Cool Stars 18 Proceedings: Touchstone Stars Splinter Session

Unable to sustain nuclear fusion in their cores due to insufficient mass, all brown dwarfs are about the radius of Jupiter, emit primarily in the infrared, and are degenerate across effective temperature, mass, and age. These observational challenges make stellar direct measurement methods such as interferometry and asteroseismology unfeasible and dynamical mass measurements very difficult for a large, diverse sample. However, spectral energy distributions (SED) with optical through mid-infrared data allow detailed spectroscopic study of these objects over a broad wavelength baseline. With the addition of parallaxes, precise bolometric luminosities (\(L_{bol}\)) become a robust direct measurement by which we can investigate the effects of different fundamental parameters on the global characteristics of brown dwarfs.

We construct 175 near-complete SEDs for the entire sequence of late-M, L and T dwarfs with optical and near-infrared (NIR) spectra from the BDNYC Data Archive combined with mid-infrared (MIR) data from the Wide-field Infrared Survey Explorer (WISE; (Wright 2008)) and the Spitzer Space Telscope (Fazio et al. 2004; Houck et al. 2004). Objects with optical signatures of low surface gravity (\(\beta\) or \(\gamma\); (Kirkpatrick 2005), (Cruz 2009)) or membership in nearby young (10-150 Myr) moving groups (NYMGs; Faherty et al. in prep) are identified as 24 percent of the sample to investigate the effects of temperature, gravity, and dust/clouds on spectral morphology. We calculate \(L_{bol}\) for our sample by integration of the SEDs, flux calibrated using parallax measurements from the Brown Dwarf Kinematics Project ((Faherty 2012)) and the literature ((Dupuy 2012), (Tinney 2003), (Vrba 2004)) or published kinematic distances ((Cruz 2003); (Faherty 2009); (Schmidt 2007), (Schmidt 2010); (Reid 2006); (Delorme 2012); (Naud 2014)).

Figure 1(a,b,c) shows \(L_{bol}\) versus selected absolute magnitudes \(M_J\), \(M_{Ks}\) and \(M_{W2}\) for 54 field age, 14 low-g, and 10 NYMG member L dwarfs of our sample. The flux of low surface gravity (low-g) L dwarfs appears to be redistributed from the NIR into the MIR, primarily from J to W2, as compared to field age Ls of the same luminosity ((Faherty 2012), (Faherty 2013); (Liu 2013); (Osorio 2014); Gizis et al., submitted). Indeed we find low-g Ls are 0.5-1 magnitudes dimmer in \(M_J\) and 0.3-0.6 magnitudes brighter in \(M_{W2}\) (Filippazzo et al., in prep). This is probably due to absorption and scattering of light to longer wavelengths by diffuse, unsettled dust in the atmospheres of young objects. Additionally, \(M_{Ks}\) magnitudes appear to be largely unaffected by surface gravity making it an ideal band from which to determine age-independent bolometric corrections for L dwarfs.

The plot of \(L_{bol}\) versus spectral type (Figure 1d) shows most low-g, young, and field objects all lie along the same sequence. Qualitatively, low-g L dwarfs have larger radii than their field age counterparts of the same \(L_{bol}\) so they must have cooler photospheres according to the Stefan-Boltzmann Law. Bolometric luminosities are one of the few direct measurements we can make for brown dwarfs for identification of substellar touchstones, however, effective temperatures (\(T_{eff}\)) can also be tightly constrained using evolutionary models while minimizing our assumptions about the sources (See Filippazzo et al., in prep).

Preliminary results suggest that confirmed young objects are 100-400 K cooler than field age L dwarfs of the same spectral type. While the uncertainties on the radii of low-g (but not necessarily young) objects are large, they still fall below the track of “normal” Ls on the \(T_{eff}\) versus spectral type plot (Filippazzo et al. in prep). Consequently, surface gravity must be taken into account when using spectral type as a proxy for \(T_{eff}\). The reliance on an age insensitive temperature-spectral type relationship ((Golimowski 2004), (Stephens 2009)) might explain why some young objects appear underluminous as compared to older L dwarfs of the same temperature.

Though the complex nature of their cool atmospheres obscures the fundamental parameters of brown dwarfs, the picture of substellar touchstones becomes clearer as larger age-calibrated samples are assembled and detection methods improve.

Bolometric luminosities of field age (light filled circles), low surface gravity (dark filled squares), and members of nearby young moving groups (unfilled triangles) are plotted against absolute J, Ks and W2 magnitudes and spectral type for a selection of 54 L dwarfs. First order polynomials fits to the field objects (light dashed line) and the young and low gravity objects (dark dashed line) show distinct tracks in \(M_J\) and \(M_{W2}\) and overlapping tracks in \(M_{Ks}\).


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