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Low-Mass Stars and Brown Dwarfs in 2MASS

The current estimate of the density of L dwarfs is ~1 per 10 square degrees, or ~4000 L dwarfs in the whole sky, for Ks<=15.0. Preliminary calculations for the mass function indicate that L dwarfs (and the methane-dominated T dwarfs) are twice as numerous as stars. However, they make up only a small fraction of the mass of the Milky Way Galaxy, a mere 15% of what the stars comprise.

Preprints and other materials for papers on low-mass stars and brown dwarfs




 

Dwarfs Cooler than ``M'': The Definition of Spectral Type ``L'' using Discoveries from the Two Micron All Sky Survey (2MASS)

J. Davy Kirkpatrick (IPAC/Caltech), I. Neill Reid (Caltech), J. Liebert (UofA), R.M. Cutri, B. Nelson, C.A. Beichman (IPAC/Caltech), C.C. Dahn, D.G. Monet (USNO-FS), J.E. Gizis, M.F. Skrutskie (UMass)

The Astrophysical Journal, 519, 802 (1999).

Abstract: Before the Two Micron All Sky Survey (2MASS) began, only six objects were known with spectral types later than M9.5 V. In the first 371 sq. deg. of actual 2MASS survey data, we have identified another twenty such objects spectroscopically confirmed using the Low Resolution Imaging Spectrograph (LRIS) at the W. M. Keck Observatory. Because the TiO and VO bands which dominate the far-optical portions of late-M spectra disappear in these cooler dwarfs, we define a new spectral class - ``L'' - where metallic oxides are replaced by metallic hydrides and neutral alkali metals as the major spectroscopic signatures. We establish classification indices and type all twenty-five L dwarfs. The twenty-sixth ``post-M9.5'' object - Gl 229B - is the prototype of a methane-dominated spectral class which we propose as class ``T''. At least five of the twenty 2MASS L dwarfs show the 6708 Å lithium doublet at low resolution, the strongest having an equivalent width of 18.5 Å. For objects this cool, the presence of lithium proves that they are substellar. Two other 2MASS objects appear to have lithium lines at the limit of our detectability, which if verified means that at least a third of our L dwarfs are bona fide brown dwarfs. All of the 2MASS brown dwarfs discovered so far have J-Ks > 1.30. We have not yet, despite deliberately searching for them, found any brown dwarfs with colors resembling Gl 229B (J-Ks ~ -0.1).

Example spectra of L dwarfs and comparisons to late-M dwarfs and Gl 229B:

Color-color diagram showing locations of M dwarfs, L dwarfs, and Gl 229B:

 

An Improved Optical Spectrum and New Model Fits of the Likely Brown Dwarf GD 165B

J. Davy Kirkpatrick (IPAC/Caltech), France Allard (Lyon), Tom Bida (Keck), Ben Zuckerman, E.E. Becklin (UCLA), Gilles Chabrier, and Isabelle Baraffe (Lyon)

The Astrophysical Journal, 519, 834 (1999).

Abstract: Long thought by some researchers to be an oddity, GD 165B has instead proven to be the first example of a class of very cool objects (the L dwarfs) which, due to dust formation in their photospheres, lack the dominant bands of TiO seen in warmer M dwarfs. Here we present an improved optical (6200-10300 Å) spectrum of GD 165B and identify its prominent spectral features. Among these are newly identified bands of FeH and the first identification of CrH bands in a dwarf. We use the latest generation of model atmospheres, which include the effects of condensation and dust opacities, to derive values of Teff =1900±100K and log(g) = 5.0±0.5 for GD 165B. We also derive a crude age of 1.2 to 5.5 Gyr for the GD 165 system. A comparison of the temperature and age of GD 165B to evolutionary models predicts GD 165B to be an object in the transition zone between stars and brown dwarfs. Further observational evidence - the discovery of lithium in other dwarfs spectroscopically similar to GD 165B and the scarcity of GD 165B-like companions found by imaging surveys - favors a substellar interpretation for this object. We argue that the weight of this observational evidence together with the known shortcomings of the evolutionary models (which do not yet include opacity by grains) indicate that GD 165B is probably a brown dwarf.

 

A 2MASS Ultracool M Dwarf Observed in a Spectacular Flare

J. Liebert (UofA), J. Davy Kirkpatrick (IPAC/Caltech), I. Neill Reid (Caltech), M.D. Fisher (UofA)

The Astrophysical Journal, 519, 345 (1999).

Abstract: 2MASSW J0149090+295613 (hereafter 2M0149) is unique among the very late M/L dwarf counterparts to 2MASS point sources in having shown, in the first set of spectra taken on 1997 Dec 7, a diverse emission line spectrum at red wavelengths, featuring an H-alpha equivalent width of 300Å. On four repeat observations, however, the spectrum was that of a more ordinary dMe object, albeit with a late spectral type of M9.5 V. Our interpretation is that the object underwent an extreme magnetic flare event. The continuum flux at the short wavelength end of the spectrum was also several times stronger during the apparent flare than in quiescence. Comparison is made with mid-type M dwarfs of higher mass which have undergone strong flares, and with another known M9.5 emission line object PC0025+0047. The 2MASS object adds to the evidence that magnetic dynamo eruptions may release more energy relative to Lbol as the stellar mass decreases; if the total flare luminosity scales with H-alpha in the same way as a well-observed event of the middle-M dwarf AD Leo, 2M0149 may have had a flare luminosity which approached or exceeded its quiescent Lbol during the brief impulse phase. In contrast, its quiescent state shows an unremarkable level of chromospheric activity, which leads us to question whether it is unique at all; rather, it may be that many of the ultracool M and L dwarfs found by DENIS and 2MASS undergo similar flares.

 

L Dwarfs and the Substellar Mass Function

I. Neill Reid (Caltech), J. Davy Kirkpatrick (IPAC/Caltech), J. Liebert, A. Burrows (UofA), J. Gizis (UMass), A. Burgasser (Caltech), C.C. Dahn, D. Monet (USNO-FS), R. Cutri, C.A. Beichman (IPAC/Caltech), M. Skrutskie (UMass)

The Astrophysical Journal, 521, 613 (1999).

Abstract: Analysis of initial observations sky surveys has shown that the resulting photometric catalogues, combined with far-red optical data, provide an extremely effective method of finding isolated, very low-temperature objects in the general field. Follow-up observations have already identified more than 25 sources with temperatures cooler than the latest M dwarfs. A comparison with detailed model predictions (Burrows & Sharp) indicates that these L dwarfs have effective temperatures between ~2000± 100 K and 1500 ± 100 K, while the available trigonometric parallax data place their luminosities at between 10-3.5 and 10-4.3 Lsun. Those properties, together with the detection of lithium in one-third of the objects, are consistent with the majority having substellar masses. The mass function cannot be derived directly, since only near-infrared photometry and spectral types are available for most sources, but we can incorporate VLM/brown dwarf models in simulations of the Solar Neighborhood population and constrain Psi(M) by comparing the predicted L-dwarf surface densities and temperature distributions against observations from the DENIS and 2MASS surveys. The data, although sparse, can be represented by a power-law mass function, Psi(M) propto M-alpha, with 1 < alpha < 2. Current results favor a value nearer the lower limit. If alpha = 1.3, then the local space density of 0.075 > M / Msun > 0.01 brown dwarfs is 0.10 systems pc-3. In that case brown dwarfs are twice as common as main-sequence stars, but contribute no more than ~15 % of the total mass of the disk.

 

A 2MASS Survey for Brown Dwarfs Towards the Hyades

John E. Gizis (U. Mass), I. Neill Reid (Caltech), David G. Monet (USNO-FS)

The Astronomical Journal, 118, 997 (1999).

Abstract: We present results of a search for very low mass stars and brown dwarfs towards the central region of the Hyades cluster. Using Two Micron All Sky Survey (2MASS) near-infrared and Second Palomar Sky Survey (POSSII) optical photometry, we select candidates corresponding to spectral types M8 through L4 at the distance of the Hyades. Spectroscopic followup data indicate that none are cluster members. The lack of members in the mass range 0.08 < M < 0.06 Msun is consistent with prior evidence that this 600 Myr old cluster is deficient in very low mass objects. Three of our objects are spectral type M8, with surface gravity sensitive features intermediate between dwarfs and giants. We interpret these objects as background brown dwarfs, perhaps associated with the Taurus star formation region.

 

Discovery of Four Field Methane (T-type) Dwarfs with 2MASS

Adam J. Burgasser, J. Davy Kirkpatrick (IPAC), Michael E. Brown (Caltech), I. Neill Reid (UPenn), John E. Gizis (UMass, Conard C. Dahn, David G. Monet (USNO-FS), Charles A. Beichman (JPL), James Liebert (UofA), Roc M. Cutri (IPAC), & Michael F. Skrutskie (UMass)

Astrophysical Journal Letters, 522, L65 (1999).

Abstract: We report the discovery of four field methane (T-type) brown dwarfs using 2MASS survey data. One additional methane dwarf, previously discovered by SDSS, was also identified. Near-infrared spectra clearly show the 1.6 and 2.2 µm CH4 absorption bands characteristic of objects with Teff ~ 1300 K, as well as broadened H2O bands at 1.4 and 1.9 µm. Comparing the spectra of these objects with Gl 229B, we propose that all new 2MASS T dwarfs are warmer than 950 K, in order from warmest to coolest: 2MASS J1217-03, J1225-27, J1047+21 and J1237+65. Based on this preliminary sample, we find a warm T dwarf surface density of 0.0022 T dwarfs deg-2, or ~ 90 warm T dwarfs over the whole sky detectable to J < 16. The resulting space density upper limit, 0.01 T dwarfs/pc3, is comparable to that of the L dwarf sample from Kirkpatrick et al. (1999).

Example spectra of the 2MASS T dwarfs and comparison to Gl 229B:

 

Two Nearby M Dwarf Binaries from 2MASS

John E. Gizis (UMass), David G. Monet (USNO-FS), I. Neill Reid (UPenn), J. Davy Kirkpatrick (IPAC), & Adam J. Burgasser (Caltech)

Monthly Notices of the Royal Astronomical Society, 311, 385 (2000).

Abstract: We report the discovery of two binary M dwarf systems in the immediate solar neighborhood using 2MASS. The first is an M6.5 companion to the nearby G star HD 86728 (Gl 376). The known properties of HD 86728 indicate that the M dwarf (Gl 376B) is old, metal-rich and only 14.9 parsecs away. The M dwarf is highly active with both H alpha and X-ray emission. Thus, Gl 376B offers the opportunity to study an old, bright, active M dwarf with known metallicity, age, and luminosity. We show that it is probable that Gl 376B is itself an unresolved pair. The other system consists of an M6.5 and an M8 dwarf with 14.5 arcseconds separation. We estimate a distance of ~16 parsecs for this very low mass pair. Stronger activity is observed in the M6.5 dwarf, supporting evidence that chromospheric activity is weakening near the hydrogen burning limit.

 

Four Nearby L Dwarfs

I. Neill Reid (UPenn), J. Davy Kirkpatrick (IPAC), John E. Gizis (UMass), Conard C. Dahn, David G. Monet (USNO-FS), Rik J. Williams (Caltech), James Liebert (UArizona), & Adam J. Burgasser (Caltech)

The Astronomical Journal, 119, 369 (2000).

Abstract: We present spectroscopic, photometric and astrometric observations of four bright L dwarfs identified in the course of the 2MASS near-infrared survey. Our spectroscopic data extend to wavelengths shortward of 5000 Ang. in the L0 dwarf 2MASSJ0746+2000 and the L4 dwarf 2MASSJ0036+1840, allowing the identification of absorption bands due to MgH and CaOH. The atomic resonance lines Ca I 4227 Ang. and Na I 5890/5896 Ang. are extremely strong, with the latter having an equivalent width of 240 Ang. in the L4 dwarf. By spectral type L5, the D lines extend over $\sim1000$Ang and absorb a substantial fraction of the flux emitted in the V band, with a corresponding effect on the (V-I) broadband colour. The KI resonance doublet at 7665/7699 Ang. increases in equivalent width from spectral type M3 to M7, but decreases in strength from M7 to L0 before broadening substantially at later types. These variations are likely driven by dust formation in these cool atmospheres.

 

BRI0021-0214: Another Surprise at the Bottom of the Main Sequence

I.N. Reid (UPenn), J.D. Kirkpatrick, J.E. Gizis (IPAC), & J. Liebert (UofA)

The Astrophysical Journal, 527, L105 (2000).

Abstract: We report the detection of an H alpha flare on the low-luminosity M9.5 dwarf BRI 0021-0214. This star has rapid rotation, v sin(i) ~ 40 km s-1, but generally shows no significant chromospheric emission. Our detection of the flare shows that a magnetic field is present, although the level of activity at maximum is 3 times lower than the mean quiescent level in early- and mid-type M dwarfs. Based on data available in the literature, we estimate that the star is in outburst for no more than 7% of the time.

 

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