Rotation and Activity in Mid-M to L Field Dwarfs

By S. Mohanty, G. Basri. 2002

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We analyze rotation velocities and chromospheric (Hα) activity, derived from high-resolution spectra, in a large sample of mid-M to L field dwarfs. The projected rotation velocity is found to increase from mid-M to L. This is consistent with a lengthening of spin-down timescale with later type, although in the L types the trend may also be a function of the observational bias toward younger objects. From M4 to M8.5 a saturation-type rotation-activity relation is seen, similar to that in earlier types, when activity is measured through either FHα or LHα/Lbol. However, we find that activity saturates at a significantly higher velocity (~10 km s-1) in the M5.5-M8.5 dwarfs than in the M4-M5 ones (≲4 km s-1). This may result from a change in the dynamo behavior with later type (see also below). We note that the saturation level in Hα emission appears to vary somewhat less with spectral type (from M4 to M8.5) when activity is measured through LHα/Lbol instead of FHα. In M9 and later dwarfs, we observe a drastic drop in activity and a sharp break in the rotation-activity connection: Hα emission levels in these dwarfs are much lower than in earlier types, and often undetectable, in spite of very rapid rotation. This may be caused by the very high resistivities in the predominantly neutral atmospheres of these dwarfs, which would damp the magnetic energy available for supporting a chromosphere. It is also possible that the rapid formation of dust in these cool atmospheres exacerbates this effect, as charged particles are soaked up by (more massive) dust grains. Finally, we note that spectral type determination from low-resolution spectra may be affected by gravity effects: cooler, lower gravity objects may mimic hotter, higher gravity ones. Therefore, it is possible that the few unsaturated fast rotators from M5.5 to M8.5 (whose presence leads us to ascribe a higher saturation velocity to these spectral types, as noted above) may actually be very low mass objects, with lower Teff (and gravity) than their spectral types suggest. If so, their behavior (low activity, fast rotation) would be compatible with that of the cool M9 and later dwarfs (and no change in dynamo behavior would have to be postulated in the M5.5-M8.5 dwarfs). This interpretation is supported by a preliminary analysis of the high-resolution spectra of these anomolous objects. It is also bolstered by the fact that a saturation-type Rossby number-activity relation is seen in the M5.5-M8.5 dwarfs when these anomalous objects are removed from the sample, while the relationship is much weaker when they are included.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/bc5a8dc23efb67e8037676936e728acdbf10846e

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Citations in the corpus

Simulations of Dynamo Action in Fully Convective Stars (2007, background): “Mohanty & Basri (2003) argued that a sample of stars ranging from M4 to M9 exhibited a common “saturation type” rotation-activity relationship, with observed activity roughly independent of rotation rate above a threshold value.”
An Empirical Criterion to Classify T Tauri Stars and Substellar Analogs Using Low-Resolution Optical Spectroscopy (2003, neutral): “A similar diagram was obtained by Mohanty & Basri (2003) for mid-M and L field objects (see their figure 7).”
Periodic Bursts of Coherent Radio Emission from an Ultracool Dwarf (2007, background): “Ultracool dwarfs, on the other hand, are thought to possess cooler, more neutral atmospheres, as evidenced by their low Hα and X-ray luminosities (Mohanty & Basri 2003; Stelzer et al. 2006), and yet can produce high levels of radio emission, many orders of magnitude brighter than predicted by this…”
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).”
Magnetic Field Generation in Fully Convective Rotating Spheres (2004, background): “However, for less massive stars (M9 dwarfs and beyond) there is a sharp decline in chromospheric magnetic activity (e.g. Gizis et al. 2000), which may be connected with dust formation and the almost fully neutral photospheres (Mohanty & Basri 2003).”
Activity and Kinematics of Ultracool Dwarfs, Including an Amazing Flare Observation (2007, background): “At spectral types beyond M6, the activity level diminishes sharply (Mart́ın et al. 1999; Gizis et al. 2000; Burgasser et al. 2002; Mohanty & Basri 2003; West et al. 2004).”
YOUNG L DWARFS IDENTIFIED IN THE FIELD: A PRELIMINARY LOW-GRAVITY, OPTICAL SPECTRAL SEQUENCE FROM L0 TO L5 (2008, background): “— High-resolution spectroscopy has shown the rotational velocities (vrot sin i) – 13 – of normal field L dwarfs to range from 10 to 60 km s−1 (Mohanty & Basri 2003; Zapatero Osorio et al. 2007).”
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): “…Galactic U,V,W comparison with the ultracool dwarf population is not possible because radial velocity measurements for ultracool dwarfs are sparse, with only 48 of the L and T dwarfs to date having been reported in the literature (e.g. Mohanty & Basri 2003, Osorio et al. 2007, Bailer-Jones 2004).”
Chromospheric Activity, Rotation, and Rotational Braking in M and L Dwarfs (2008, methodology): “In the right panel, we show them together with the data from Delfosse et al. (1998), Reiners & Basri (2007), and Mohanty & Basri (2003).”
A VOLUME-LIMITED SAMPLE OF 63 M7–M9.5 DWARFS. II. ACTIVITY, MAGNETISM, AND THE FADE OF THE ROTATION-DOMINATED DYNAMO (2009, methodology): “Among ultracool dwarfs, the existence of a rotationactivity connection was probed by Reid et al. (2002), Electronic address: Ansgar.Reiners@phys.uni-goettingen.de Electronic address: basri@berkeley.edu * Emmy Noether Fellow Mohanty & Basri (2003), and Reiners (2007).”
The stellar rotation–activity relationship in fully convective M dwarfs (2018, background): “However, such objects have been observed to exhibit intense magnetic activity such as chromospheric Hα (e.g. Hawley 1993; Mohanty & Basri 2003), coronal radio (White, Kundu & Jackson 1989), X-ray emission (Wright et al. 2011), and strong large-scale surface magnetic fields as evidenced by Zeeman…”
COLORS AND KINEMATICS OF L DWARFS FROM THE SLOAN DIGITAL SKY SURVEY (2010, methodology): “Ten L dwarfs with spectra in the SDSS database also have radial velocities measured with high resolution spectroscopy (Mohanty & Basri 2003; Bailer-Jones 2004; Blake et al. 2007).”
CONSTRAINING THE AGE–ACTIVITY RELATION FOR COOL STARS: THE SLOAN DIGITAL SKY SURVEY DATA RELEASE 5 LOW-MASS STAR SPECTROSCOPIC SAMPLE (2007, neutral): “Although previous studies have claimed evidence for a rotation-activity relation extending past the M3 convective transition and into the brown dwarf regime (Delfosse et al. 1998; Mohanty & Basri 2003), these studies may be biased by samples of nearby, young, active M dwarfs.”
Large-scale magnetic topologies of late M dwarfs★: Magnetic topologies of late M dwarfs (2010, methodology): “2), in agreement with the previously published values (RV = 9 kms−1 in Delfosse et al. 1998; RV = 10.1 km s−1 in Mohanty & Basri 2003).”
A PHOTOMETRIC VARIABILITY SURVEY OF FIELD K AND M DWARF STARS WITH HATNet (2009, background): “This result suggests that the distribution of starspots on late M dwarfs is uncorrelated with rotation period over a large period range and is perhaps at odds with Hα/ v sin i studies which indicate a drop in activity for very late M-dwarf stars with v sin i 10 km s−1 (e.g., Mohanty & Basri 2003).”