arXiv subjects

C. A. L. Bailer-Jones

Publications and source records attributed to C. A. L. Bailer-Jones.

At least 109 records · Page 6Linked to original sources

arXiv2007

Estimation of stellar atmospheric parameters from SDSS/SEGUE spectra

We present techniques for the estimation of stellar atmospheric parameters (Teff,logg,[Fe/H]) for stars from the SDSS/SEGUE survey. The atmospheric parameters are derived from the observed medium-resolution (R=2000) stellar spectra using non-linear regression models trained either on (1) pre-classified observed data or (2) synthetic stellar spectra. In the first case we use our models to automate and generalize parametrization produced by a preliminary version of the SDSS/SEGUE Spectroscopic Parameter Pipeline (SSPP). In the second case we directly model the mapping between synthetic spectra (derived from Kurucz model atmospheres) and the atmospheric parameters, independently of any intermediate estimates. After training, we apply our models to various samples of SDSS spectra to derive atmospheric parameters, and compare our results with those obtained previously by the SSPP for the same samples. We obtain consistency between the two approaches, with RMS deviations of 150K in Teff, 0.35dex in logg, and 0.22dex in [Fe/H]. The models are applied to pre-processed spectra, either via Principal Components Analysis or a Wavelength Range Selection method, which employs a subset of the full 3850-9000A spectral range. This is both for computational reasons, and because it delivers higher accuracy. From an analysis of cluster candidates with available SDSS spectroscopy (M15, M13, M2, and NGC2420), we find evidence for small systematic offsets in Teff and/or logg for the atmospheric parameter estimates from the model trained on real data with the SSPP. Thus, this model turns out to derive more precise, but less accurate, atmospheric parameters than the model trained on synthetic data.

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arXiv2006

Pleiades low-mass brown dwarfs: the cluster L dwarf sequence

We present a search for low-mass brown dwarfs in the Pleiades open cluster. The identification of Pleiades members fainter and cooler than those currently known allows us to constrain evolutionary models for L dwarfs and to extend the study of the cluster mass function to lower masses. We conducted a 1.8 deg^2 near-infrared J-band survey at the 3.5m Calar Alto Telescope, with completeness J~19.0. The detected sources were correlated with those of previously available optical I-band images (completeness I~22). Using a J versus I-J colour-magnitude diagram, we identified 18 faint red L-type candidates, with magnitudes 17.4 3.2. If Pleiades members, their masses would span ~0.040-0.020 M_Sol. We performed follow-up HKs-band imaging to further confirm their cluster membership by photometry and proper motion. Out of 11 IJ candidates with proper motion measurements, we find six cluster members, two non-members and three whose membership is uncertain and depends on the intrinsic velocity dispersion of Pleiades brown dwarfs. This dispersion (>4 mas yr^-1) is at least four times that of cluster stars with masses >1 M_Sol. Five of the seven other IJ candidates are discarded because their J-Ks colours are bluer than those of confirmed members. The J versus I-J sequence of the L-type candidates at J>18 is not as red as theoretical models predict; it rather follows the field L-dwarf sequence translated to the cluster distance. This sequence overlapping, also observed in the J versus J-H and J-K diagrams, suggests that Pleiades and field L dwarfs may have similar spectral energy distributions and luminosities, and thus possibly similar radii. Also, we find alpha=0.5+-0.2 for a power-law approximation dN/dM propor. M^-alpha of the survey mass spectrum in the mass range 0.5-0.026 M_Sol.

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arXiv2005

The design and performance of the Gaia photometric system

The European Gaia astrometry mission is due for launch in 2011. Gaia will rely on the proven principles of ESA's Hipparcos mission to create an all-sky survey of about one billion stars throughout our Galaxy and beyond, by observing all objects down to 20th magnitude. Through its massive measurement of stellar distances, motions and multi-colour photometry it will provide fundamental data necessary for unravelling the structure, formation and evolution of the Galaxy. This paper presents the design and performance of the broad- and medium-band set of photometric filters adopted as the baseline for Gaia. The nineteen selected passbands (extending from the ultraviolet to the far-red), the criteria, and the methodology on which this choice has been based are discussed in detail. We analyse the photometric capabilities for characterizing the luminosity, temperature, gravity and chemical composition of stars. We also discuss the automatic determination of these physical parameters for the large number of observations involved, for objects located throughout the entire Hertzsprung-Russell diagram. Finally, the capability of the photometric system to deal with the main Gaia science case is outlined.

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arXiv2005

Proper motion Pleiades candidate L-type brown dwarfs

We present results of an optical and near-infrared (IR) 1.8 deg^2 survey in the Pleiades open cluster to search for substellar objects. From optical I-band images from the CFHT and J-band images from the 3.5 m CAHA Telescope, we identify 18 faint and very red L brown dwarf candidates, with I> 20.9 and I-J> 3.2. The follow-up observations of nine objects in the H- and Ks-bands confirm that eight belong to the IR sequence of the cluster and the proper motion measurements of seven candidates confirm that they are Pleiades members. A preliminary estimation of the substellar mass spectrum dN/dM in the form of a power law M^-alpha provides alpha=0.57+-0.14. We extrapolate this function to estimate the number of very low-mass brown dwarfs and planetary mass objects that could be present in the cluster down to 1 M_Jup. Sensitive searches combining far red and near infrared observations may unveal these objects in a near future.

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arXiv2005

The Disappearing Act of KH 15D: Photometric Results from 1995 to 2004

We present results from the most recent (2002-2004) observing campaigns of the eclipsing system KH 15D, in addition to re-reduced data obtained at Van Vleck Observatory (VVO) between 1995 and 2000. Phasing nine years of photometric data shows substantial evolution in the width and depth of the eclipses. The most recent data indicate that the eclipses are now approximately 24 days in length, or half the orbital period. These results are interpreted and discussed in the context of the recent models for this system put forward by Winn et al. and Chiang & Murray-Clay. A periodogram of the entire data set yields a highly significant peak at 48.37 +/- 0.01 days, which is in accord with the spectroscopic period of 48.38 +/- 0.01 days determined by Johnson et al. Another significant peak, at 9.6 days, was found in the periodogram of the out-of-eclipse data at two different epochs. We interpret this as the rotation period of the visible star and argue that it may be tidally locked in pseudosynchronism with its orbital motion. If so, application of Hut's theory implies that the eccentricity of the orbit is e = 0.65 +/- 0.01. Analysis of the UVES/VLT spectra obtained by Hamilton et al. shows that the v sin(i) of the visible star in this system is 6.9 +/- 0.3 km/sec. Using this value of v sin(i) and the measured rotation period of the star, we calculate the lower limit on the radius to be R = (1.3 +/- 0.1), R_Sun, which concurs with the value obtained by Hamilton et al. from its luminosity and effective temperature. Here we assume that i = 90 degrees since it is likely that the spin and orbital angular momenta vectors are nearly aligned.

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arXiv2005

Analysis of medium resolution spectra by automated methods - application to M55 and omega Centauri

We have employed feedforward neural networks trained on synthetic spectra in the range 3800 to 5600 AA with resolutions of 2-3 AA to determine metallicities from spectra of about 1000 main-sequence turn-off, subgiant and red giant stars in the globular clusters M55 and omega Cen. The overall metallicity accuracies are of the order of 0.15 to 0.2 dex. In addition, we tested how well the stellar parameters logg and Teff can be retrieved from such data without additional colour or photometric information. We find overall uncertainties of 0.3 to 0.4 dex for logg and 140 to 190 K for Teff. In order to obtain some measure of uncertainty for the determined values of [Fe/H], logg and Teff, we applied the bootstrap method for the first time to neural networks for this kind of parametrization problem. The distribution of metallicities for stars in omega Cen clearly shows a large spread in agreement with the well known multiple stellar populations in this cluster.

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arXiv2005

Object classification and the determination of stellar parameters with Gaia

Gaia will observe more than one billion objects brighter than V=20, including stars, asteroids, galaxies and quasars. As Gaia performs real time detection (i.e. without an input catalogue) the intrinsic properties of most of these objects will not be known a priori. An integral part of the Gaia data processing is therefore to classify everything observed. This will be based primarily on multiband photometry provided by Gaia, but should also make optimal use of the high resolution spectroscopy (for brighter stars) and the parallaxes. In addition to a broad classification, we can also determine fundamental stellar parameters, in particular effective temperature, metallicity and the line-of-sight interstellar extinction. Such information will be essential for fully exploiting the astrometric part of the Gaia catalogue for stellar population studies. However, extracting this information is a significant challenge, and will need to make use of appropriate multidimensional data analysis techniques. I outline some of the problems and the strategies being developed to tackle them.

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arXiv2005

Design of the Gaia photometric systems for stellar parametrization using a population-based optimizer

Large, deep surveys must typically rely on multiband photometry rather than spectroscopy for determining the astrophysical properties (APs) of stars. Yet designing an optimal photometric system for a wide range of objects is complex, because it must trade off conflicting scientific requirements. I present a new method for designing photometric systems and apply it to the Gaia Galactic survey satellite, which will observe one billion stars brighter than V=20. The principle is to optimally sample stellar spectra in order to best determine the APs (e.g. Teff, [Fe/H], interstellar extinction). By considering a filter system as a set of free parameters (central wavelengths, FWHM etc.), it may be designed by optimizing a figure-of-merit (FoM) with respect to these parameters. The FoM is a measure of how well the filter system can vectorially 'separate' stars in the data space to avoid AP degeneracies. The resulting systems show some interesting features, in particular broad, overlapping filters, which may be desirable from a multivariate classification perspective. These systems are competitive with others proposed for Gaia.

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arXiv2004

Microarcsecond astrometry with Gaia: the solar system, the Galaxy and beyond

Gaia is an all sky, high precision astrometric and photometric satellite of the European Space Agency (ESA) due for launch in 2010-2011. Its primary mission is to study the composition, formation and evolution of our Galaxy. Gaia will measure parallaxes and proper motions of every object in the sky brighter than V=20, amounting to a billion stars, galaxies, quasars and solar system objects. It will achieve an astrometric accuracy of 10muas at V=15 - corresponding to a distance accuracy of 1% at 1kpc. With Gaia, tens of millions of stars will have their distances measured to a few percent or better. This is an improvement over Hipparcos by several orders of magnitude in the number of objects, accuracy and limiting magnitude. Gaia will also measure radial velocities for source brighter than V~17. To characterize the objects, each object is observed in 15 medium and broad photometric bands with an onboard CCD camera. With these capabilities, Gaia will make significant advances in a wide range of astrophysical topics. These include a detailed kinematical map of stellar populations, stellar structure and evolution, the discovery and characterization of thousands of exoplanetary systems and General Relativity on large scales. I give an overview of the mission, its operating principles and its expected scientific contributions. For the latter I provide a quick look in five areas on increasing scale size in the universe: the solar system, exosolar planets, stellar clusters and associations, Galactic structure and extragalactic astronomy.

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arXiv2004

Variability and rotation of ultra cool dwarfs

Over the past few years monitoring programs have shown ultra cool dwarfs (UCDs) to be photometrically variable. Of the 60 sources monitored in the field and some 120 monitored in clusters, about 40% show variability in both cases. For mid to late M dwarfs in young (<100 Myr) clusters, this variability is generally periodic with amplitudes of up to a few tenths of a magnitude and periods of between a few hours and several days. For older field dwarfs (covering late M, L and T types) this variability is often nonperiodic with smaller amplitudes (up to 0.1 mag in I) and timescales of order hours. The former may be attributed to the rotational modulation of magnetically-induced photospheric spots, as seen in higher mass T Tauri stars. The nonperiodic variability, on the other hand, may be caused by a rapid evolution of surface features (which 'mask' the otherwise observable rotational modulation). This could be related to the formation and dissipation of inhomogeneities in dust clouds in the photospheres of UCDs.

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arXiv2004

Spectroscopic rotation velocities of L dwarfs from VLT/UVES and their comparison with periods from photometric monitoring

The variability and rotation of ultra cool dwarfs (UCDs) provide important information on their atmospheres and evolution. As part of an ongoing program to investigate this, projected rotation speeds, vsini, are presented for 16 field UCDs (M9V-L7.5V). This doubles the number of L dwarfs for which vsini has been measured. All targets are found to have vsini between 10 and 40 km/s, confirming that L dwarfs are rapid rotators. From the random distribution of the rotation axes, i, one-sided confidence intervals are placed on the rotation periods of individual object. These are compared with published period data obtained from photometric monitoring. From this, the period of 31 hrs for the L0.5 dwarf 2M0746+2000 published by Gelino et al. (2002) may be ruled out as the rotation period. The period of 11.2 +/- 0.8 hrs for the L1.5 dwarf 2M1145+2317 obtained by Bailer-Jones & Mundt (2001) is plausibly the rotation period. The inclination of the rotation axis is i = 62-90 deg with an expectation value of 76 deg. Alternatively the data set a lower limit on the radius of 0.1 Rsol. Similarly, the period of 2.7 +/- 0.1 hrs for 2M1334+1940 is the likely rotation period; the inclination is i = 27-44 deg (mean i = 34 deg). Where no variability or period was detected by the monitoring programs the likely reason is low contrast surface features. However, in three cases variability but no period was detected, despite sensitivity to the likely rotation periods inferred from vsini. This reinforces the `masking hypothesis', the idea that the evolution of photospheric features obscures regular modulation of the light curve. As has been previously discussed, a likely candidate for such features is inhomogeneous dust clouds.

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arXiv2004

Evolutionary design of photometric systems and its application to Gaia

Designing a photometric system to best fulfil a set of scientific goals is a complex task, demanding a compromise between conflicting requirements and subject to various constraints. A specific example is the determination of stellar astrophysical parameters (APs) - effective temperature, metallicity etc. - across a wide range of stellar types. I present a novel approach to this problem which makes minimal assumptions about the required filter system. By considering a filter system as a set of free parameters it may be designed by optimizing some figure-of-merit (FoM) with respect to these parameters. In the example considered, the FoM is a measure of how well the filter system can `separate' stars with different APs. This separation is vectorial in nature, in the sense that the local directions of AP variance are preferably mutually orthogonal to avoid AP degeneracy. The optimization is carried out with an evolutionary algorithm, which uses principles of evolutionary biology to search the parameter space. This model, HFD (Heuristic Filter Design), is applied to the design of photometric systems for the Gaia space astrometry mission. The optimized systems show a number of interesting features, not least the persistence of broad, overlapping filters. These HFD systems perform as least as well as other proposed systems for Gaia, although inadequacies remain in all. The principles underlying HFD are quite generic and may be applied to filter design for numerous other projects, such as the search for specific types of objects or photometric redshift determination.

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arXiv2004

A rotational and variability study of a large sample of PMS stars in NGC 2264

We present the results of an extensive search for periodic and irregular variable pre-main sequence (PMS) stars in the young (2-4 Myr) open cluster NGC 2664, based on photometric monitoring using the Wide Field Imager (WFI) on the 2.2m telescope on La Silla (Chile). In total, about 10600 stars with I_c magnitudes between 9.8mag and 21mag have been monitored in our 34x33 arcmin field. Time series data were obtained in the I_c band in 44 nights between Dec. 2000 and March 2001; altogether we obtained 88 data points per star. Using two different time series analysis techniques (Scargle and CLEAN) we found 543 periodically variable stars with periods between 0.2 days and 15 days. Also, 484 irregular variable stars were identified using a chi-squared-test. In addition we have carried out nearly simultaneous observations in V, R_c and a narrow-band Halpha filter. The photometric data enable us to reject background and foreground stars from our sample of variable stars according to their location in the I_c vs. (R_c-I_c) colour-magnitude and (R_c-Halpha) vs. (R_c-I_c) colour-colour diagram. We could in the end identify 405 periodically variable and 184 irregular variable PMS stars as cluster members using these two different tests. In addition 35 PMS stars for which no significant variablilty were detected could be identified as members using an Halpha emission index criterion. This yields to a total of 624 PMS stars in NGC 2264, of which only 182 were known before. Most of the newly found PMS stars are fainter than I_c = 16 and of late spectral type (>M2). (abridged)

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arXiv2003

Automated Determination of Stellar Parameters from Simulated Dispersed Images for DIVA

We have assessed how well stellar parameters (T_eff, logg and [Fe/H]) can be retrieved from low-resolution dispersed images to be obtained by the DIVA satellite. Although DIVA is primarily an all-sky astrometric mission, it will also obtain spectrophotometric information for about 13 million stars (operational limiting magnitude V ~ 13.5 mag). Constructional studies foresee a grating system yielding a dispersion of ~200nm/mm on the focal plane (first spectral order). For astrometric reasons there will be no cross dispersion which results in the overlapping of the first to third diffraction orders. The one-dimensional, position related intensity function is called a DISPI (DISPersed Intensity). We simulated DISPIS from synthetic spectra (...) for a limited range of metallicites i.e. our results are for [Fe/H] in the range -0.3 to 1 dex. We show that there is no need to deconvolve these low resolution signals in order to obtain basic stellar parameters. Using neural network methods and by including simulated data of DIVA's UV telescope, we can determine T_eff to an average accuracy of about 2% for DISPIS from stars with 2000 K < T_eff < 20000 K and visual magnitudes of V=13 mag (end of mission data). logg can be determined for all temperatures with an accuracy better than 0.25 dex for magnitudes brighter than V=12 mag. For low temperature stars with 2000 K < T_eff < 5000 K and for metallicities in the range -0.3 to +1 dex a determination of [Fe/H] is possible (to better than 0.2 dex) for these magnitudes. Additionally we examined the effects of extinction E(B-V) on DISPIS and found that it can be determined to better than 0.07 mag for magnitudes brighter than V=14 mag if the UV information is included.

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arXiv2002

Limits on the infrared photometric monitoring of brown dwarfs

Recent monitoring programs of ultra cool field M and L dwarfs (low mass stars or brown dwarfs) have uncovered low amplitude photometric I-band variations which may be associated with an inhomogeneous distribution of photospheric condensates. Further evidence hints that this distribution may evolve on very short timescales, specifically of order a rotation period or less. In an attempt to study this behaviour in more detail, we have carried out a pilot program to monitor three L dwarfs in the near infrared where these objects are significantly brighter than at shorter wavelengths. We present a robust data analysis method for improving the precision and reliability of infrared photometry. No significant variability was detected in either the J or Km bands in 2M1439 and SDSS1203 above a peak-to-peak amplitude of 0.04 mag (0.08 mag for 2M1112). The main limiting factor in achieving lower detection limits is suspected to be second order extinction effects in the Earth's atmosphere, on account of the very different colours of the target and reference stars. Suggestions are given for overcoming such effects which should improve the sensitivity and reliability of infrared variability searches.

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arXiv2002

Dust clouds or magnetic spots? Exploring the atmospheres of L dwarfs with time-resolved spectrophotometry

I present the results of a program to spectrophotometrically monitor the L1.5 dwarf 2MASSW J1145572+231730 to identify the cause of photospheric variability in ultra cool dwarfs. Plausible candidates are magnetically-induced star spots and inhomogeneous photospheric dust clouds. Based on the atmospheric models and synthetic spectra of Allard et al. (2001), the expected signatures of these phenomena in the 0.5-2.5 um wavelength region are presented and discussed. Near infrared spectra of 2M1145 were obtained along with a nearby reference star observed simultaneously in the spectrograph slit. No convincing variability over a 54 hour interval is found in any one of several colour indices designed to be most sensitive to dust- and spot-related variability. Nonetheless, a significant correlation between the variability of two colour indices is found. This is slightly more consistent with the dust-related variability model than the cool spot one considered. Based on the theoretically predicted signatures and the median errors in the colour indices (0.03-0.05 magnitudes), upper limits are placed on the coverage of possible spots and clouds. Assuming the L dwarf to be best modelled by a dusty atmosphere at 1900K, coherent clear clouds are limited to a coverage of 10-15% of the projected surface area and 200K cooler spots to a 20% coverage. A larger coverage of many small features varying incoherently cannot be ruled out with this method. A lower effective temperature restricts coherent clear clouds to be much smaller; a higher temperature allows both clouds and spots to be larger. These upper limits are consistent with the two separate variability detections in the I-band reported by Bailer-Jones & Mundt (2001).

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arXiv2002

Determination of stellar parameters with GAIA

The GAIA Galactic survey satellite will obtain photometry in 15 filters of over 10^9 stars in our Galaxy across a very wide range of stellar types. No other planned survey will provide so much photometric information on so many stars. I examine the problem of how to determine fundamental physical parameters (Teff, log g, [Fe/H] etc.) from these data. Given the size, multidimensionality and diversity of this dataset, this is a challenging task beyond any encountered so far in large-scale stellar parametrization. I describe the problems faced (initial object identification, interstellar extinction, multiplicity, missing data etc.) and present a framework in which they can can be addressed. A probabilistic approach is advocated on the grounds that it can take advantage of additional information (e.g. priors and data uncertainties) in a consistent and useful manner, as well as give meaningful results in the presence of poor or degenerate data. Furthermore, I suggest an approach to parametrization which can use the other information GAIA will acquire, in particular the parallax, which has not previously been available for large-scale multidimensional parametrization. Several of the problems identified and ideas suggested will be relevant to other large surveys, such as SDSS, DIVA, FAME, VISTA and LSST, as well as stellar parametrization in a virtual observatory.

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arXiv2001

Optical spectroscopy of isolated planetary mass objects in the sigma Orionis cluster

We have obtained low resolution optical spectra of 15 isolated planetary mass objects (IPMOs) in the sigma Orionis cluster, and derived spectral types by comparison with nearby M and L dwarfs. The spectral types are in the range late M - mid L, in agreement with our expectations based on colors and magnitudes for bona fide members. Therefore, most of these objects have masses below the deuterium burning limit. About 2/3 show H(alpha) in emission at our spectral resolution. From our spectroscopic and photometric data, we infer that three IPMOs in this sample may be binaries with components of similar masses. These results confirm that the substellar mass function of the sigma Orionis cluster, in the form dN/dM, keeps rising in the planetary domain.

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