67 research outputs found
Studying cool dwarfs and their mass function using infrared surveys
Anyone seeking to understand the physical processes of star formation must make
reference to the spectrum of masses (mass function) produced by such processes. The
low mass end of this mass function in particular is poorly constrained.The two leading environments in which to measure the mass function are in open
clusters and star forming regions and in the solar neighbourhood. Studies of some star
forming regions claim to constrain the mass function down to a few tens of Jupiter
masses. However the validity of results in young clusters and star forming regions has
been called into question, with the accuracy of the evolutionary models that all studies
are based on being considered debatable at young ages. Hence a study in the field,
where there is a spread of ages, provides a useful, possibly more robust, m ethod for
measuring the mass function.Low mass stars and brown dwarfs in the field are usually identified by either infrared
surveys or by proper motion surveys. Here these techniques are combined for the first
time to produce an infrared proper m otion survey. This has produced a sample of over
7000 low mass objects with proper motions greater than 0.1” /y r. It has also found
several com m on proper motion binary systems and SIPS1259-4336, an M8.5 dwarf
within lOpc.This sample on its own will not yield a constraint on the mass function. In the field
the range of ages and the luminosity evolution of objects leads to the mass function
being intertwined with the stellar birthrate. Here this problem is dealt with by detailed
simulations. By drawing ages from a birthrate and masses from a mass function,
simulated objects can have space positions, velocities and absolute magnitudes assigned
using a simple model of the Galaxy, evolutionary models of low mass objects and
empirical relations. These can then be converted into observables and passed through a survey selection mechanism. By varying the underlying birthrate and mass function
the effect these have 011 the survey results can be found. These results can then be
compared with those of the actual survey to constrain the mass function and birthrate.Here the mass function parameter a is found to be 0.66 ± 0 .4 4 in the range O.2M⨀ >
m > O.O75M⨀ , that the birthrate parameter ß is —0.01 ±0.10 and that the space density
of objects in the mass range O.O9M⨀ < m < O.1M⨀ is 0.0053 ± 0.0002/pc³. It should
be noted that due to noise in the probability surface, the constraint on ß should be
treated with caution.Finally the results of the future infrared surveys by UKIDSS are simulated and the
potential constraints that could be set by them outlined
Touchstone Stars: Highlights from the Cool Stars 18 Splinter Session
We present a summary of the splinter session on "touchstone stars" -- stars
with directly measured parameters -- that was organized as part of the Cool
Stars 18 conference. We discuss several methods to precisely determine cool
star properties such as masses and radii from eclipsing binaries, and radii and
effective temperatures from interferometry. We highlight recent results in
identifying and measuring parameters for touchstone stars, and ongoing efforts
to use touchstone stars to determine parameters for other stars. We conclude by
comparing the results of touchstone stars with cool star models, noting some
unusual patterns in the differences.Comment: Proceedings of the 18th Cambridge Workshop on Cool Stars, Stellar
Systems, and the Sun, Eds G. van Belle & H. Harri
The sub-stellar birth rate from UKIDSS
We present a new sample of mid-L to mid-T dwarfs with effective temperatures of 11001700 K selected from the UKIDSS Large Area Survey (LAS) and confirmed with infrared spectra from X-shooter/Very Large Telescope. This effective temperature range is especially sensitive to the formation history of Galactic brown dwarfs and allows us to constrain the form of the sub-stellar birth rate, with sensitivity to differentiate between a flat (stellar like) birth rate and an exponentially declining form. We present the discovery of 63 new L and T dwarfs from the UKIDSS LAS DR7, including the identification of 12 likely unresolved binaries, which form the first complete sub-set from our programme, covering 495 square degrees of sky, complete to J = 18.1. We compare our results for this sub-sample with simulations of differing birth rates for objects of masses 0.10-0.03 M-circle dot and ages 1-10 Gyr. We find that the more extreme birth rates (e. g. a halo type form) can likely be excluded as the true form of the birth rate. In addition, we find that although there is substantial scatter we find a preference for a mass function, with a power-law index a in the range -1 <alpha <0 that is consistent (within the errors) with the studies of late T dwarfs.Peer reviewe
Prospecting in ultracool dwarfs : Measuring the metallicities of mid- and late-m dwarfs
© 2014. The American Astronomical Society. All rights reserved.Metallicity is a fundamental parameter that contributes to the physical characteristics of a star. The low temperatures and complex molecules present in M dwarf atmospheres make it difficult to measure their metallicities using techniques that have been commonly used for Sun-like stars. Although there has been significant progress in developing empirical methods to measure M dwarf metallicities over the last few years, these techniques have been developed primarily for early- to mid-M dwarfs. We present a method to measure the metallicity of mid- to late-M dwarfs from moderate resolution (R ∼ 2000) K-band (≃ 2.2 μm) spectra. We calibrate our formula using 44 wide binaries containing an F, G, K, or early-M primary of known metallicity and a mid- to late-M dwarf companion. We show that similar features and techniques used for early-M dwarfs are still effective for late-M dwarfs. Our revised calibration is accurate to ∼0.07 dex for M4.5-M9.5 dwarfs with -0.58 <[Fe/H] <+0.56 and shows no systematic trends with spectral type, metallicity, or the method used to determine the primary star metallicity. We show that our method gives consistent metallicities for the components of M+M wide binaries. We verify that our new formula works for unresolved binaries by combining spectra of single stars. Lastly, we show that our calibration gives consistent metallicities with the Mann et al. study for overlapping (M4-M5) stars, establishing that the two calibrations can be used in combination to determine metallicities across the entire M dwarf sequence.Peer reviewe
Weather on the Nearest Brown Dwarfs: Resolved Simultaneous Multi-Wavelength Variability Monitoring of WISE J104915.57-531906.1AB
We present two epochs of MPG/ESO 2.2m GROND simultaneous 6-band ()
photometric monitoring of the closest known L/T transition brown dwarf binary
WISE J104915.57-531906.1AB. We report here the first resolved variability
monitoring of both the T0.5 and L7.5 components. We obtained 4 hours of focused
observations on the night of UT 2013-04-22, as well as 4 hours of defocused
(unresolved) observations on the night of UT 2013-04-16. We note a number of
robust trends in our light curves. The and light curves appear to be
anticorrelated with and for the T0.5 component and in the unresolved
lightcurve. In the defocused dataset, appears correlated with and
and anticorrelated with and , while in the focused dataset we measure
no variability for at the level of our photometric precision, likely due to
evolving weather phenomena. In our focused T0.5 component lightcurve, the
band lightcurve displays a significant phase offset relative to both and
. We argue that the measured phase offsets are correlated with atmospheric
pressure probed at each band, as estimated from 1D atmospheric models. We also
report low-amplitude variability in and intrinsic to the L7.5
component.Comment: 14 pages, 5 figures, accepted to ApJ Letter
Four new T dwarfs identified in PanSTARRS 1 commissioning data
A complete well-defined sample of ultracool dwarfs is one of the key science
programs of the Pan-STARRS 1 optical survey telescope (PS1). Here we combine
PS1 commissioning data with 2MASS to conduct a proper motion search
(0.1--2.0\arcsec/yr) for nearby T dwarfs, using optical+near-IR colors to
select objects for spectroscopic followup. The addition of sensitive far-red
optical imaging from PS1 enables discovery of nearby ultracool dwarfs that
cannot be identified from 2MASS data alone. We have searched 3700 sq. deg. of
PS1 y-band (0.95--1.03 um) data to y19.5 mag (AB) and J16.5
mag (Vega) and discovered four previously unknown bright T dwarfs. Three of the
objects (with spectral types T1.5, T2 and T3.5) have photometric distances
within 25 pc and were missed by previous 2MASS searches due to more restrictive
color selection criteria. The fourth object (spectral type T4.5) is more
distant than 25 pc and is only a single-band detection in 2MASS. We also
examine the potential for completing the census of nearby ultracool objects
with the PS1 3 survey.Comment: 25 pages, 8 figures, 5 table, AJ accepted, updated to comply with
Pan-STARRS1 naming conventio
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