Chromospheric Activity, Rotation, and Rotational Braking in M and L Dwarfs

By A. Reiners, G. Basri. 2008

208 citations recorded by the source corpus.

We present results from a high-resolution spectroscopic survey of 45 L dwarfs, which includes both very low mass stars and brown dwarfs. Our spectra allow us to derive a significant number of new rotational velocities, and discover a slowly rotating (in projected velocity) L dwarf that allows more accurate measurement of spectroscopic rotations for these objects. We measure chromospheric activity (and often its variability) through the Hα emission line. Our primary new result is good evidence that magnetic braking dominates the angular momentum evolution of even brown dwarfs, although spindown times appear to increase as mass decreases. We confirm that activity decreases as effective temperature decreases, although a larger fraction of L dwarfs are active than has previously been reported. Essentially all active objects are also variable. We confirm the lack of a rotation-activity connection for L dwarfs. We find a minimum limit for rotational velocities that increases with later spectral types, rising from near zero in older mid-M stars to more than 20 km s−1 for mid-L objects. There is strong evidence that all L dwarfs are rapid rotators. We derive a braking law that can depend on either temperature or mass which can explain all the rotational results and provides an age dependence for the angular momentum evolution. It is clear that angular momentum loss mechanisms in smaller and cooler objects become more inefficient, starting at the fully convective boundary.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/8baff255770e482128f651918317883f2a878196

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In Astronomy and Astrophysics

1. M. Marley
2. Michael C. Liu
3. K. Luhman
4. A. Reiners
5. D. Charbonneau
6. Jon Fortney
7. A. Burgasser
8. D. Saumon

Citations in the corpus

• A VOLUME-LIMITED SAMPLE OF 63 M7–M9.5 DWARFS. II. ACTIVITY, MAGNETISM, AND THE FADE OF THE ROTATION-DOMINATED DYNAMO (2009, methodology): “Hα Emission Our measurement of Hα emission follows the procedure outlined in Reiners & Basri (2008).”
• LARGE-AMPLITUDE VARIATIONS OF AN L/T TRANSITION BROWN DWARF: MULTI-WAVELENGTH OBSERVATIONS OF PATCHY, HIGH-CONTRAST CLOUD FEATURES (2012, background): “As noted earlier, the derived 7.721±0.005 hr periodicity would be consistent with rotation rates of ultracool dwarfs (∼2- 12 hrs Reiners & Basri 2008) inferred from v sin i data, whereas a double-peaked light curve with a 15.44 hr period would correspond to a slower rotator (this is not…”
• AN ALL-SKY CATALOG OF BRIGHT M DWARFS (2011, result): “In contrast to late-type M stars, the vast majority of early M dwarfs are not rapid rotators or chromospherically active (WHα > 1Å) (Bochanski et al. 2005; West et al. 2008; Reiners & Basri 2008) and are thus amenable to these search techniques.”
• HYDROGEN GREENHOUSE PLANETS BEYOND THE HABITABLE ZONE (2011, background): “However, these stars, especially those of mid- to late-M spectral type, may vary greatly in their EUV flux (Reiners & Basri 2008; Browning et al. 2010).”
• THE 0.8–14.5 μm SPECTRA OF MID-L TO MID-T DWARFS: DIAGNOSTICS OF EFFECTIVE TEMPERATURE, GRAIN SEDIMENTATION, GAS TRANSPORT, AND SURFACE GRAVITY (2009, background): “We note that rotational velocities have been reported by Zapatero Osorio et al. (2006) and Reiners & Basri (2008) for seven L dwarfs in our sample (Table 2), but no correlation between fsed and rotation is apparent in this small sample.”
• Magnetic Fields of Nondegenerate Stars (2009, background): “…from empirical modelling of distributions of, e.g., rotation periods in young open clusters of various ages (e.g., Bouvier 2007), or activity levels andli e broadening at various ages or vertical distances from the Galactic plane (e.g., Delfosse et al.1998; West et al. 2008; Reiners & Basri 2008).”
• DETECTING PLANETS AROUND VERY LOW MASS STARS WITH THE RADIAL VELOCITY METHOD (2009, methodology): “For example, Reiners & Basri (2008) show that rapid rotation is more frequent in cooler M dwarfs.”
• Chromospheric activity catalogue of 4454 cool stars (2018, result): “This result is consistent with previous work on low-mass stars (Mohanty & Basri 2003; Reiners & Basri 2008).”
• Observations of Cool-Star Magnetic Fields (2012, background): “It is well known that early-M dwarfs (M0 – M3) in the field are generally much less active and slower-rotating than later, fully convective M stars (e.g., Delfosse et al., 1998; Reiners and Basri, 2008; Reiners et al., 2012).”
• ON THE ANGULAR MOMENTUM EVOLUTION OF FULLY CONVECTIVE STARS: ROTATION PERIODS FOR FIELD M-DWARFS FROM THE MEarth TRANSIT SURVEY (2010, methodology): “Previous studies have used tangential velocity (e.g. Reiners & Basri 2008), but this does not take into account the bulk motions of the various Galactic populations along each line of sight through the Galaxy.”
• THE NIRSPEC ULTRACOOL DWARF RADIAL VELOCITY SURVEY (2010, methodology): “Comparing our V sin i measurements to those from Reiners & Basri (2008) for objects with spectral types later than or equal to L4 with a two-sided K-S test indicates a probability of 31% that the two samples are drawn from the same underlying distribution.”
• The Formation and Early Evolution of Low-Mass Stars and Brown Dwarfs (2012, background): “Measurements for older brown dwarfs in the field indicate that they do eventually spin down, but on much longer timescales than stars (Blake, Charbonneau & Russel 2010; Irwin et al. 2011; Mohanty & Basri 2003; Reiners & Basri 2008; Scholz et al. 2011; Zapatero Osorio et al. 2006).”
• THE EVOLUTION OF CATACLYSMIC VARIABLES AS REVEALED BY THEIR DONOR STARS (2011, background): “Broadly speaking, there is evidence for systematic changes in both magnetic field strength and topology with spectral type (e.g., Reiners & Basri 2008, 2009, 2010; Donati et al. 2008; Morin et al. 2010).”
• RADIUS-DEPENDENT ANGULAR MOMENTUM EVOLUTION IN LOW-MASS STARS. I (2011, background): “…data for low-mass stars has been gathered over the past decades, both in the field and in clusters of various ages, in order to determine the precise rules underlying the qualitative picture of angular momentum evolution painted above (Barnes 2007; Reiners & Basri 2008; Irwin & Bouvier 2009).”