62 research outputs found
Photometric monitoring of the doubly imaged quasar UM673: possible evidence for chromatic microlensing
We present the results of two-band CCD photometric monitoring of the
gravitationally lensed quasar Q 0142-100 (UM 673).The data, obtained at ESO-La
Silla with the 1.54 m Danish telescope in the Gunn i-band (October 1998 -
September 1999) and in the Johnson V-band (October 1998 to December 2001), were
analyzed using three different photometric methods. The light-curves obtained
with all methods show variations, with a peak-to-peak amplitude of 0.14
magnitude in . Although it was not possible to measure the time delay
between the two lensed QSO images, the brighter component displays possible
evidence for microlensing: it becomes bluer as it gets brighter, as expected
under the assumption of differential magnification of a quasar accretion diskComment: Accepted for publication in Astronomy & Astrophysics; 8 pages, 7
figure
Time delay between images of the lensed quasar UM673
We study brightness variations in the double lensed quasar UM673 (Q0142-100)
with the aim of measuring the time delay between its two images. In the paper
we combine our previously published observational data of UM673 obtained during
the 2003 - 2005 seasons at the Maidanak Observatory with archival and recently
observed Maidanak and CTIO UM673 data. We analyze the V, R and I-band light
curves of the A and B images of UM673, which cover ten observational seasons
from August 2001 to November 2010. We also analyze the time evolution of the
difference in magnitudes between images A and B of UM673 over more than ten
years. We find that the quasar exhibits both short-term (with amplitude of \sim
0.1 mag in the R band) and high-amplitude (\sim 0.3 mag) long-term variability
on timescales of about several months and several years, respectively. These
brightness variations are used to constrain the time delay between the images
of UM673. From cross-correlation analysis of the A and B quasar light curves
and error analysis we measure the mean time delay and its error of 89 \pm11
days. Given the input time delay of 88 days, the most probable value of the
delay that can be recovered from light curves with the same statistical
properties as the observed R-band light curves of UM673 is 95{+5/-16}{+14/-29}
days (68 and 95 % confidence intervals). Analysis of the V - I color variations
and V, R and I-band magnitude differences of the quasar images does not show
clear evidence of the microlensing variations between 1998 and 2010.Comment: Submitted to A&A, 11 pages, 9 figure
<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 â a realisation of the Tycho-Gaia Astrometric Solution (TGAS) â 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 âŒ3000 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 yrâ1 for the proper motions. A systematic component of âŒ0.3 mas should be added to the parallax uncertainties. For the subset of âŒ94 000 HIPPARCOS stars in the primary data set, the proper motions are much more precise at about 0.06 mas yrâ1. For the secondary astrometric data set, the typical uncertainty of the positions is âŒ10 mas. The median uncertainties on the mean G-band magnitudes range from the mmag level to âŒ0.03 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
Metallicity of M dwarfs II. A comparative study of photometric metallicity scales
Stellar parameters are not easily derived from M dwarf spectra, which are
dominated by complex bands of diatomic and triatomic molecules and not well
described at the line by line level by atmospheric models. M dwarf
metallicities are therefore most commonly derived through less direct
techniques. Several recent publications propose calibrations that provide the
metallicity of an M dwarf from its Ks band absolute magnitude and its V-Ks
color, but disagree at the \pm0.1 dex level. We compare these calibrations on a
sample of 23 M dwarfs, which we select as wide (> 5 arcsec) companions of F-,
G- or K- dwarfs with metallicities measured on a homogeneous scale, and which
we require to have V band photometry measured to better than \sim0.03
magnitude. We find that the Schlaufman & Laughlin (2010) calibration has lowest
offsets and residuals against our sample, and use our improved statistics to
marginally refine that calibration. With more strictly selected photometry than
in previous studies, the dispersion around the calibration is well in excess of
the [Fe/H] and photometric uncertainties. This suggests that the origin of the
remaining dispersion is astrophysical rather than observational.Comment: paper accepted in A&A, 12 pages, 3 figures; A&A REF: AA/2011/1811
Properties of planets in binary systems. The role of binary separation
The statistical properties of planets in binaries were investigated. Any
difference to planets orbiting single stars can shed light on the formation and
evolution of planetary systems. As planets were found around components of
binaries with very different separation and mass ratio, it is particularly
important to study the characteristics of planets as a function of the
effective gravitational influence of the companion. A compilation of planets in
binary systems was made; a search for companions orbiting stars recently shown
to host planets was performed, resulting in the addition of two further binary
planet hosts (HD 20782 and HD 109749). The probable original properties of the
three binary planet hosts with white dwarfs companions were also investigated.
Using this updated sample of planets in binaries we performed a statistical
analysis of the distributions of planet mass, period, and eccentricity,
fraction of multiplanet systems, and stellar metallicity for planets orbiting
components of tight and wide binaries and single stars. The only highly
significant difference revealed by our analysis concerns the mass distribution
of short-period planets. Massive planets in short period orbits are found in
most cases around the components of rather tight binaries. The properties of
exoplanets orbiting the components of wide binaries are compatible with those
of planets orbiting single stars, except for a possible greater abundance of
high-eccentricity planets. The previously suggested lack of massive planets
with P>100 days in binaries is not confirmed. We conclude that the presence of
a stellar companion with separation smaller than 100-300 AU is able to modify
the formation and/or migration and/or the dynamical evolution history of giant
planets while wide companions play a more limited roleComment: A&A, in pres
Gaia Data Release 1: Testing parallaxes with local Cepheids and RR Lyrae stars
Context. Parallaxes for 331 classical Cepheids, 31 Type II Cepheids, and 364 RR Lyrae stars in common between Gaia and the Hipparcos and Tycho-2 catalogues are published in Gaia Data Release 1 (DR1) as part of the Tycho-Gaia Astrometric Solution (TGAS). Aims. In order to test these first parallax measurements of the primary standard candles of the cosmological distance ladder, which involve astrometry collected by Gaia during the initial 14 months of science operation, we compared them with literature estimates and derived new period-luminosity (PL), period-Wesenheit (PW) relations for classical and Type II Cepheids and infrared PL, PL-metallicity (PLZ), and optical luminosity-metallicity (M V -[Fe/H]) relations for the RR Lyrae stars, with zero points based on TGAS. Methods. Classical Cepheids were carefully selected in order to discard known or suspected binary systems. The final sample comprises 102 fundamental mode pulsators with periods ranging from 1.68 to 51.66 days (of which 33 with Ï Î© /Ω < 0.5). The Type II Cepheids include a total of 26 W Virginis and BL Herculis stars spanning the period range from 1.16 to 30.00 days (of which only 7 with Ï Î© /Ω < 0.5). The RR Lyrae stars include 200 sources with pulsation period ranging from 0.27 to 0.80 days (of which 112 with Ï Î© /Ω < 0.5). The new relations were computed using multi-band (V,I,J,K s ) photometry and spectroscopic metal abundances available in the literature, and by applying three alternative approaches: (i) linear least-squares fitting of the absolute magnitudes inferred from direct transformation of the TGAS parallaxes; (ii) adopting astrometry-based luminosities; and (iii) using a Bayesian fitting approach. The last two methods work in parallax space where parallaxes are used directly, thus maintaining symmetrical errors and allowing negative parallaxes to be used. The TGAS-based PL,PW,PLZ, and M V - [Fe/H] relations are discussed by comparing the distance to the Large Magellanic Cloud provided by different types of pulsating stars and alternative fitting methods. Results. Good agreement is found from direct comparison of the parallaxes of RR Lyrae stars for which both TGAS and HST measurements are available. Similarly, very good agreement is found between the TGAS values and the parallaxes inferred from the absolute magnitudes of Cepheids and RR Lyrae stars analysed with the Baade-Wesselink method. TGAS values also compare favourably with the parallaxes inferred by theoretical model fitting of the multi-band light curves for two of the three classical Cepheids and one RR Lyrae star, which were analysed with this technique in our samples. The K-band PL relations show the significant improvement of the TGAS parallaxes for Cepheids and RR Lyrae stars with respect to the Hipparcos measurements. This is particularly true for the RR Lyrae stars for which improvement in quality and statistics is impressive. Conclusions. TGAS parallaxes bring a significant added value to the previous Hipparcos estimates. The relations presented in this paper represent the first Gaia-calibrated relations and form a work-in-progress milestone report in the wait for Gaia-only parallaxes of which a first solution will become available with Gaia Data Release 2 (DR2) in 2018. © ESO, 2017
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