arXiv subjects

J. Bouvier

Publications and source records attributed to J. Bouvier.

At least 163 records · Page 9Linked to original sources

arXiv2005

Exploring the Lower Mass Function in the young open Cluster IC 4665

We present a study of the young (30-100Myr) open cluster IC 4665 with the aim to determine the shape of the mass function well into the brown dwarf regime. We photometrically select 691 low-mass stellar and 94 brown dwarf candidate members over an area of 3.82 square degrees centred on the cluster. K-band follow-up photometry and Two-Micron All-Sky Survey data allow a first filtering of contaminant objects from our catalogues. A second filtering is performed for the brightest stars using proper motion data provided by the Tycho-2 and UCAC2 public catalogues. Contamination by the field population for the lowest mass objects is estimated using same latitude control fields. We fit observed surface densities of various cluster populations with King profiles and find a consistent tidal radius of 1.0 degree. The presence of possible mass segregation is discussed. In most respects investigated, IC 4665 is similar to other young open clusters at this age: (1) a power law fit to the mass function between 1 and 0.04Msun results in best fit for a slope of -0.6; (2) a cusp in the mass function is noticed at about the substellar boundary with respect to the power law description, the interpretation of which is discussed; (3) a fraction between 10-19% for BDs with M > 0.03Msun to total members; (4) a best-fit lognormal function to the full mass distribution shows an average member mass of 0.32Msun, if IC 4665 has an age of 50Myr.

astro-ph↗

arXiv2005

A Hubble Space Telescope ACS Search for Brown Dwarf Binaries in the Pleiades Open Cluster

We present the results of a high-resolution imaging survey for brown dwarf binaries in the Pleiades open cluster. The observations were carried out with the Advance Camera for Surveys onboard the Hubble Space Telescope. Our sample consists of 15 bona-fide brown dwarfs. We confirm 2 binaries and detect their orbital motion, but we did not resolve any new binary candidates in the separation range between 5.4AU and 1700AU and masses in the range 0.035--0.065~Msun. Together with the results of our previous study (Martin et al., 2003), we can derive a visual binary frequency of 13.3$^{+13.7}_{-4.3}$\% for separations greater than 7~AU masses between 0.055--0.065~M$_{\sun}$ and mass ratios between 0.45--0.9$<q<$1.0. The other observed properties of Pleiades brown dwarf binaries (distributions of separation and mass ratio) appear to be similar to their older counterparts in the field.

astro-ph↗

arXiv2005

A near-infrared survey for new low-mass members in Alpha Per

We present a near-infrared (K'-band) survey of 0.7 square degree area in the Alpha Persei open cluster (age = 90 Myr, distance = 182 pc) carried out with the Omega-Prime camera on the Calar Alto 3.5-m telescope. Combining optical data (Rc and Ic) obtained with the KPNO/MOSA detector and presented in Stauffer et al. (1999) with the K' observations, a sample of new candidate members has been extracted from the optical-infrared colour-magnitude diagram. The location of these candidates in the colour-colour diagram suggests that two-thirds of them are actually reddened background giants. About 20 new candidate members with masses between 0.3 and 0.04 Msun are added to the 400 known Alpha Per cluster members. If they are indeed Alpha Per members, four of the new candidates would be brown dwarfs. We discuss the advantages and drawbacks of the near-infrared survey as compared to the optical selection method. We also describe the outcome of optical spectroscopy obtained with the Twin spectrograph on the Calar Alto 3.5-m telescope for about 30 candidates, including selected members from the optical sample presented in Barrado y Navascues et al. (2002) and from our joint optical/infrared catalogue. These results argue in favour of the optical selection method for this particular cluster.

astro-ph↗

arXiv2005

A deep, wide-field search for substellar members in NGC 2264

We report the first results of our ongoing campaign to discover the first brown dwarfs (BD) in NGC 2264, a young (3 Myr), populous star forming region for which our optical studies have revealed a very high density of potential candidates - 236 in $<$ 1 deg$^2$ - from the substellar limit down to at least $\sim$ 20 M$_{\rm Jup}$ for zero reddening. Candidate BD were first selected using wide field ($I,z$) band imaging with CFH12K, by reference to current theoretical isochrones. Subsequently, 79 (33%) of the $I,z$ sample were found to have near-infrared 2MASS photometry ($JHK_s$ $\pm$ 0.3 mag. or better), yielding dereddened magnitudes and allowing further investigation by comparison with the location of NextGen and DUSTY isochrones in colour-colour and colour-magnitude diagrams involving various combinations of $I$,$J$,$H$ and $K_s$. We discuss the status and potential substellarity of a number of relatively unreddened (A$_{\rm v}$ $<$ 5) likely low-mass members in our sample, but in spite of the depth of our observations in $I,z$, we are as yet unable to unambiguously identify substellar candidates using only 2MASS data. Nevertheless, there are excellent arguments for considering two faint (observed $I$ $\sim$ 18.4 and 21.2) objects as cluster candidates with masses respectively at or rather below the hydrogen burning limit. More current candidates could be proven to be cluster members with masses around 0.1 M$_{\odot}$ {\it via} gravity-sensitive spectroscopy, and deeper near-infrared imaging will surely reveal a hitherto unknown population of young brown dwarfs in this region, accessible to the next generation of deep near-infrared surveys.

astro-ph↗

arXiv2004

The Collinder 69 cluster in the context of the Lambda Orionis SFR

The Lambda Orionis Star Forming Region is a complex structure which includes the Col 69 (Lambda Orionis) cluster and the B30 & B35 dark clouds. We have collected deep optical photometry and spectroscopy in the central cluster of the SFR (Col 69), and combined with 2MASS IR data, in order to derive the Initial Mass Function of the cluster, in the range 50-0.02 M(sun). In addition, we have studied the H(alpha) and lithium equivalent widths, and the optical-infrared photometry, to derive an age (5+-2 Myr) for Col 69, and to compare these properties to those of B30 & B35 members.

astro-ph↗

arXiv2004

A possible third component in the L dwarf binary system DENIS-P J020529.0-115925 discovered with the Hubble Space Telescope

We present results showing that the multiple system DENIS-P J020529.0-115925 is likely to be a triple system of brown dwarfs. The secondary of this previously known binary system shows a clear elongation on six images obtained at six different epochs. Significant residuals remain after PSF subtraction on these images, characteristic of multiplicity, and indicating that the secondary is probably a double itself. Dual-PSF fitting shows that the shape of the secondary is consistent with that of a binary system. These measurements show that the probability that DENIS-P J020529.0-115925 is a triple system is very high. The photometric and spectroscopic properties of the system are consistent with the presence of three components with spectral types L5, L8 and T0.

astro-ph↗

arXiv2004

Lambda Orionis: A 0.02-50 Msun Initial Mass Function

The Lambda Orionis Star Forming Region is dominated by the O8 III star lambda^1 Ori. Among other structures, it includes a CO and a dust ring whose diameter is about nine deg, the S264 HII region, a large number of IRAS sources, the Barnard 30 and 35 dark clouds and a cluster associated with the central star, the Lambda Ori cluster (Collinder 69). We derive the initial mass function for this cluster (~5 Myr), covering several orders of magnitude in mass (50 - 0.02 Msun).

astro-ph↗

arXiv2004

The 0.03-10Mo mass function of young open clusters

We report the present day mass functions (PDMFs) of 3 young open clusters over a mass range from 30 Jupiter masses to 10~\msunn. The PDMFs of the 3 clusters are remarkably similar, suggesting little impact of specific conditions (stellar density, metallicity, early dynamical evolution) on the mass distribution. Functional forms are provided to allow quantitative comparison with MFs derived in other environments.

astro-ph↗

arXiv2004

Multiple protostellar systems. I. A deep near infrared survey of Taurus and Ophiuchus protostellar objects

(Abridged) We performed a deep infrared imaging survey of 63 embedded young stellar objects (YSOs) located in the Taurus and Ophiuchus clouds to search for companions. The sample includes Class I and flat infrared spectrum protostellar objects. We find 17 companions physically bound to 15 YSOs with angular separations in the range 0.8-10" (110-1400 AU) and derive a companion star fraction of 23+/-9 % and 29+/-7 % for embedded YSOs in Taurus and Ophiuchus, respectively. In spite of different properties of the clouds and especially of the prestellar cores, the fraction of wide companions, 27+/-6 % for the combined sample, is identical in the two star-forming regions. This suggests that the frequency and properties of wide multiple protostellar systems are not very sensitive to specific initial conditions. Comparing the companion star fraction of the youngest YSOs still surrounded by extended envelopes to that of more evolved YSOs, we find evidence for a possible evolution of the fraction of wide multiple systems, which seems to decrease by a factor of about 2 on a timescale of about 10^5 yr. Somewhat contrary to model predictions, we do not find evidence for a sub-clustering of embedded sources at this stage on a scale of a few 100 AU that could be related to the formation of small-N protostellar clusters. Possible interpretations for this discrepancy are discussed.

astro-ph↗

arXiv2004

The Pleiades mass function: models versus observations

Two stellar-dynamical models of binary-rich embedded proto-Orion-Nebula-type clusters that evolve to Pleiades-like clusters are studied with an emphasis on comparing the stellar mass function with observational constraints. By the age of the Pleiades (about 100 Myr) both models show a similar degree of mass segregation which also agrees with observational constraints. This thus indicates that the Pleiades is well relaxed and that it is suffering from severe amnesia. It is found that the initial mass function (IMF) must have been indistinguishable from the standard or Galactic-field IMF for stars with mass m < 2 Mo, provided the Pleiades precursor had a central density of about 10^4.8 stars/pc^3. A denser model with 10^5.8 stars/pc^3 also leads to reasonable agreement with observational constraints, but owing to the shorter relaxation time of the embedded cluster it evolves through energy equipartition to a mass-segregated condition just prior to residual-gas expulsion. This model consequently looses preferentially low-mass stars and brown dwarfs (BDs), but the effect is not very pronounced. The empirical data indicate that the Pleiades IMF may have been steeper than Salpeter for stars with m > 2 Mo.

astro-ph↗

arXiv2004

First determination of the dynamical mass of a binary L dwarf

We present here the results of astrometric, photometric and spectroscopic observations leading to the determination of the orbit and dynamical masses of the binary L dwarf 2MASSW J0746425+2000321. High angular resolution observations spread over almost 4 years and obtained with the Hubble Space Telescope (HST), the ESO Very Large Telescope (VLT), and a the W. M. Keck Observatory (Keck) allow us to cover 36% of the period, corresponding to 60% of the orbit, and, for the first time, to derive a precise estimate of the total and individual masses of such a late-type object. We find an orbital period of 3850.9$^{+904}_{-767}$ days. The corresponding total mass is 0.146$^{+0.016}_{-0.006}$ M$_{\sun}$, with uncertainties depending on the distance. Spatially resolved low resolution optical (550--1025 nm) spectra have been obtained with HST/STIS, allowing us to measure the spectral types of the two components (L0$\pm$0.5 for the primary and L1.5$\pm$0.5 for the secondary). We also present precise photometry of the individual components measured on the high angular resolution images obtained with HST/ACS and WFPC2 (visible), VLT/NACO (J, H and Ks bands) and Keck I (Ks) band). These spectral and photometric measurements enable us to estimate their effective temperatures and mass ratio, and to place the object accurately in a H--R diagram. The binary system is most likely formed by a primary with a mass of 0.085$\pm$0.010 M$_{\sun}$ and a secondary with a mass of 0.066$\pm$0.006 M$_{\sun}$, thus clearly substellar, for an age of approximately 300$\pm$150 Myr. H$α$ variability indicates chromospheric and/or magnetic activity.

astro-ph↗

arXiv2004

The substellar population of the young cluster lambda Orionis

By collecting optical and infrared photometry and low resolution spectroscopy, we have identified a large number of low mass stars and brown dwarf candidates belonging to the young cluster (~5 Myr) associated with the binary star lambda Orionis. The lowest mass object found is a M8.5 with an estimated mass of 0.02 Msun (~0.01 Msun for objects without spectroscopic confirmation). For those objects with spectroscopy, the measured strength of the Halpha emission line follows a distribution similar to other clusters with the same age range, with larger equivalent widths for cooler spectral types. Three of the brown dwarfs have Halpha emission equivalent widths of order 100 Å, suggestive that they may have accretion disks and thus are the substellar equivalent of Classical T Tauri stars. We have derived the Initial Mass Function for the cluster. For the substellar regime, the index of the mass spectrum is alpha=0.60$+-0.06, very similar to other young associations.

astro-ph↗

arXiv2003

Hall effect in the normal state of high Tc cuprates

We propose a model for explaining the dependence in temperature of the Hall effect of high Tc cuprates in the normal state in various materials. They all show common features: a decrease of the Hall coefficient RH with temperature and a universal law, when plotting RH(T)/RH(T0) versus T/T0, where T0 is defined from experimental results. This behaviour is explained by using the well known electronic band structure of the CuO2 plane, showing saddle points at the energies ES in the directions (0,+/-pi) and (+/-pi,0). We remark that in a magnetic field, for energies E>ES the carrier orbits are hole-like and for E<ES they are electron-like, giving opposite contributions to RH. We are abble to fit the experimental results for a wide range of hole doping, and to fit the universal curve. For us kb*T0 is simply EF-ES, where EF is the Fermi level varying with the doping.

cond-mat.supr-con↗

arXiv2003

On the universal outcome of star-formation: Is there a link between stars and brown-dwarfs?

(abridged) The recent evidence obtained by Briceno et al. that star-formation in Taurus-Auriga (TA) may be producing significantly fewer brown dwarfs (BDs) per star than the ONC is investigated by setting up a realistic model stellar plus BD population and explicitly taking into account a high binary proportion and dynamical evolution in the TA groups and the ONC. The Briceno result is reproduced almost exactly despite an identical IMF in both systems because many BD-BD and star-BD binaries are disrupted in the ONC thus freeing BDs, while the TA groups remain unevolved dynamically. However, the resulting populations do not have the correct star-star, star-BD and expecially BD-BD binary properties, even if a variable BD IMF is allowed for. The conclusion is therefore that BDs need to be added as a separate population which has its own binary properties. Such an extra population can have various origins which are briefly discussed in this contribution but more fully in an associated paper.

astro-ph↗

arXiv2003

Magnetically channeled accretion in T Tauri stars : a dynamical process

We review observational evidence and open issues related to the process of magnetospheric accretion in T Tauri stars. Emphasis is put on recent numerical simulations and observational results which suggest that the interaction between the stellar magnetosphere and the inner accretion disk is a highly time dependent process on timescales ranging from hours to months.

astro-ph↗

arXiv2003

Constraints on the disk geometry of the T Tauri star AA Tau from linear polarimetry

We have simultaneously monitored the photometric and polarimetric variations of the Classical T Tauri star AA Tau during the fall of 2002. We combine these data with previously published polarimetric data covering two earlier epochs. The phase coverage is complete, although not contiguous. AA Tau clearly shows cyclic variations coupled with the rotation of the system. The star-disk system produces a repeatable polarisation curve where the polarisation increases with decreasing brightness. The data fit well with the model put forward by Bouvier et al. (1999) where AA Tau is viewed almost edge-on and its disk is actively dumping material onto the central star via magnetospheric accretion. The inner edge of the disk is deformed by its interaction with the tilted magnetosphere, producing eclipses as it rotates and occults the photosphere periodically. From the shape of the polarisation curve in the QU-Plane we confirm that the accretion disk is seen at a large inclination, almost edge-on, and predict that its position angle is PA~90 deg., i.e., that the disk's major axis is oriented in the East-West direction.

astro-ph↗

arXiv2003

Eclipses by circumstellar material in the T Tauri star AA Tau. II. Evidence for non-stationary magnetospheric accretion

We report the results of a synoptic study of the photometric and spectroscopic variability of the classical T Tauri star AA Tau on timescales ranging from a few hours to several weeks. Emission lines show both infall and outflow signatures and are well reproduced by magnetospheric accretion models with moderate mass accretion rates and high inclinations. The veiling shows variations that indicate the presence of 2 rotationally modulated hot spots corresponding to the two magnetosphere poles. It correlates well with the HeI line flux, with B-V and the V excess flux. We have indications of a time delay between the main emission lines and veiling, the lines formed farther away preceding the veiling changes. The time delay we measure is consistent with accreted material propagating downwards the accretion columns at free fall velocity from a distance of about 8 Rstar. We also report periodic radial velocity variations of the photospheric spectrum which might point to the existence of a 0.02 Msun object orbiting the star at a distance of 0.08 AU. During a few days, the variability of the system was strongly reduced and the line fluxes and veiling severely depressed. We argue that this episode of quiescence corresponds to the temporary disruption of the magnetic configuration at the disk inner edge. The radial velocity variations of inflow and outflow diagnostics in the Halpha profile yield further evidence for large scale variations of the magnetic configuration on a timescale of a month. These results may provide the first clear evidence for large scale instabilities developping in T Tauri magnetospheres as the magnetic field lines are twisted by differential rotation between the star and the inner disk.

astro-ph↗

arXiv2003

A Census of the Young Cluster IC 348

We present a new census of the stellar and substellar members of the young cluster IC 348. We have obtained images at I and Z for a 42'x28' field encompassing the cluster and have combined these measurements with previous optical and near-infrared photometry. From spectroscopy of candidate cluster members appearing in these data, we have identified 122 new members, 15 of which have spectral types of M6.5-M9, corresponding to masses of 0.08-0.015 M_sun by recent evolutionary models. The latest census for IC 348 now contains a total of 288 members, 23 of which are later than M6 and thus are likely to be brown dwarfs. From an extinction-limited sample of members (A_V<=4) for a 16'x14' field centered on the cluster, we construct an IMF that is unbiased in mass and nearly complete for M/M_sun>=0.03 (<=M8). In logarithmic units where the Salpeter slope is 1.35, the mass function for IC 348 rises from high masses down to a solar mass, rises more slowly down to a maximum at 0.1-0.2 M_sun, and then declines into the substellar regime. In comparison, the similarly-derived IMF for Taurus from Briceno et al. and Luhman et al. rises quickly to a peak near 0.8 M_sun and steadily declines to lower masses. The distinctive shapes of the IMFs in IC 348 and Taurus are reflected in the distributions of spectral types, which peak at M5 and K7, respectively. These data provide compelling, model-independent evidence for a significant variation of the IMF with star-forming conditions.

astro-ph↗