Analysis of Keck HIRES spectra of early L-type dwarfs

We present analyses of high resolution and medium resolution spectra of early L dwarfs. We used our latest set of model atmospheres to reproduce and analyze the observed features. We can model the optical flux and the atomic line profiles with the best accuracy to date. The models used to reproduce the observations include dust condensation and dust opacities. Compared to previous studies using older models we find that our dust treatment is much improved. The derived parameters for the objects are well in the expected range for old very low mass objects. This is also supported by the absence of Li in most of the objects. For the objects showing Li we can be almost certain that those are brown dwarfs. However, a spectral analysis in general, and this one in particular can only very roughly determine mass and age.Comment: AASTeX5.0. 26 pages, including all figures, Accepted for Ap

Hokupa'a-Gemini Discovery of Two Ultracool Companions to the Young Star HD 130948

We report the discovery of two faint ultracool companions to the nearby (d~17.9 pc) young G2V star HD 130948 (HR 5534, HIP 72567) using the Hokupa'a adaptive optics instrument mounted on the Gemini North 8-meter telescope. Both objects have the same common proper motion as the primary star as seen over a 7 month baseline and have near-IR photometric colors that are consistent with an early-L classification. Near-IR spectra taken with the NIRSPEC AO instrument on the Keck II telescope reveal K I lines, FeH, and water bandheads. Based on these spectra, we determine that both objects have spectral type dL2 with an uncertainty of 2 spectral subclasses. The position of the new companions on the H-R diagram in comparison with theoretical models is consistent with the young age of the primary star (<0.8 Gyr) estimated on the basis of X-ray activity, lithium abundance and fast rotation. HD 130948 B and C likely constitute a pair of young contracting brown dwarfs with an orbital period of about 10 years, and will yield dynamical masses for L dwarfs in the near future.Comment: 10 pages, 3 figures, (13 total pages

The First Substellar Subdwarf? Discovery of a Metal-poor L Dwarf with Halo Kinematics

We present the discovery of the first L-type subdwarf, 2MASS J05325346+8246465. This object exhibits enhanced collision-induced H$_2$ absorption, resulting in blue NIR colors ($J-K_s = 0.26{\pm}0.16$). In addition, strong hydride bands in the red optical and NIR, weak TiO absorption, and an optical/J-band spectral morphology similar to the L7 DENIS 0205$-$1159AB imply a cool, metal-deficient atmosphere. We find that 2MASS 0532+8246 has both a high proper motion, $\mu$ = 2\farcs60\pm0\farcs15 yr$^{-1}$, and a substantial radial velocity, $v_{rad} = -195{\pm}11$ km s$^{-1}$, and its probable proximity to the Sun (d = 10--30 pc) is consistent with halo membership. Comparison to subsolar-metallicity evolutionary models strongly suggests that 2MASS 0532+8246 is substellar, with a mass of 0.077 $\lesssim$ M $\lesssim$ 0.085 M_{\sun} for ages 10--15 Gyr and metallicities $Z = 0.1-0.01$ Z_{\sun}. The discovery of this object clearly indicates that star formation occurred below the Hydrogen burning mass limit at early times, consistent with prior results indicating a flat or slightly rising mass function for the lowest-mass stellar subdwarfs. Furthermore, 2MASS 0532+8246 serves as a prototype for a new spectral class of subdwarfs, additional examples of which could be found in NIR proper motion surveys.Comment: 9 pages, 3 figures, accepted to Ap

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.Comment: To appear in Astronomy & Astrophysics; 10 page

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).Comment: Astronomy & Astrophysics, in press. (Minor typos corrected.

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$150 Myr. H$\alpha$ variability indicates chromospheric and/or magnetic activity.Comment: accepted for publication in A&A 16 pages, 7 figures, 6 table

Detection of brown dwarf-like objects in the core of NGC3603

We use near-infrared data obtained with the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope to identify objects having the colors of brown dwarfs (BDs) in the field of the massive galactic cluster NGC 3603. These are identified through use of a combination of narrow and medium band filters spanning the J and H bands, and which are particularly sensitive to the presence of the 1.3-1.5{\mu}m H2O molecular band - unique to BDs. We provide a calibration of the relationship between effective temperature and color for both field stars and for BDs. This photometric method provides effective temperatures for BDs to an accuracy of {\pm}350K relative to spectroscopic techniques. This accuracy is shown to be not significantly affected by either stellar surface gravity or uncertainties in the interstellar extinction. We identify nine objects having effective temperature between 1700 and 2200 K, typical of BDs, observed J-band magnitudes in the range 19.5-21.5, and that are strongly clustered towards the luminous core of NGC 3603. However, if these are located at the distance of the cluster, they are far too luminous to be normal BDs. We argue that it is unlikely that these objects are either artifacts of our dataset, normal field BDs/M-type giants or extra-galactic contaminants and, therefore, might represent a new class of stars having the effective temperatures of BDs but with luminosities of more massive stars. We explore the interesting scenario in which these objects would be normal stars that have recently tidally ingested a Hot Jupiter, the remnants of which are providing a short-lived extended photosphere to the central star. In this case, we would expect them to show the signature of fast rotation.Comment: 26 Pages, 8 Figures, Accepted for publication on Ap

Individual rules for trail pattern formation in Argentine ants (Linepithema humile)

We studied the formation of trail patterns by Argentine ants exploring an empty arena. Using a novel imaging and analysis technique we estimated pheromone concentrations at all spatial positions in the experimental arena and at different times. Then we derived the response function of individual ants to pheromone concentrations by looking at correlations between concentrations and changes in speed or direction of the ants. Ants were found to turn in response to local pheromone concentrations, while their speed was largely unaffected by these concentrations. Ants did not integrate pheromone concentrations over time, with the concentration of pheromone in a 1 cm radius in front of the ant determining the turning angle. The response to pheromone was found to follow a Weber's Law, such that the difference between quantities of pheromone on the two sides of the ant divided by their sum determines the magnitude of the turning angle. This proportional response is in apparent contradiction with the well-established non-linear choice function used in the literature to model the results of binary bridge experiments in ant colonies (Deneubourg et al. 1990). However, agent based simulations implementing the Weber's Law response function led to the formation of trails and reproduced results reported in the literature. We show analytically that a sigmoidal response, analogous to that in the classical Deneubourg model for collective decision making, can be derived from the individual Weber-type response to pheromone concentrations that we have established in our experiments when directional noise around the preferred direction of movement of the ants is assumed.Comment: final version, 9 figures, submitted to Plos Computational Biology (accepted

A spectroscopic and proper motion search of Sloan Digital Sky Survey : red subdwarfs in binary systems

Red subdwarfs in binary systems are crucial for both model calibration and spectral classification. We search for red subdwarfs in binary systems from a sample of high proper motion objects with Sloan Digital Sky Survey spectroscopy. We present here discoveries from this search, as well as highlight several additional objects of interest. We find 30 red subdwarfs in wide binary systems including: two with spectral type of esdM5.5, 6 companions to white dwarfs and 3 carbon-enhanced red subdwarfs with normal red subdwarf companions. 15 red subdwarfs in our sample are partially resolved close binary systems. With this binary sample, we estimate the low limit of the red subdwarf binary fraction of similar to 10 per cent. We find that the binary fraction goes down with decreasing masses and metallicities of red subdwarfs. A spectroscopic esdK7 subdwarf + white dwarf binary candidate is also reported. 30 new M subdwarfs have spectral type of >= M6 in our sample. We also derive relationships between spectral types and absolute magnitudes in the optical and near-infrared for M and L subdwarfs, and we present an M subdwarf sample with measured U, V, W space velocities.Peer reviewe