182 research outputs found
Investigating Cepheid Carinae's Cycle-to-cycle Variations via Contemporaneous Velocimetry and Interferometry
Baade-Wesselink-type (BW) techniques enable geometric distance measurements
of Cepheid variable stars in the Galaxy and the Magellanic clouds. The leading
uncertainties involved concern projection factors required to translate
observed radial velocities (RVs) to pulsational velocities and recently
discovered modulated variability. We carried out an unprecedented observational
campaign involving long-baseline interferometry (VLTI/PIONIER) and spectroscopy
(Euler/Coralie) to search for modulated variability in the long-period (P
35.5 d) Cepheid Carinae. We determine highly precise angular diameters
from squared visibilities and investigate possible differences between two
consecutive maximal diameters, . We characterize the
modulated variability along the line-of-sight using 360 high-precision RVs.
Here we report tentative evidence for modulated angular variability and confirm
cycle-to-cycle differences of Carinae's RV variability. Two successive
maxima yield = 13.1 0.7 (stat.) {\mu}as for
uniform disk models and 22.5 1.4 (stat.) {\mu}as (4% of the total angular
variation) for limb-darkened models. By comparing new RVs with 2014 RVs we show
modulation to vary in strength. Barring confirmation, our results suggest the
optical continuum (traced by interferometry) to be differently affected by
modulation than gas motions (traced by spectroscopy). This implies a previously
unknown time-dependence of projection factors, which can vary by 5% between
consecutive cycles of expansion and contraction. Additional interferometric
data are required to confirm modulated angular diameter variations. By
understanding the origin of modulated variability and monitoring its long-term
behavior, we aim to improve the accuracy of BW distances and further the
understanding of stellar pulsations.Comment: Accepted for publication in MNRAS. 19 pages, 13 figures, 10 table
Large-amplitude variables in Gaia Data Release 2. Multi-band variability characterization
The second data release (DR2) of Gaia provides mean photometry in three bands
for 1.4 billion sources, but light curves and variability properties are
available for only 0.5 million of them. Here, we provide a census of
large-amplitude variables with amplitudes larger than 0.2 mag in the
band for objects with mean brightnesses between 5.5 and 19 mag. To achieve
this, we rely on variability amplitude proxies in , and
computed from the uncertainties on the magnitudes published in DR2. We then
apply successive filters to identify two subsets containing respectively
sources with reliable mean and (for studies using colours)
and sources having compatible amplitude proxies in , and
(for multi-band variability studies). The full catalogue gathers
large-amplitude variable candidates, and the two subsets with increased levels
of purity contain respectively and sources. A
multi-band variability analysis of the catalogue shows that different types of
variable stars can be globally categorized in four groups according to their
colour and blue-to-red amplitude ratios as determined from the ,
and amplitude proxies. The catalogue constitutes the first census of
Gaia large-amplitude variable candidates, extracted from the public DR2
archive. The overview presented here illustrates the added-value of the mission
for multi-band variability studies even at this stage when epoch photometry is
not yet available for all sources. (Abridged abstract)Comment: Final version, A&A, in press. Main text: 20 pages, 26 figures. Four
appendixe
The astrometric Gaia-FUN-SSO observation campaign of 99 942 Apophis
Astrometric observations performed by the Gaia Follow-Up Network for Solar
System Objects (Gaia-FUN-SSO) play a key role in ensuring that moving objects
first detected by ESA's Gaia mission remain recoverable after their discovery.
An observation campaign on the potentially hazardous asteroid (99 942) Apophis
was conducted during the asteroid's latest period of visibility, from
12/21/2012 to 5/2/2013, to test the coordination and evaluate the overall
performance of the Gaia-FUN-SSO . The 2732 high quality astrometric
observations acquired during the Gaia-FUN-SSO campaign were reduced with the
Platform for Reduction of Astronomical Images Automatically (PRAIA), using the
USNO CCD Astrograph Catalogue 4 (UCAC4) as a reference. The astrometric
reduction process and the precision of the newly obtained measurements are
discussed. We compare the residuals of astrometric observations that we
obtained using this reduction process to data sets that were individually
reduced by observers and accepted by the Minor Planet Center. We obtained 2103
previously unpublished astrometric positions and provide these to the
scientific community. Using these data we show that our reduction of this
astrometric campaign with a reliable stellar catalog substantially improves the
quality of the astrometric results. We present evidence that the new data will
help to reduce the orbit uncertainty of Apophis during its close approach in
2029. We show that uncertainties due to geolocations of observing stations, as
well as rounding of astrometric data can introduce an unnecessary degradation
in the quality of the resulting astrometric positions. Finally, we discuss the
impact of our campaign reduction on the recovery process of newly discovered
asteroids.Comment: Accepted for publication in A&
Multi-epoch high-resolution spectroscopy of SN2011fe - Linking the progenitor to its environment
This study attempts to establish a link between the reasonably well known
nature of the progenitor of SN2011fe and its surrounding environment. This is
done with the aim of enabling the identification of similar systems in the vast
majority of the cases, when distance and epoch of discovery do not allow a
direct approach. To study the circumstellar environment of SN2011fe we have
obtained high-resolution spectroscopy of SN2011fe on 12 epochs, from 8 to 86
days after the estimated date of explosion, targeting in particular at the time
evolution of CaII and NaI. Three main absorption systems are identified from
CaII and NaI, one associated to the Milky Way, one probably arising within a
high-velocity cloud, and one most likely associated to the halo of M101. The
Galactic and host galaxy reddening, deduced from the integrated equivalent
widths (EW) of the NaI lines are E(B-V)=0.011+/-0.002 and E(B-V)=0.014+/-0.002
mag, respectively. The host galaxy absorption is dominated by a component
detected at the same velocity measured from the 21-cm HI line at the projected
SN position (~180 km/s). During the ~3 months covered by our observations, its
EW changed by 15.6+/-6.5 mA. This small variation is shown to be compatible
with the geometric effects produced by therapid SN photosphere expansion
coupled to the patchy fractal structure of the ISM. The observed behavior is
fully consistent with ISM properties similar to those derived for our own
Galaxy, with evidences for structures on scales <100 AU. SN2011fe appears to be
surrounded by a "clean" environment. The lack of blue-shifted, time-variant
absorption features is fully consistent with the progenitor being a binary
system with a main-sequence, or even another degenerate star.Comment: 12 pages, 6 figures, submitted to Astronomy and Astrophysic
<i>Gaia</i> Data Release 1. Summary of the astrometric, photometric, and survey properties
Context. At about 1000 days after the launch of Gaia we present the first Gaia data release, Gaia DR1, consisting of astrometry and photometry for over 1 billion sources brighter than magnitude 20.7.
Aims. A summary of Gaia DR1 is presented along with illustrations of the scientific quality of the data, followed by a discussion of the limitations due to the preliminary nature of this release.
Methods. The raw data collected by Gaia during the first 14 months of the mission have been processed by the Gaia Data Processing and Analysis Consortium (DPAC) and turned into an astrometric and photometric catalogue.
Results. Gaia DR1 consists of three components: a primary astrometric data set which contains the positions, parallaxes, and mean proper motions for about 2 million of the brightest stars in common with the HIPPARCOS and Tycho-2 catalogues – a realisation of the Tycho-Gaia Astrometric Solution (TGAS) – and a secondary astrometric data set containing the positions for an additional 1.1 billion sources. The second component is the photometric data set, consisting of mean G-band magnitudes for all sources. The G-band light curves and the characteristics of ∼3000 Cepheid and RR-Lyrae stars, observed at high cadence around the south ecliptic pole, form the third component. For the primary astrometric data set the typical uncertainty is about 0.3 mas for the positions and parallaxes, and about 1 mas yr−1 for the proper motions. A systematic component of ∼0.3 mas should be added to the parallax uncertainties. For the subset of ∼94 000 HIPPARCOS stars in the primary data set, the proper motions are much more precise at about 0.06 mas yr−1. For the secondary astrometric data set, the typical uncertainty of the positions is ∼10 mas. The median uncertainties on the mean G-band magnitudes range from the mmag level to ∼0.03 mag over the magnitude range 5 to 20.7.
Conclusions. Gaia DR1 is an important milestone ahead of the next Gaia data release, which will feature five-parameter astrometry for all sources. Extensive validation shows that Gaia DR1 represents a major advance in the mapping of the heavens and the availability of basic stellar data that underpin observational astrophysics. Nevertheless, the very preliminary nature of this first Gaia data release does lead to a number of important limitations to the data quality which should be carefully considered before drawing conclusions from the data
Gaia Data Release 3: Processing and validation of BP/RP low-resolution spectral data
(Abridged) Blue (BP) and Red (RP) Photometer low-resolution spectral data is
one of the exciting new products in Gaia Data Release 3 (Gaia DR3). We
calibrate about 65 billion individual transit spectra onto the same mean BP/RP
instrument through a series of calibration steps, including background
subtraction, calibration of the CCD geometry and an iterative procedure for the
calibration of CCD efficiency as well as variations of the line-spread function
and dispersion across the focal plane and in time. The calibrated transit
spectra are then combined for each source in terms of an expansion into
continuous basis functions. Time-averaged mean spectra covering the optical to
near-infrared wavelength range [330, 1050] nm are published for approximately
220 million objects. Most of these are brighter than G = 17.65 but some BP/RP
spectra are published for sources down to G = 21.43. Their signal- to-noise
ratio varies significantly over the wavelength range covered and with magnitude
and colour of the observed objects, with sources around G = 15 having S/N above
100 in some wavelength ranges. The top-quality BP/RP spectra are achieved for
sources with magnitudes 9 < G < 12, having S/N reaching 1000 in the central
part of the RP wavelength range. Scientific validation suggests that the
internal calibration was generally successful. However, there is some evidence
for imperfect calibrations at the bright end G < 11, where calibrated BP/RP
spectra can exhibit systematic flux variations that exceed their estimated flux
uncertainties. We also report that due to long-range noise correlations, BP/RP
spectra can exhibit wiggles when sampled in pseudo-wavelength.Comment: Submitted to A&
Total eclipse of the heart: the AM CVn Gaia14aae/ASSASN-14cn
We report the discovery and characterization of a deeply eclipsing AM CVn-system, Gaia14aae (=ASSASN-14cn). Gaia14aae was identified independently by the All-Sky Automated Survey for Supernovae (ASAS-SN; Shappee et al.) and by the Gaia Science Alerts project, during two separate outbursts. A third outburst is seen in archival Pan-STARRS-1 (PS1; Schlafly et al.; Tonry et al.; Magnier et al.) and ASAS-SN data. Spectroscopy reveals a hot, hydrogen-deficient spectrum with clear double-peaked emission lines, consistent with an accreting double-degenerate classification. We use follow-up photometry to constrain the orbital parameters of the system. We find an orbital period of 49.71min, which places Gaia14aae at the long period extremum of the outbursting AM CVn period distribution. Gaia14aae is dominated by the light from its accreting white dwarf (WD). Assuming an orbital inclination of 90° for the binary system, the contact phases of the WD lead to lower limits of 0.78 and 0.015M⊙ on the masses of the accretor and donor, respectively, and a lower limit on the mass ratio of 0.019. Gaia14aae is only the third eclipsing AM CVn star known, and the first in which the WD is totally eclipsed. Using a helium WD model, we estimate the accretor's effective temperature to be 12 900 ± 200K. The three outburst events occurred within four months of each other, while no other outburst activity is seen in the previous 8yr of Catalina Real-time Transient Survey (CRTS; Drake et al.), Pan-STARRS-1 and ASAS-SN data. This suggests that these events might be rebrightenings of the first outburst rather than individual event
Gaia Data Release 3: The Galaxy in your preferred colours: Synthetic photometry from Gaia low-resolution spectra
Gaia Data Release 3 provides novel flux-calibrated low-resolution spectrophotometry for '220 million sources in the wavelength range 330 nm ≤ λ ≤ 1050 nm (XP spectra). Synthetic photometry directly tied to a flux in physical units can be obtained from these spectra for any passband fully enclosed in this wavelength range. We describe how synthetic photometry can be obtained from XP spectra, illustrating the performance that can be achieved under a range of different conditions - for example passband width and wavelength range - as well as the limits and the problems affecting it. Existing top-quality photometry can be reproduced within a few per cent over a wide range of magnitudes and colour, for wide and medium bands, and with up to millimag accuracy when synthetic photometry is standardised with respect to these external sources. Some examples of potential scientific application are presented, including the detection of multiple populations in globular clusters, the estimation of metallicity extended to the very metal-poor regime, and the classification of white dwarfs. A catalogue providing standardised photometry for 2.2×108sources in several wide bands of widely used photometric systems is provided (Gaia Synthetic Photometry Catalogue; GSPC) as well as a catalogue of '105 white dwarfs with DA/non-DA classification obtained with a Random Forest algorithm (Gaia Synthetic Photometry Catalogue for White Dwarfs; GSPC-WD)
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Gaia Early Data Release 3: Gaia photometric science alerts
Context. Since July 2014, the Gaia mission has been engaged in a high-spatial-resolution, time-resolved, precise, accurate astrometric, and photometric survey of the entire sky. Aims. We present the Gaia Science Alerts project, which has been in operation since 1 June 2016. We describe the system which has been developed to enable the discovery and publication of transient photometric events as seen by Gaia. Methods. We outline the data handling, timings, and performances, and we describe the transient detection algorithms and filtering procedures needed to manage the high false alarm rate. We identify two classes of events: (1) sources which are new to Gaia and (2) Gaia sources which have undergone a significant brightening or fading. Validation of the Gaia transit astrometry and photometry was performed, followed by testing of the source environment to minimise contamination from Solar System objects, bright stars, and fainter near-neighbours. Results. We show that the Gaia Science Alerts project suffers from very low contamination, that is there are very few false-positives. We find that the external completeness for supernovae, CE = 0.46, is dominated by the Gaia scanning law and the requirement of detections from both fields-of-view. Where we have two or more scans the internal completeness is CI = 0.79 at 3 arcsec or larger from the centres of galaxies, but it drops closer in, especially within 1 arcsec. Conclusions. The per-Transit photometry for Gaia transients is precise to 1% at G = 13, and 3% at G = 19. The per-Transit astrometry is accurate to 55 mas when compared to Gaia DR2. The Gaia Science Alerts project is one of the most homogeneous and productive transient surveys in operation, and it is the only survey which covers the whole sky at high spatial resolution (subarcsecond), including the Galactic plane and bulge. © S. T. Hodgkin et al. 2021
The Catalina Surveys Periodic Variable Star Catalog
We present ~47,000 periodic variables found during the analysis of 5.4
million variable star candidates within a 20,000 square degree region covered
by the Catalina Surveys Data Release-1 (CSDR1). Combining these variables with
type-ab RR Lyrae from our previous work, we produce an on-line catalog
containing periods, amplitudes, and classifications for ~61,000 periodic
variables. By cross-matching these variables with those from prior surveys, we
find that > 90% of the ~8,000 known periodic variables in the survey region are
recovered. For these sources we find excellent agreement between our catalog
and prior values of luminosity, period and amplitude, as well as
classification.
We investigate the rate of confusion between objects classified as contact
binaries and type-c RR Lyrae (RRc's) based on periods, colours, amplitudes,
metalicities, radial velocities and surface gravities. We find that no more
than few percent of these variables in these classes are misidentified. By
deriving distances for this clean sample of ~5,500 RRc's, we trace the path of
the Sagittarius tidal streams within the Galactic halo. Selecting 146
outer-halo RRc's with SDSS radial velocities, we confirm the presence of a
coherent halo structure that is inconsistent with current N-body simulations of
the Sagittarius tidal stream. We also find numerous long-period variables that
are very likely associated within the Sagittarius tidal streams system.
Based on the examination of 31,000 contact binary light curves we find
evidence for two subgroups exhibiting irregular lightcurves. One subgroup
presents significant variations in mean brightness that are likely due to
chromospheric activity. The other subgroup shows stable modulations over more
than a thousand days and thereby provides evidence that the O'Connell effect is
not due to stellar spots.Comment: Accepted ApJS, 43 pages, 9 tables, 44 figures (some at reduced
resolution
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