28 research outputs found
The Physical Model in Action: Quality Control for X-Shooter
The data reduction pipeline for the VLT 2nd generation instrument X-Shooter
uses a physical model to determine the optical distortion and derive the
wavelength calibration. The parameters of this model describe the positions,
orientations, and other physical properties of the optical components in the
spectrograph. They are updated by an optimisation process that ensures the best
possible fit to arc lamp line positions. ESO Quality Control monitors these
parameters along with all of the usual diagnostics. This enables us to look for
correlations between inferred physical changes in the instrument and, for
example, instrument temperature sensor readings.Comment: 9 pages, 8 figures, style files included, Proc. of Observatory
Operations: Strategies, Processes, and Systems III, eds. D.R. Silva, A.B.
Peck, B.T. Soifer, SPIE 7737, 1
Quality control and instruments monitoring for
ABSTRACT The VLTI Data Flow Operations consist in monitoring the performance of the different VLTI instruments offered to the community, in verifying the quality of the calibration and scientific data and their associated products. Since the beginning of MIDI (April 2004) and AMBER (October 2005) Service Mode Operations, scientific as well as calibration data have been accumulated to monitor the instruments and the quality of the observations on different time scales and under different conditions or system configurations. In this presentation, we will describe the Quality Control procedures and give some statistics and results on the different parameters used for instrument monitoring for time scales from hours to years in the case of MIDI. We will show that this includes parameters extracted directly from the instruments (Instrumental Transfer Function, Flux stability, Image Quality, Detector stability …) and parameters extracted from some of the sub-systems associated to the instruments (Adaptive Optics, telescopes used …). We will discuss the development of the monitoring of the instruments once more instrument modes or sub-systems such as PRIMA are offered to the community
SHOTGLAS I: The ultimate spectroscopic census of extreme horizontal branch stars in Centauri
The presence of extreme horizontal branch (EHB) and blue hook stars in some
Galactic globular clusters (GGCs) constitutes one of the remaining mysteries of
stellar evolution. In this paper, we focus on Cen, a peculiar, massive
GGC that hosts multiple stellar populations. We use non-LTE model atmospheres
to derive atmospheric parameters (Teff, log g and N(He)/N(H)) and spectroscopic
masses for 152 EHB stars in the cluster. This constitutes the largest
spectroscopic sample of EHB stars ever analyzed in a GGC and represents ~20% of
the EHB population of Cen. We also search for close binaries among
these stars based on radial velocity variations. Our results show that the EHB
population of Cen is divided into three spectroscopic groups that are
very distinct in the Teff - helium abundance plane. The majority of our sample
consists of sdOB stars that have roughly solar or super-solar atmospheric
helium abundances. It is these objects that constitute the blue hook at
18.5 mag in the Cen color-magnitude diagram. Interestingly, the
helium-enriched sdOBs do not have a significant counterpart population in the
Galactic field. Another major difference between the EHB stars in Cen
and the field is the fraction of close binaries. From our radial velocity
survey we identify two binary candidates and estimate an EHB close binary
fraction of ~5% in Cen. This low fraction is in line with findings for
other GGCs, but in sharp contrast to the situation in the field, where around
50% of the sdB stars reside in close binaries. Finally, the mass distribution
derived is very similar for all three spectroscopic groups, however the average
mass (0.38 Msun) is lower than that expected from stellar evolution theory.
While this mass conundrum has previously been noted for EHB stars in
Cen, it so far appears to be unique to that cluster.Comment: 25 pages, Accepted for publication in A&
Hot UV-bright stars of galactic globular clusters
Context. We have performed a census of the UV-bright population in 78 globular clusters using wide-field UV telescopes. This population includes a variety of phases of post-horizontal branch (HB) evolution, including hot post-asymptotic giant branch (AGB) stars, and post-early AGB stars. There are indications that old stellar systems like globular clusters produce fewer post-(early) AGB stars than currently predicted by evolutionary models, but observations are still scarce. Aims. We wish to derive effective temperatures, surface gravities, and helium abundances of the luminous hot UV-bright stars in these clusters to determine their evolutionary status and compare the observed numbers to predictions from evolutionary theory. Methods. We obtained FORS2 spectroscopy of eleven of these UV-selected objects (covering a range of -2.3†< †[Fe/H]†< †-1.0), which we (re-)analysed together with previously observed data. We used model atmospheres of different metallicities, including super-solar ones. Where possible, we verified our atmospheric parameters using UV spectrophotometry and searched for metal lines in the optical spectra. We calculated evolutionary sequences for four metallicity regimes and used them together with information about the HB morphology of the globular clusters to estimate the expected numbers of post-AGB stars. Results. We find that metal-rich model spectra are required to analyse stars hotter than 40 000 K. Seven of the eleven new luminous UV-bright stars are post-AGB or post-early AGB stars, while two are evolving away from the HB, one is a foreground white dwarf, and another is a white dwarf merger. Taking into account published information on other hot UV-bright stars in globular clusters, we find that the number of observed hot post-AGB stars generally agrees with the predicted values, although the numbers are still low. Conclusions. Spectroscopy is clearly required to identify the evolutionary status of hot UV-bright stars. For hotter stars, metal-rich model spectra are required to reproduce their optical and UV spectra, which may affect the flux contribution of hot post-AGB stars to the UV spectra of evolved populations. While the observed numbers of post-AGB and post-early AGB stars roughly agree with the predictions, our current comparison is affected by low number statistics.Fil: Moehler, Sabine. European Southern Observatory; AlemaniaFil: Landsman, W. B.. National Aeronautics and Space Administration; Estados UnidosFil: Lanz, T.. Observatoire de la Cote D'Azur; FranciaFil: Miller Bertolami, Marcelo Miguel. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Centro CientĂfico TecnolĂłgico Conicet - La Plata. Instituto de AstrofĂsica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias AstronĂłmicas y GeofĂsicas. Instituto de AstrofĂsica La Plata; Argentin