Rotational Velocities of Individual Components in Very Low Mass Binaries

Abstract

We present rotational velocities for individual components of 11 very low mass (VLM) binaries with spectral types between M7 and L7.5. These results are based on observations taken with the near-infrared spectrograph, NIRSPEC, and the Keck II laser guide star adaptive optics system. We find that the observed sources tend to be rapid rotators (v sin i > 10 km s −1 ), consistent with previous seeing-limited measurements of VLM objects. The two sources with the largest v sin i, LP 349−25B and HD 130948C, are rotating at ∼30% of their break-up speed, and are among the most rapidly rotating VLM objects known. Furthermore, five binary systems, all with orbital semimajor axes 3.5 AU, have component v sin i values that differ by greater than 3σ . To bring the binary components with discrepant rotational velocities into agreement would require the rotational axes to be inclined with respect to each other, and that at least one component is inclined with respect to the orbital plane. Alternatively, each component could be rotating at a different rate, even though they have similar spectral types. Both differing rotational velocities and inclinations have implications for binary star formation and evolution. We also investigate possible dynamical evolution in the triple system HD 130948A−BC. The close binary brown dwarfs B and C have significantly different v sin i values. We demonstrate that components B and C could have been torqued into misalignment by the primary star, A, via orbital precession. Such a scenario can also be applied to another triple system in our sample, GJ 569A−Bab. Interactions such as these may play an important role in the dynamical evolution of VLM binaries. Finally, we note that two of the binaries with large differences in component v sin i, LP 349−25AB and 2MASS 0746+20AB, are also known radio sources.

High-Precision Radio and Infrared Astrometry of LSPM J1314+1320ab. Ii. Testing Pre-Main-Sequence Models at the Lithium Depletion Boundary With Dynamical Masses

“…Given that we have shown that the component masses are within 2% of each other for this coeval, co-compositional binary system, one likely explanation for the divergent behavior is a difference in the angular momentum evolution of the two components. Projected rotational velocities (v i sin ) of other very-low-mass binaries have hinted at such differences in angular momentum evolution (e.g., Konopacky et al 2012), but such measurements have yet to be obtained for LSPMJ1314+1320AB. The components of LSPMJ1314+1320AB are now the nearest, lowest mass pre-main-sequence stars with direct mass measurements.…”

Linear polarization of rapidly rotating ultracool dwarfs

“…We selected 18 bright (typically J < 14.1 mag) ultracool dwarfs with spectral types ranging from M7 through T2 and published projected rotational velocities, v sin i ≥ 30 km s −1 (Mohanty & Basri 2003;Zapatero Osorio et al 2006;Reiners & Basri 2008;Blake et al 2010;Reiners & Basri 2010;Konopacky et al 2012;Deshpande et al 2012). All dwarfs are observable from northern astronomical observatories and sufficiently bright at near-infrared wavelengths to achieve accurate polarimetric photometry (σ P ≤ 0.2%) using short exposures and 4-m class telescopes.…”

“…As seen from the figure, our sample includes some of the fastest ultracool rotators known to date. In the target sample, there are three binary dwarfs, of which two (LP 349−25AB and LP 415−20AB) are resolved, and v sin i values are available for each component separately (Konopacky et al 2012). The components of the pairs have similar spectral types (Table 1) and magnitudes: ΔJ = 0.84 ± 0.15 mag for LP 415-20 (Siegler et al 2003), and ΔJ = 0.35 ± 0.03 mag for LP 349-25 (Dupuy et al 2010).…”

“…Additionally, ultracool dwarfs have high values of projected rotational velocities (v sin i), indicating that they are indeed rapid rotators (Blake et al 2010;Konopacky et al 2012;Mohanty & Basri 2003;Reiners & Basri 2008Zapatero Osorio et al 2006). This result combined with convection and the presence of dust could give rise to intricated atmospheric dynamics, likely generating periodic and non-periodic photometric variability as seen in some late-M, L, and T dwarfs (Bailer-Jones & Mundt 2001;Martín et al 2001;Koen 2004;Buenzli et al 2012;Khandrika et al 2013).…”

Long-termKs-band photometric monitoring of L dwarfs

“…However, L dwarfs are known to be fast rotators with spectroscopic rotational velocities ranging from ∼10 through ∼90 km s −1 at nearly one-third of the break-up velocity (Konopacky et al 2012, and references therein). By adopting a radius of 0.1-0.3 R , which is the size predicted for L-type sources with ages between 50 Myr and a few Gyr by evolutionary models (Chabrier et al 2000), the expected rotation periods of our targets lie in the interval 1-35 h. Our data do not sample these short periods of time, rather they cover hundreds (>750) to tens of thousands of rotation loops.…”