New Neighbors from 2MASS: Activity and Kinematics at the Bottom of the Main Sequence

By J. Gizis, D. Monet, I. Neill Reid, J. D. Kirkpatrick, J. Liebert, Rik J. Williams IPACCaltech, Usno Flagstaff, U. Pennsylvania, U. Arizona, Caltech. 2000

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We have combined 2MASS and POSS II data in a search for nearby ultracool (later than M6.5) dwarfs with Ks < 12. Spectroscopic follow-up observations identify 53 M7–M9.5 dwarfs and seven L dwarfs. The observed space density is 0.0045 ± 0.0008 M8–M9.5 dwarfs per cubic parsec, without accounting for biases, consistent with a mass function that is smooth across the stellar/substellar limit. We show the observed frequency of Hα emission peaks at ∼100% for M7 dwarfs and then decreases for cooler dwarfs. In absolute terms, however, as measured by the ratio of Hα to bolometric luminosity, none of the ultracool M dwarfs can be considered very active compared to earlier M dwarfs, and we show that the decrease that begins at spectral type M6 continues to the latest L dwarfs. We find that flaring is common among the coolest M dwarfs and estimate the frequency of flares at 7% or higher. We show that the kinematics of relatively active (EW > 6 Å) ultracool M dwarfs are consistent with an ordinary old disk stellar population, while the kinematics of inactive ultracool M dwarfs are more typical of a 0.5 Gyr old population. The early L dwarfs in the sample have kinematics consistent with old ages, suggesting that the hydrogen-burning limit is near spectral types L2–L4. We use the available data on M and L dwarfs to show that chromospheric activity drops with decreasing mass and temperature and that at a given (M8 or later) spectral type, the younger field (brown) dwarfs are less active than many of the older, more massive field stellar dwarfs. Thus, contrary to the well-known stellar age-activity relationship, low activity in field ultracool dwarfs can be an indication of comparative youth and substellar mass.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/55136e44b337865cb7e1af6e318eddce6b9a20a5

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Frequent Flaring in the TRAPPIST-1 System—Unsuited for Life? (2017, background): “TRAPPIST-1 indeed seems to exhibit chromospheric activity as shown by its Hα emission level (Gizis et al. 2000; Reiners & Basri 2010) and significant X-ray and EUV (XUV) radiation (Wheatley et al. 2017).”
Meeting the Cool Neighbors. IX. The Luminosity Function of M7-L8 Ultracool Dwarfs in the Field (2006, background): “; (6) Luyten (1979b, NLTT); (7) Gizis et al. (2000); (8) Forveille et al. (2005); (9) Cruz et al. (2003, Paper III); (10) This Paper; (11) Kendall et al. (2003); (12) Liebert et al. (2003); (13) Delfosse et al. (1997); (14) Koerner et al. (1999); (15) Kirkpatrick et al. (1997); (16) Wilson (2002);…”
Detection of Nine M8.0-L0.5 Binaries: The Very Low Mass Binary Population and Its Implications for Brown Dwarf and Very Low Mass Star Formation (2003, background): “5 stars with Ks < 12 mag from the list of Gizis et al. (2000).”
A Census of the Chamaeleon I Star-forming Region (2004, background): “For the case of Hα, I have selected a boundary that approximates the upper limit of emission strengths observed in field dwarfs in the diagram of log W λ (Hα) versus spectral type in Figure 15 (Stauffer & Hartmann 1986; Hawley et al. 1996; Delfosse et al. 1998; Gizis et al. 2000, 2002).”
MEETING THE COOL NEIGHBORS. X. ULTRACOOL DWARFS FROM THE 2MASS ALL-SKY DATA RELEASE (2008, background): “Most were identified from follow-up observations of extremely red sources from the 2MASS survey (K00; Gizis et al, 2000; Gizis et al, 2002), the DENIS survey (Mart́ın et al, 1999; Phan-Bao et al, 2001) and the SDSS survey (Fan et al, 2000; Hawley et al, 2002).”
The Initial Mass Function in the Taurus Star-forming Region (2002, neutral): “5 V, 2MASSs J0024246 015820 and 2MASSs J1733189+463359 (Reid et al. 1999a; Gizis et al. 2000); and for M10+ III, IO Vir (Kirkpatrick, Henry, & Irwin 1997).”
Identification of New M Dwarfs in the Solar Neighborhood (2003, background): “Gizis et al. (2000) added the cooler dwarfs and found a decline in activity at spectral types later than M7.”
COLORS AND KINEMATICS OF L DWARFS FROM THE SLOAN DIGITAL SKY SURVEY (2010, methodology): “The presence and strength of Hα emission is used as an indicator of chromospheric activity in low mass stars and brown dwarfs (e.g., Gizis et al. 2000; West et al. 2004; Schmidt et al. 2007).”
Hubble Space Telescope NICMOS Observations of T Dwarfs: Brown Dwarf Multiplicity and New Probes of the L/T Transition (2006, background): “Both signatures are rare in very late-type L dwarfs and T dwarfs, as the strength and frequency of Hα emission plummets across the L dwarf regime (Gizis et al. 2000; Kirkpatrick et al. 2000; Mohanty & Basri 2003; West et al. 2004), while Li I absorption becomes increasingly difficult to detect…”
THE BROWN DWARF KINEMATICS PROJECT I. PROPER MOTIONS AND TANGENTIAL VELOCITIES FOR A LARGE SAMPLE OF LATE-TYPE M, L, AND T DWARFS (2008, background): “In the past decade a number of groups have conducted astrometric surveys of UCDs, including subsets of low mass objects (e.g. Vrba et al. 2004, Dahn et al. 2002, Gizis et al. 2000, Tinney et al. 2003, Schmidt et al. 2007, Jameson et al. 2007, Osorio et al. 2007, and West et al. 2008, 2006).”
New Low-Mass Members of the Taurus Star-forming Region (2003, background): “…the next section, we find log (LHα/Lbol) = −2.6±0.12, which is slightly higher than the strongest emission observed in late-M dwarfs (Schneider et al. 1991; Mart́ın et al. 1999; Liebert et al. 1999) and two orders of magnitude greater than the typical values for these objects (Gizis et al. 2000).”
Radio Observations of a Large Sample of Late M, L, and T Dwarfs: The Distribution of Magnetic Field Strengths (2006, background): “However, beyond spectral type M7 there is a precipitous drop in H and X-ray persistent activity, and only a few percent of the objects exhibit flares (e.g., Reid et al. 1999; Gizis et al. 2000; Rutledge et al. 2000; Liebert et al. 2003; West et al. 2004).”
Flaring up All Over—Radio Activity in Rapidly Rotating Late M and L Dwarfs (2001, background): “It shows evidence for weak persistent Hα emission, as well as Hα flares, with an increase of∼ 30 in the line equivalent width (Martín, Rebolo & Magazzu 1994; Gizis et al. 2000).”
New spectral types L and T (2005, background): “…et al. 2000 0 3 SDSS early-T “missing link” dwarfs Kirkpatrick et al. 2000 64 0 more 2MASS L dwarfs Burgasser et al. 2000b 0 1 2MASSI J0559191−140448 Gizis et al. 2000 4 0 more 2MASS L dwarfs Zapatero Osorio et al. 2000 10 0 more SOri L dwarfs Gizis, Kirkpatrick, & Wilson 2001 1 0 GJ…”
THE HAWAII INFRARED PARALLAX PROGRAM. II. YOUNG ULTRACOOL FIELD DWARFS (2016, neutral): “…(28) Esposito et al. (2013); (29) Faherty et al. (2012); (30) Faherty et al. (2013); (31) Gagné et al. (2014a); (32) Gagné et al. (2015a); (33) Gizis et al. (2000); (34) Gizis (2002); (35) Goldman et al. (2010); (36) Guenther et al. (2001); (37) Hawley et al. (2002); (38) Henry et al.…”
The Spectra of T Dwarfs. II. Red Optical Data (2003, background): “Gizis et al. (2000) have noted that for late-type M and L field dwarfs, H emission is generally found among the older and hence more massive objects.”
Multiplicity of Nearby Free-floating Ultracool Dwarfs: A Hubble Space Telescope WFPC2 Search for Companions (2003, methodology): “The photometric distance given by Gizis et al. (2000) is 22.9 pc considering a single object.”