia Data Release 1
Context. On September 2016 the first data from Gaia were released (DR1). The
first release included photometry for over 10^9 sources in the very broad G
system. Aims. To test the correspondence between G magnitudes in DR1 and the
synthetic equivalents derived using spectral energy distributions from observed
and model spectrophotometry. To correct the G passband curve and to measure the
zero point in the Vega system. Methods. I have computed the synthetic G and
Tycho-2 BV photometry for a sample of stars using the Next Generation Spectral
Library (NGSL) and the Hubble Space Telescope (HST) CALSPEC spectroscopic
standards. Results. I have found that the nominal G passband curve is too blue
for the DR1 photometry, as shown by the presence of a color term in the
comparison between observed and synthetic magnitudes. A correction to the
passband applying a power law in lambda with an exponent of 0.783 eliminates
the color term. The corrected passband has a Vega zero point of 0.0700.004
magnitudes.Comment: Four-page letter, accepted for publication in A&
The little-studied cluster Berkeley 90. II. The foreground ISM
Context: Nearly one century after their discovery, the carrier(s) of Diffuse
Interstellar Bands is/are still unknown and there are few sightlines studied in
detail for a large number of DIBs. Aims: We want to study the ISM sightlines
towards LS III +46 11 and LS III +46 12, two early-O-type stellar systems, and
LS III +46 11 B, a mid-B-type star. The three targets are located in the
stellar cluster Berkeley 90 and have a high extinction. Methods: We use the
multi-epoch high-S/N optical spectra presented in paper I (Ma\'iz Apell\'aniz
et al. 2015), the extinction results derived there, and additional spectra.
Results: We have measured equivalent widths, velocities, and FWHMs for a large
number of absorption lines in the rich ISM spectrum in front of Berkeley 90.
The absorbing ISM has at least two clouds at different velocities, one with a
lower column density (thinner) in the K I lines located away from Berkeley 90
and another one with a higher column density (thicker) associated with the
cluster. The first cloud has similar properties for both O-star sightlines but
the second one is thicker for LS III +46 11. The comparison between species
indicate that the cloud with a higher column density has a denser core,
allowing us to classify the DIBs in a sigma-zeta scale, some of them for the
first time. The LS III +46 12 sightline also has a high-velocity redshifted
component.Comment: Accepted for publication in A&
The VLT-FLAMES Tarantula Survey XVI. The optical+NIR extinction laws in 30 Doradus and the photometric determination of the effective temperatures of OB stars
Context: The commonly used extinction laws of Cardelli et al. (1989) have
limitations that, among other issues, hamper the determination of the effective
temperatures of O and early B stars from optical+NIR photometry. Aims: We aim
to develop a new family of extinction laws for 30 Doradus, check their general
applicability within that region and elsewhere, and apply them to test the
feasibility of using optical+NIR photometry to determine the effective
temperature of OB stars. Methods: We use spectroscopy and NIR photometry from
the VLT-FLAMES Tarantula Survey and optical photometry from HST/WFC3 of 30
Doradus and we analyze them with the software code CHORIZOS using different
assumptions such as the family of extinction laws. Results: We derive a new
family of optical+NIR extinction laws for 30 Doradus and confirm its
applicability to extinguished Galactic O-type systems. We conclude that by
using the new extinction laws it is possible to measure the effective
temperatures of OB stars with moderate uncertainties and only a small bias, at
least up to E(4405-5495) ~ 1.5 mag.Comment: Accepted for publication in A&A. Revised version corrects language
and fixes typos (one of them caught by David Nicholls). Figure 4 has poor
quality due to the size restrictions imposed by arXi
Lucky Spectroscopy, an equivalent technique to Lucky Imaging. Spatially resolved spectroscopy of massive close visual binaries using the William Herschel Telescope
CONTEXT: Many massive stars have nearby companions whose presence hamper
their characterization through spectroscopy. AIMS: We want to obtain spatially
resolved spectroscopy of close massive visual binaries to derive their spectral
types. METHODS: We obtain a large number of short long-slit spectroscopic
exposures of five close binaries under good seeing conditions, select those
with the best characteristics, extract the spectra using multiple-profile
fitting, and combine the results to derive spatially separated spectra.
RESULTS: We demonstrate the usefulness of Lucky Spectroscopy by presenting the
spatially resolved spectra of the components of each system, in two cases with
separations of only ~0.3". Those are delta Ori Aa+Ab (resolved in the optical
for the first time) and sigma Ori AaAb+B (first time ever resolved). We also
spatially resolve 15 Mon AaAb+B, zeta Ori AaAb+B (both previously resolved with
GOSSS, the Galactic O-Star Spectroscopic Survey), and eta Ori AaAb+B, a system
with two spectroscopic B+B binaries and a fifth visual component. The systems
have in common that they are composed of an inner pair of slow rotators orbited
by one or more fast rotators, a characteristic that could have consequences for
the theories of massive star formation.Comment: Accepted for publication in A&A, 7 page
Optical-NIR dust extinction towards Galactic O stars
[ABRIDGED] Context. O stars are excellent tracers of the intervening ISM
because of their high luminosity, blue intrinsic SED, and relatively
featureless spectra. We are currently conducting GOSSS, which is generating a
large sample of O stars with accurate spectral types within several kpc of the
Sun. Aims. To obtain a global picture of the properties of dust extinction in
the solar neighborhood based on optical-NIR photometry of O stars with accurate
spectral types. Methods. We have processed a photometric set with the CHORIZOS
code to measure the amount and type of extinction towards 562 O-type stellar
systems. We have tested three different families of extinction laws and
analyzed our results with the help of additional archival data. Results. The
Ma\'iz Apell\'aniz et al. (2014) family of extinction laws provides a better
description of Galactic dust that either the Cardelli et al. (1989) or
Fitzpatrick (1999) families, so it should be preferentially used. In many cases
O stars and late-type stars experience similar amounts of extinction at similar
distances but some O stars are located close to the molecular clouds left over
from their births and have larger extinctions than the average for nearby
late-type populations. In qualitative terms, O stars experience a more diverse
extinction than late-type stars, as some are affected by the small-grain-size,
low-R_5495 effect of molecular clouds and others by the large-grain-size,
high-R_5495 effect of H II regions. Late-type stars experience a narrower range
of grain sizes or R_5495, as their extinction is predominantly caused by the
average, diffuse ISM. We propose that the reason for the existence of
large-grain-size, high-R_5495 regions in the ISM in the form of H II regions
and hot-gas bubbles is the selective destruction of small dust grains by EUV
photons and possibly by thermal sputtering by atoms or ions.Comment: 33 pages, accepted for publication in A&A. This second version
includes language editing, other minor changes to the text, and the solution
to formatting issues in the main tabl
A reanalysis of the Gaia Data Release 2 photometric sensitivity curves using HST/STIS spectrophotometry
CONTEXT: The second Gaia data release (DR2) took place on April 2018. DR2
included photometry for more than 1.3 10^9 sources in G, BP, and RP. Even
though Gaia DR2 photometry is very precise, there are currently three
alternative definitions of the sensitivity curves that show significative
differences. AIMS: The aim of this paper is to improve the quality of the input
calibration data to produce new compatible definitions of the three bands and
to identify the reasons for the discrepancies between previous definitions.
METHODS: We have searched the HST archive for STIS spectra with G430L+G750L
data obtained with wide apertures and combined them with the CALSPEC library to
produce a high quality SED library of 122 stars with a broad range of colors,
including three very red stars. We have used it to compute new sensitivity
curves for G, BP, and RP using a functional analytical formalism. RESULTS: The
new curves are significantly better than the two previous attempts, REV and
WEI. For G we confirm the existence of a systematic bias in magnitude and
correct a color term present in REV. For BP we confirm the need to define two
magnitude ranges with different sensitivity curves and measure the cut between
them at G = 10.87 with a significant increase in precision. The new curves also
fit the data better than either REV or WEI. For RP we obtain a sensitivity
curve that better fits the STIS spectra and we find that the differences with
previous attempts reside in a systematic effect between ground-based and HST
spectral libraries. Additional evidence from color-color diagrams indicate that
the new sensitivity curve is more accurate. Nevertheless, there is still room
for improvement in the accuracy of the sensitivity curves because of the
current dearth of good-quality red calibrators: adding more to the sample
should be a priority before Gaia data release 3 takes place.Comment: Accepted for publication in A&A, revised version with minor change
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