131 research outputs found
The LSST AGN Data Challenge: Selection methods
Development of the Rubin Observatory Legacy Survey of Space and Time (LSST)
includes a series of Data Challenges (DC) arranged by various LSST Scientific
Collaborations (SC) that are taking place during the projects preoperational
phase. The AGN Science Collaboration Data Challenge (AGNSCDC) is a partial
prototype of the expected LSST AGN data, aimed at validating machine learning
approaches for AGN selection and characterization in large surveys like LSST.
The AGNSC-DC took part in 2021 focusing on accuracy, robustness, and
scalability. The training and the blinded datasets were constructed to mimic
the future LSST release catalogs using the data from the Sloan Digital Sky
Survey Stripe 82 region and the XMM-Newton Large Scale Structure Survey region.
Data features were divided into astrometry, photometry, color, morphology,
redshift and class label with the addition of variability features and images.
We present the results of four DC submitted solutions using both classical and
machine learning methods. We systematically test the performance of supervised
(support vector machine, random forest, extreme gradient boosting, artificial
neural network, convolutional neural network) and unsupervised (deep embedding
clustering) models when applied to the problem of classifying/clustering
sources as stars, galaxies or AGNs. We obtained classification accuracy 97.5%
for supervised and clustering accuracy 96.0% for unsupervised models and 95.0%
with a classic approach for a blinded dataset. We find that variability
features significantly improve the accuracy of the trained models and
correlation analysis among different bands enables a fast and inexpensive first
order selection of quasar candidatesComment: Accepted by ApJ. 21 pages, 14 figures, 5 table
A binary engine fuelling HD87643' s complex circumstellar environment, using AMBER/VLTI
International audienceContext. The star HD 87643, exhibiting the “B[e] phenomenon”, has one of the most extreme infrared excesses for this object class. It harbours a large amount of both hot and cold dust, and is surrounded by an extended reflection nebula. Aims. One of our major goals was to investigate the presence of a companion in HD87643. In addition, the presence of close dusty material was tested through a combination of multi-wavelength high spatial resolution observations. Methods. We observed HD 87643 with high spatial resolution techniques, using the near-IR AMBER/VLTI interferometer with baselines ranging from 60 m to 130 m and the mid-IR MIDI/VLTI interferometer with baselines ranging from 25 m to 65 m. These observations are complemented by NACO/VLT adaptive-optics-corrected images in the K and L-bands, ESO-2.2m optical Wide-Field Imager large-scale images in the B, V and R-bands, Results. We report the direct detection of a companion to HD 87643 by means of image synthesis using the AMBER/VLTI instrument. The presence of the companion is confirmed by the MIDI and NACO data, although with a lower confidence. The companion is separated by ∼ 34 mas with a roughly north-south orientation. The period must be large (several tens of years) and hence the orbital parameters are not determined yet. Binarity with high eccentricity might be the key to interpreting the extreme characteristics of this system, namely a dusty circumstellar envelope around the primary, a compact dust nebulosity around the binary system and a complex extended nebula witnessing past violent ejections
A spectroscopic study of southern (candidate) gamma Doradus stars. I. Time series analysis
We present results of a spectroscopic study of 37 southern (candidate) gamma
Doradus stars based on echelle spectra. The observed spectra were
cross-correlated with the standard template spectrum of an F0-type star for an
easier detection of binary and intrinsic variations. We identified 15 objects
as spectroscopic binaries, including 7 new ones, and another 3 objects are
binary suspects. At least 12 objects show composite spectra. We could determine
the orbital parameters for 9 binaries, of which 4 turn out to be ellipsoidal
variables. For 6 binaries, we estimated the expected time-base of the orbital
variations. Clear profile variations are observed for 17 objects, pointing
towards stellar pulsation. For 8 of them, we have evidence that the main
spectroscopic and photometric periods coincide. Our results, in combination
with prior knowledge from the literature, lead to the classification of 10
objects as new bona-fide gamma Doradus stars, 1 object as new bona-fide delta
Scuti star, and 8 objects as constant stars. Finally, we determined the
projected rotational velocity with two independent methods. The resulting vsini
values range from 3 to 135 km/s. For the bona-fide gamma Doradus stars, the
majority has vsini below 60 km/s.Comment: 13 pages (+ 10 pages online material), 10 (+16) figures. Accepted for
publication by A&
Long-term photometric monitoring with the Mercator telescope. Frequencies and multicolour amplitudes of Doradus stars
Gamma Doradus stars are excellent targets for asteroseismology since the
gravity modes present in these stars probe the deep stellar interiors. Mode
identification will improve the knowledge of these stars considerably. A
selected group of Gamma Doradus stars and some candidates were observed with
the Mercator telescope to find and/or confirm the periodicities in the light
variations and to derive reliable amplitude ratios in different pass bands. A
frequency analysis was performed on all new data obtained in the Geneva
photometric system. In order to have more reliable and accurate frequencies,
the new data were combined with similar data from the literature and with
Hipparcos observations. A set of frequencies that minimized the the residuals
in a harmonic fit was searched for while allowing means and amplitudes to vary
from one observation set to another. Frequencies and amplitudes in the
photometric passbands of the Geneva system are given for 21 Gamma Doradus
stars. We report the discovery of HD 74504 as a newly found Gamma Doradus star.
Our study provides the first extensive multicolour database for the
understanding of gravity modes in F-type stars.Comment: Electronic tables (5 to 25) not included in this versio
MATISSE, perspective of imaging in the mid-infrared at the VLTI
International audienceMATISSE is foreseen as a mid-infrared spectro-interferometer combining the beams of up to four UTs/ATs of the Very Large Telescope Interferometer (VLTI) of the European Southern Observatory. The related science case study demonstrates the enormous capability of a new generation mid-infrared beam combiner. MATISSE will constitute an evolution of the two-beam interferometric instrument MIDI. MIDI is a very successful instrument which offers a perfect combination of spectral and angular resolution. New characteristics present in MATISSE will give access to the mapping and the distribution of the material (typically dust) in the circumstellar environments by using a wide mid-infrared band coverage extended to L, M and N spectral bands. The four beam combination of MATISSE provides an efficient UV-coverage : 6 visibility points are measured in one set and 4 closure phase relations which can provide aperture synthesis images in the mid-infrared spectral regime
Investigating the spectroscopic, magnetic and circumstellar variability of the O9 subgiant star HD 57682
The O9IV star HD 57682, discovered to be magnetic within the context of the
MiMeS survey in 2009, is one of only eight convincingly detected magnetic
O-type stars. Among this select group, it stands out due to its sharp-lined
photospheric spectrum. Since its discovery, the MiMeS Collaboration has
continued to obtain spectroscopic and magnetic observations in order to refine
our knowledge of its magnetic field strength and geometry, rotational period,
and spectral properties and variability. In this paper we report new ESPaDOnS
spectropolarimetric observations of HD 57682, which are combined with
previously published ESPaDOnS data and archival H{\alpha} spectroscopy. This
dataset is used to determine the rotational period (63.5708 \pm 0.0057 d),
refine the longitudinal magnetic field variation and magnetic geometry (dipole
surface field strength of 880\pm50 G and magnetic obliquity of 79\pm4\circ as
measured from the magnetic longitudinal field variations, assuming an
inclination of 60\circ), and examine the phase variation of various lines. In
particular, we demonstrate that the H{\alpha} equivalent width undergoes a
double-wave variation during a single rotation of the star, consistent with the
derived magnetic geometry. We group the variable lines into two classes: those
that, like H{\alpha}, exhibit non-sinusoidal variability, often with multiple
maxima during the rotation cycle, and those that vary essentially sinusoidally.
Based on our modelling of the H{\alpha} emission, we show that the variability
is consistent with emission being generated from an optically thick, flattened
distribution of magnetically-confined plasma that is roughly distributed about
the magnetic equator. Finally, we discuss our findings in the magnetospheric
framework proposed in our earlier study.Comment: 21 pages, 19 figures, Accepted for publication in MNRA
An asymmetry detected in the disk of Kappa CMa with the AMBER/VLTI
International audienceAims. We study the geometry and kinematics of the circumstellar environment of the Be star Kappa CMa in the Br gamma emission line and its nearby continuum. Methods. We use the VLTI/AMBER instrument operating in the K band which provides a spatial resolution of about 6 mas with a spectral resolution of 1500 to study the kinematics within the disk and to infer its rotation law. In order to obtain more kinematical constraints we also use an high spectral resolution Pa beta line profile obtain in December 2005 at the Observatorio do Pico do Dios, Brazil and we compile V/R line profile variations and spectral energy distribution data points from the literature. Results. Using differential visibilities and differential phases across the Br gamma line we detect an asymmetry in the disk. Moreover, we found that kappa CMa seems difficult to fit within the classical scenario for Be stars, illustrated recently by alpha Arae observations, i.e. a fast rotating B star close to its breakup velocity surrounded by a Keplerian circumstellar disk with an enhanced polar wind. Finally we discuss the possibility for kappa CMa to be a critical rotator with a Keplerian rotating disk and try to see if the detected asymmetry can be interpreted within the "one-armed" viscous disk framework
The LSST Era of Supermassive Black Hole Accretion Disk Reverberation Mapping
peer reviewedThe Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST) will detect an unprecedentedly large sample of actively accreting supermassive black holes with typical accretion disk (AD) sizes of a few light days. This brings us to face challenges in the reverberation mapping (RM) measurement of AD sizes in active galactic nuclei using interband continuum delays. We examine the effect of LSST cadence strategies on AD RM using our metric AGN_TimeLagMetric. It accounts for redshift, cadence, the magnitude limit, and magnitude corrections for dust extinction. Running our metric on different LSST cadence strategies, we produce an atlas of the performance estimations for LSST photometric RM measurements. We provide an upper limit on the estimated number of quasars for which the AD time lag can be computed within 0 1000 sources in each deep drilling field (DDF; (10 deg2)) in any filter, with the redshift distribution of these sources peaking at z ≍ 1. We find the LSST observation strategies with a good cadence (≲5 days) and a long cumulative season (~9 yr), as proposed for LSST DDF, are favored for the AD size measurement. We create synthetic LSST light curves for the most suitable DDF cadences and determine RM time lags to demonstrate the impact of the best cadences based on the proposed metric
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