YOUNG L DWARFS IDENTIFIED IN THE FIELD: A PRELIMINARY LOW-GRAVITY, OPTICAL SPECTRAL SEQUENCE FROM L0 TO L5

By Kelle Cruz, J. D. Kirkpatrick, A. Burgasser. 2008

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We present an analysis of 23 L dwarfs whose optical spectra display unusual features. Twenty-one were uncovered during our search for nearby, late-type objects using the Two Micron All Sky Survey, while two were identified in the literature. The unusual spectral features, notably weak FeH molecular absorption and weak Na i and K i doublets, are attributable to low gravity and indicate that these L dwarfs are young, low-mass brown dwarfs. We use these data to expand the spectral classification scheme for L0- to L5-type dwarfs to include three gravity classes. Most of the low-gravity L dwarfs have southerly declinations and distance estimates within 60 pc. Their implied youth, on-sky distribution, and distances suggest that they are members of nearby, intermediate-age (∼10–100 Myr), loose associations such as the β Pictoris moving group, the Tucana/Horologium Association, and the AB Doradus moving group. At an age of 30 Myr and with effective temperatures from 1500 to 2400 K, evolutionary models predict masses of 11–30 MJupiter for these objects. One object, 2M 0355+11, with J − Ks = 2.52 ± 0.03, is the reddest L dwarf found in the field and its late spectral type and spectral features indicative of a very low gravity suggest it might also be the lowest mass field L dwarf. However, before ages and masses can be confidently adopted for any of these low-gravity L dwarfs, additional kinematic observations are needed to confirm cluster membership.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/8590e6ba209e1dd085c6391175f9995742885e67

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DISCOVERIES FROM A NEAR-INFRARED PROPER MOTION SURVEY USING MULTI-EPOCH TWO MICRON ALL-SKY SURVEY DATA (2010, methodology): “As has been true in other studies (Kirkpatrick et al. 2008; Cruz et al. 2009), these young field objects tend to be found in the southern sky where, not coincidentally, most of the young associations are concentrated.”
WEATHER ON OTHER WORLDS. II. SURVEY RESULTS: SPOTS ARE UBIQUITOUS ON L AND T DWARFS (2014, methodology): “The youth of three of these (2MASS J04210718-6306022 [L5β], 2MASSI J1726000+153819 [L3β], and 2MASSW J2208136+292121 [L3γ]) is discussed in Cruz et al. (2009) and Gagné et al. (2014).”
A NEAR-INFRARED SPECTROSCOPIC STUDY OF YOUNG FIELD ULTRACOOL DWARFS (2013, methodology): “To date, these studies have mainly focused on the optical spectroscopic properties of low-gravity M and L field dwarfs (Cruz et al. 2009; Kirkpatrick et al. 2008).”
THE HAWAII INFRARED PARALLAX PROGRAM. II. YOUNG ULTRACOOL FIELD DWARFS (2016, background): “2MASS J03552337+1133437 (L5γ/L3 vl-g): This object has the reddest J −K color in our sample, and both Cruz et al. (2009) and Allers & Liu (2013a) characterize it as very low gravity.”
COLORS AND KINEMATICS OF L DWARFS FROM THE SLOAN DIGITAL SKY SURVEY (2010, background): “Additionally, a majority of the spectroscopically young red outliers have positions coincident with young moving groups (Cruz et al. 2009), while many of the spectroscopically peculiar blue outliers have fast velocities consistent with being members of an older population.”
The Field Substellar Mass Function Based on the Full-sky 20 pc Census of 525 L, T, and Y Dwarfs (2020, background): “…Kirkpatrick et al. 2000, (106) Bardalez Gagliuffi, et al. 2014, (107) Reid, et al. 2000, (108) Knapp et al. 2004, (109) Wilson, et al. 2003, (110) Cruz et al. 2009, (111) Kendall et al. 2007, (112) Reid et al. 2008, (113) Liebert, et al. 2003, (114) Schneider, et al. 2014, (115) Cruz, et al.…”
THE HAWAII INFRARED PARALLAX PROGRAM. I. ULTRACOOL BINARIES AND THE L/T TRANSITION, (2012, background): “…that would benefit from a larger set of distance measurements: young field brown dwarfs (e.g., Kirkpatrick et al. 2006; Allers et al. 2009; Cruz et al. 2009), the coldest brown dwarfs (.500 K, e.g., Lucas et al. 2010; Cushing et al. 2011), and the L/T transition (e.g., Liu et al. 2006;…”
THE MASS–RADIUS(–ROTATION?) RELATION FOR LOW-MASS STARS (2010, background): “…parallaxes have led to the measurement of highly precise luminosities (Deacon et al. 2005; Henry et al. 2006; Lepine et al. 2009), and temperature and gravity calibrations (while still systematically uncertain) have been significantly refined (Luhman et al. 2003; Lyo et al. 2008; Cruz et al. 2009).”
FUNDAMENTAL PARAMETERS AND SPECTRAL ENERGY DISTRIBUTIONS OF YOUNG AND FIELD AGE OBJECTS WITH MASSES SPANNING THE STELLAR TO PLANETARY REGIME (2015, background): “Since it is well documented that the colors of ultracool dwarfs are sensitive to age (Kirkpatrick et al. 2006, 2008; Cruz et al. 2009; Faherty et al. 2009; Allers et al. 2010; Bihain et al. 2010; Faherty et al. 2012; Schmidt et al. 2010), we derived one polynomial for the field age objects (MFLD)…”
BANYAN. VII. A NEW POPULATION OF YOUNG SUBSTELLAR CANDIDATE MEMBERS OF NEARBY MOVING GROUPS FROM THE BASS SURVEY (2015, methodology): “If this system is younger than∼ 200 Myr, the individual mass of each component would be well below the deuterium burning limit, which would make it a remarkable benchmark system to understand the properties of planetary-mass objects.”
THE DISK POPULATION OF THE TAURUS STAR-FORMING REGION (2009, background): “…& Steeghs 2004; Luhman 2004a; Lyo et al. 2004; Song et al. 2004; Zuckerman & Song 2004; Mamajek 2005; Scholz et al. 2005; Looper et al. 2007; Luhman et al. 2008a; Kastner et al. 2008) and young late-type dwarfs in the solar neighborhood (Kirkpatrick et al. 2006, 2008; Cruz et al. 2007, 2009).”
SpeX SPECTROSCOPY OF UNRESOLVED VERY LOW MASS BINARIES. I. IDENTIFICATION OF 17 CANDIDATE BINARIES STRADDLING THE L DWARF/T DWARF TRANSITION (2009, background): “…Muench et al. 2007), and sources specifically noted to have peculiar spectra associated with low surface gravities, subsolar metallicites or unusual cloud properties (e.g., Lucas & Roche 2000; Burgasser et al. 2003a; Knapp et al. 2004; Chiu et al. 2006; Cruz et al. 2007, 2009; Looper et al. 2008b).”
BANYAN. II. VERY LOW MASS AND SUBSTELLAR CANDIDATE MEMBERS TO NEARBY, YOUNG KINEMATIC GROUPS WITH PREVIOUSLY KNOWN SIGNS OF YOUTH (2013, background): “2MASS J23225299–6151275 is an L2γ BD with signs of low gravity in its optical spectrum and NIR colors unusually red for its spectral type (Reid et al. 2008a, Cruz et al. 2009, Faherty et al. 2013b).”
AN INFRARED/X-RAY SURVEY FOR NEW MEMBERS OF THE TAURUS STAR-FORMING REGION (2009, methodology): “We classified this object through a comparison to standard L dwarfs (Kirkpatrick et al. 1999) and young L dwarfs in the field (τ ∼ 10-100 Myr, Kirkpatrick et al. 2006; Cruz et al. 2009), arriving at a spectral type of L0 (see Figure 2).”