Publications (18)0 Total impact
-
A. S. Rajpurohit, C. Reyle,
M. Schultheis,
Ch. Leinert,
F. Allard,
D. Homeier,
T. Ratzka,
P. Abraham,
B. Moster,
S. Witte,
N. Ryde
[show abstract]
[hide abstract]
ABSTRACT: LHS1070 is a nearby multiple system of low mass stars. It is an important
source of information for probing the low mass end of the main sequence, down
to the hydrogen-burning limit. The primary of the system is a mid-M dwarf and
two components are late-M to early L dwarfs, at the star-brown dwarf
transition. Hence LHS1070 is a valuable object to understand the onset of dust
formation in cool stellar atmospheres.This work aims at determining the
fundamental stellar parameters of LHS1070 and to test recent model atmospheres:
BT-Dusty,BT-Settl, DRIFT, and MARCS models.Unlike in previous studies, we have
performed a chi^2-minimization comparing well calibrated optical and infrared
spectra with recent cool star synthetic spectra leading to the determination of
the physical stellar parameters Teff, radius, and log g for each of the three
components of LHS1070.
08/2012;
-
[show abstract]
[hide abstract]
ABSTRACT: The European Southern Observatory's (ESO) Visible and Infrared Survey
Telescope for Astronomy (VISTA) is a 4-m class survey telescope for wide-field
near-infrared imaging. VISTA is currently running a suite of six public
surveys, which will shortly deliver their first Europe wide public data
releases to ESO. The VISTA Kilo-degree Infrared Galaxy Survey (VIKING) forms a
natural intermediate between current wide shallow, and deeper more concentrated
surveys, by targeting two patches totalling 1500 sq.deg in the northern and
southern hemispheres with measured 5-sigma limiting depths of Z ~ 22.4, Y ~
21.4, J ~ 20.9, H ~ 19.9 and Ks ~19.3 (Vega). This architecture forms an ideal
working parameter space for the discovery of a significant sample of 6.5 <= z
<= 7.5 quasars. In the first data release priority has been placed on small
areas encompassing a number of fields well sampled at many wavelengths, thereby
optimising science gains and synergy whilst ensuring a timely release of the
first products. For rare object searches e.g. high-z quasars, this policy is
not ideal since photometric selection strategies generally evolve considerably
with the acquisition of data. Without a reasonably representative data set
sampling many directions on the sky it is not clear how a rare object search
can be conducted in a highly complete and efficient manner.
In this paper, we alleviate this problem by supplementing initial data with a
realistic model of the spatial, luminosity and colour distributions of sources
known to heavily contaminate photometric quasar selection spaces, namely dwarf
stars of spectral type M, L and T. We use this model along with a subset of
available data to investigate contamination of quasar selection space by cool
stars and galaxies and lay down a set of benchmark selection constraints that
limit contamination to reasonable levels whilst maintaining high
completeness...
11/2011;
-
[show abstract]
[hide abstract]
ABSTRACT: We present a comparison of low-resolution spectra of 60 stars covering the
whole M-dwarf sequence. Using the most recent PHOENIX BT-Settl stellar model
atmospheres (see paper by F. Allard, in this book) we do a first quantitative
compari- son to our observed spectra in the wavelength range 550-950 nm. We
perform a first confrontation between models and observations and we assign an
effective tempera- tures to the observed M-dwarfs. Teff-spectral type relations
are then compared with the published ones. This comparison also aims at
improving the models' opacities.
02/2011;
-
S. K. Leggett,
L. Albert,
E. Artigau,
Ben Burningham,
X. Delfosse,
P. Delorme,
T. Forveille,
P. W. Lucas,
M. S. Marley,
D. J. Pinfield, C. Reyle,
D. Saumon,
R. L. Smart,
S. J. Warren
[show abstract]
[hide abstract]
ABSTRACT: Mid-infrared data, including Spitzer warm-IRAC [3.6] and [4.5] photometry, is critical for understanding the cold population of brown dwarfs now being found, objects which have more in common with planets than stars. As effective temperature (T_eff) drops from 800 K to 400 K, the fraction of flux emitted beyond 3 microns increases rapidly, from about 40% to >75%. This rapid increase makes a color like H-[4.5] a very sensitive temperature indicator, and it can be combined with a gravity- and metallicity-sensitive color like H-K to constrain all three of these fundamental properties, which in turn gives us mass and age for these slowly cooling objects. Determination of mid-infrared color trends also allows better exploitation of the WISE mission by the community. We use new Spitzer Cycle 6 IRAC photometry, together with published data, to present trends of color with type for L0 to T10 dwarfs. We also use the atmospheric and evolutionary models of Saumon & Marley to investigate the masses and ages of 13 very late-type T dwarfs, which have H-[4.5] > 3.2 and T_eff ~ 500 K to 750 K. Comment: To be published in the on-line version of the Proceedings of Cool Stars 16 (ASP Conference Series). This is an updated version of Leggett et al. 2010 ApJ 710 1627; a photometry compilation is available at http://www.gemini.edu/staff/sleggett
12/2010;
-
[show abstract]
[hide abstract]
ABSTRACT: Aims: Low-resolution optical spectroscopic observations for 78 very low-mass star and brown dwarf candidates that have been photometrically selected using the DENIS survey point source catalogue. Methods: Spectral types are derived for them using measurements of the PC3 index. They range from M6 to L4. H_alpha emission and NaI subordinate doublet (818.3 nm and 819.9 nm) equivalent widths are measured in the spectra. Spectroscopic indices of TiO, VO, CrH and FeH molecular features are also reported. Results: A rule-of-thumb criterion to select young very low-mass objects using the NaI doublet equivalent width is given. It is used to confirm seven new members of the Upper Sco OB association and two new members of the R Cr-A star-forming region. Four of our field objects are also classified as very young, but are not members of any known nearby young association. The frequency of lower-gravity young objects in our field ultracool sample is 8.5%. Our results provide the first spectroscopic classification for 38 ultracool dwarfs in the solar vicinity with spectrophotometric distances in the range 17 pc to 65 pc (3 of them are new L dwarfs within 20 pc). Comment: 25 pages, 10 figures, 7 tables, Accepted by A&A
04/2010;
-
[show abstract]
[hide abstract]
ABSTRACT: The bulge is a region of the Galaxy of tremendous interest for understanding galaxy formation. However measuring photometry and kinematics in it raises several inherent issues, such as severe crowding and high extinction in the visible. Using the Besancon Galaxy model and a 3D extinction map, we estimate the stellar density as a function of longitude, latitude and apparent magnitude and we deduce the possibility of reaching and measuring bulge stars with Gaia. We also present an ongoing analysis of the bulge using the Canada-France-Hawaii Telescope.
01/2009;
-
C. J. Willott,
P. Delorme, C. Reyle,
L. Albert,
J. Bergeron,
D. Crampton,
X. Delfosse,
T. Forveille,
J. B. Hutchings,
R. J. McLure,
A. Omont,
D. Schade
[show abstract]
[hide abstract]
ABSTRACT: We present imaging and spectroscopic observations for six quasars at z>5.9 discovered by the Canada-France High-z Quasar Survey (CFHQS). The CFHQS contains sub-surveys with a range of flux and area combinations to sample a wide range of quasar luminosities at z~6. The new quasars have luminosities 10 to 75 times lower than the most luminous SDSS quasars at this redshift. The least luminous quasar, CFHQS J0216-0455 at z=6.01, has absolute magnitude M_1450=-22.21, well below the likely break in the luminosity function. This quasar is not detected in a deep XMM-Newton survey showing that optical selection is still a very efficient tool for finding high redshift quasars. Comment: 7 pages, 4 figures, AJ, in press
01/2009;
-
[show abstract]
[hide abstract]
ABSTRACT: The Canada-France Brown Dwarf Survey is a wide eld survey for cool brown dwarfs conducted with the MegaCam camera on the CFHT telescope. Our objectives are to nd ultracool brown dwarfs and to constrain the eld brown dwarf mass function from a large and homogeneous sample of L and T dwarfs. We identify candidates in CFHT/Megacam i' and z' images and follow them up with pointed NIR imaging on several telescopes. Our survey has to date found 50 T dwarfs candidates and 170 L or late M dwarf candidates drawn from a larger sample of 1300 candidates with typical ultracool dwarfs i'-z' colours, found in 900 square degrees. We currently have completed the NIR follow-up on a large part of the survey for all candidates from the latest T dwarfs known to the late L color range. This allows us to build on a complete and well de ned sample of ultracool dwarfs to investigate the luminosity function of eld L and T dwarfs. Comment: Cool Stars XV conference. to appear in proceedings of Cool Stars XV Conference
12/2008;
-
[show abstract]
[hide abstract]
ABSTRACT: Context. Thanks to recent large scale surveys in the near infrared such as 2MASS, the galactic plane that most suffers from extinction is revealed and its overall structure can be studied. Aims. This work aims at constraining the structure of the Milky Way external disc as seen in 2MASS data, and in particular the warp. Methods. We use the Two Micron All Sky Survey (hereafter 2MASS) along with the Stellar Population Synthesis Model of the Galaxy, developed in Besancon, to constrain the external disc parameters such as its scale length, its cutoff radius, and the slope of the warp. In order to properly interpret the observations, the simulated stars are reddened using a three dimensional extinction map. The shape of the stellar warp is then compared with previous results and with similar structures in gas and dust. Results. We find new constraints on the stellar disc, which is shown to be asymmetrical, similar to observations of HI. The positive longitude side is found to be easily modelled with a S shape warp but with a slope significantly smaller than the slope seen in the HI warp. At negative longitudes, the disc presents peculiarities which are not well reproduced by any simple model. Finally, comparing with the warp seen in the dust, it seems to follow a slope intermediate between the gas and the stars. Comment: 9 pages. Accepted for publication in Astronomy and Astrophysics
12/2008;
-
[show abstract]
[hide abstract]
ABSTRACT: (abridged) By comparing the distribution of dust and gas in the central regions of the Galaxy, we aim to obtain new insights into the properties of the offset dust lanes leading the bar's major axis in the Milky Way. On the one hand, the molecular emission of the dust lanes is extracted from the observed CO l-b-V distribution according to the interpretation of a dynamical model. On the other hand, a three dimensional extinction map of the Galactic central region constructed from near-infrared observations is used as a tracer of the dust itself and clearly reveals dust lanes in its face-on projection. Comparison of the position of both independent detections of the dust lanes is performed in the (l, b) plane. These two completely independent methods are used to provide a coherent picture of the dust lanes in the Milky Way bar. In both the gas and dust distributions, the dust lanes are found to be out of the Galactic plane, appearing at negative latitudes for l > 0 deg and at positive latitudes for l < 0 deg. However, even though there is substantial overlap between the two components, they are offset from one another with the dust appearing to lie closer to the b = 0 deg plane. Two scenarios are proposed to explain the observed offset. The first involves grain destruction by the bar shock and reformation downstream. Due to the decrease in velocity caused by the shock, this occurs at lower z. The second assumes that the gas and dust remain on a common tilted plane, but that the molecular gas decouples from the Milky Way's magnetic field, itself strong enough to resist the shear of the bar's shock. The diffuse gas and dust remain coupled to the field and are carried further downstream. This second scenario has recently been suggested in order to explain observations of the barred galaxy NGC 1097. Comment: 4 pages, 5 figures, accepted for publication in Astronomy and Astrophysics letters
11/2007;
-
[show abstract]
[hide abstract]
ABSTRACT: We present spectra of 59 nearby stars candidates, M dwarfs and white dwarfs, previously identified using high proper motion catalogues and the DENIS database. We review the existing spectral classification schemes and spectroscopic parallax calibrations in the near-infrared $J$-band and derive spectral types and distances of the nearby candidates. 42 stars have spectroscopic distances smaller than 25 pc, three of them being white dwarfs. Two targets lie within 10 pc, one M8 star at 10.0 pc (APMPM J0103-3738), and one M4 star at 8.3 pc (LP 225-57). One star, LHS 73, is found to be among the few subdwarfs lying within 20 pc. Furthermore, together with LHS 72, it probably belongs to the closest pair of subdwarfs we know. Comment: 10 pages, 7 figures, accepted for publication in MNRAS
09/2006;
-
[show abstract]
[hide abstract]
ABSTRACT: The Two Micron All Sky Survey, along with the Stellar Population Synthesis
Model of the Galaxy, developed in Besancon, is used to calculate the extinction
distribution along different lines of sight. By combining many lines of sight,
the large scale distribution of interstellar material can be deduced. The
Galaxy model is used to provide the intrinsic colour of stars and their
probable distances, so that the near infrared colour excess, and hence the
extinction, may be calculated and its distance evaluated. Such a technique is
dependent on the model used, however we are able to show that moderate changes
in the model parameters result in insignificant changes in the predicted
extinction. This technique has now been applied to over 64000 lines of sight,
each separated by 15 arcmin, in the inner Galaxy (|l|<=100 deg, |b|<=10 deg).
Using our extinction map, we have derived the main characteristics of the large
scale structure of the dust distribution: scale height and warp of the ISM disc
as well as the angle of the dust in the Galactic Bar. This resulting extinction
map will be useful for studies of the inner Galaxy and its stellar populations.
04/2006;
-
[show abstract]
[hide abstract]
ABSTRACT: In this paper we review some recent detections of wide binary brown dwarf systems and discuss them in the context of the multiplicity properties of very low-mass stars and brown dwarfs. Comment: 2 pages, 1 figure (new version with minor corrections); to appear in the proceedings of the workshop "Ultra-low mass star formation and evolution", to be published in AN
09/2005;
-
[show abstract]
[hide abstract]
ABSTRACT: We report our discovery of a nearby wide binary system, LP 714-37 AB. Phan-Bao et al. identified LP 714-37 as a mid-M dwarf during a cross-identification of the NLTT and DENIS catalogues. Our CCD images resolve the system into a binary with a projected angular separation of 1.8 arcsec, or 33 AU, and low-resolution optical spectra give spectral types of M5.5 (LP 714-37A) and M7.5 (LP 714-37B). This makes LP 714-37 AB one of very few widely separated (separation > 30 AU) very low-mass binary systems known in the field, and we discuss it in the context of the multiplicity properties of very low-mass stars and brown dwarfs. Comment: Accepted by A&A Letters
06/2005;
-
[show abstract]
[hide abstract]
ABSTRACT: We present spectra of 36 nearby star candidates and 3 red giant candidates, identified in the DENIS database by Phan-Bao et al. (2003). 32 of the dwarf candidates are indeed nearby red dwarfs, with spectral types from M5.5 to M8.5. Out of 11 targets with low proper motion (mu < 0.1$ arc-sec/yr) but a Reduced Proper Motion above an inclusive threshold, 9 are red dwarfs. The 4 contaminants are all reddened F-K main sequence stars, and they could have been eliminated by checking for some well known high latitude molecular clouds. These stars might be of interest as probes of interstellar absorption. For the red dwarfs we derive spectral types and spectroscopic distances, using a new calibration of the PC3 spectral index to absolute magnitudes in the I, J, H and K photometric bands. We confirm 2 new members of the 12 pc volume (2 new M8.5), and one M7.5 NLTT object closer than 10pc. We also show that one quarter of the stars with photometric distances under 30 pc have too small a proper motion for inclusion in the NLTT catalog. Comment: Accepted by A&A
06/2005;
-
[show abstract]
[hide abstract]
ABSTRACT: Having ruled out the possibility that stellar objects are the main contributor of the dark matter embedding galaxies, microlensing experiments cannot exclude the hypothesis that a significant fraction of the Milky Way dark halo might be made of MACHOs with masses in the range $0.5-0.8 \msun$. Ancient white dwarfs are generally considered the most plausible candidates for such MACHOs. We report the results of a search for such white dwarfs in a proper motion survey covering a 0.16 sqd field at three epochs at high galactic latitude, and 0.938 sqd at two epochs at intermediate galactic latitude (VIRMOS survey), using the CFH telescope. Both surveys are complete to I = 23, with detection efficiency fading to 0 at I = 24.2. Proper motion data are suitable to separate unambiguously halo white dwarfs identified by belonging to a non rotating system. No candidates were found within the colour-magnitude-proper motion volume where such objects can be safely discriminated from any standard population as well as from possible artefacts. In the same volume, we estimate the maximum white dwarf halo fraction compatible with this observation at different significance levels if the halo is at least 14 gigayears old and under different ad hoc initial mass functions. Our data alone rules out a halo fraction greater than 14% at 95% confidence level. Combined with two previous investigations exploring comparable volumes pushes the limit below 4 % (95% confidence level) or below 1.3% (64% confidence), this implies that if baryonic dark matter is present in galaxy halos, it is not, or it is only marginally in the form of faint hydrogen white dwarfs. Comment: accepted in Astronomy and Astrophysics (19-05-2004)
03/2004;
-
[show abstract]
[hide abstract]
ABSTRACT: We present new nearby stars extracted from a proper motion catalogue and having a DENIS counterpart. Their distances and spectral type are estimated using the DENIS colours. 107 stars are within 50 pc. 31 stars among them have previously measured distances. In addition, 40 stars may enter within the 50 pc limit depending on which population they belong to. 6 stars among them have already measured distances. 5 objects, LHS5045, LP225-57, LP831-45, LHS1767, and WT792, are probably closer than 15 pc, with LP225-57 at 9.5 pc. Most of these stars are M-type while 4 stars are white dwarfs. 88 M-dwarfs are disc stars, 14 belong to the thick disc and 1 to the spheroid. Comment: 9 pages, 3 figures, accepted for publication in A&A
05/2002;
-
[show abstract]
[hide abstract]
ABSTRACT: As part of a global analysis of deep star counts to constrain scenarii of galaxy formation and evolution, we investigate possible links between the galactic spheroid and the dark matter halo. A wide set of deep star counts at high and intermediate galactic latitudes is used to determine the large scale density law of the spheroid. Assuming a power density law, the exponent, flattening, local density and IMF slope of this population are estimated. The estimation is checked for robustness against contamination of star counts by the thick disc population. Contamination effects are derived from a model of population synthesis under a broad variety of thick disc parameters. The parameter fit is based on a maximum likelihood criterion. The best fit spheroid density law has a flattening of 0.76, a power index of 2.44. There is a significant degeneracy between these two parameters. The data are also compatible with a slightly less flattened spheroid (c/a = 0.85), in combination with a larger power index (2.75). A flatter spheroid (c/a = 0.6) with a power index of 2 is not excluded either. We also constrain the spheroid IMF slope \alpha to be 1.9 +/- 0.2, leading to a local density of 1.64 10$^{-4}$ stars pc$^{-3}$ and a mass density of 4.15 10$^{-5}$ \Msun pc$^{-3}$. With this slope the expected mass density of brown dwarfs in the halo makes a negligible part of the dark matter halo, as already estimated from microlensing surveys. So, as star count data progresses in depth and extent, the picture of the spheroid star population that comes out points to a shape quite compatible with what we know about the distribution of baryonic dark matter if it is made of stellar remnants, suggesting a common dynamical origin.
05/2000;
