Rotation and Activity in Late-type M Dwarfs

Abstract

We have examined the relationship between rotation and activity in 14 late-type (M6-M7) M dwarfs, using high resolution spectra taken at the Keck Observatory and flux-calibrated spectra from the Sloan Digital Sky Survey. Most are inactive at a spectral type where H-alpha emission has previously seen to be very common. We used the cross-correlation technique to quantify the rotational broadening; six of the stars in our sample have vsini>3.5 km/s. Three of these stars do not exhibit H-alpha emission, despite rotating at velocities where previous work has observed strong levels of magnetic field and stellar activity. Our results suggest that rotation and activity in late-type M dwarfs may not always be linked, and open several additional possibilities including a rotation dependant activity threshold, or a Maunder-minimum phenomenon in fully convective stars. Comment: 4 pages, 2 figures, to appear in proceedings of Cool Stars 15

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arXiv:0810.0061v1 [astro-ph] 1 Oct 2008
Rotation and Activity in Late-type M Dwarfs
Andrew A. West∗,†and Gibor Basri†
∗MIT Kavli Institute for Astrophysics and Space Research, 77 Massachusetts Avenue, Cambridge,
MA 02139
†Astronomy Department, University of California, 601 Campbell Hall, Berkeley, CA, 94720-3411
Abstract. We have examined the relationship between rotation and activity in 14 late-type (M6-
M7) M dwarfs, using high resolution spectra taken at the Keck Observatory and flux-calibrated
spectra from the Sloan Digital Sky Survey. Most are inactive at a spectral type where Hα emission
has previously seen to be very common. We used the cross-correlation technique to quantify the
rotational broadening; six of the stars in our sample have vsini≥3.5 kms−1. Three of these stars do
not exhibit Hα emission, despite rotating at velocities where previous work has observed strong
levels of magnetic field and stellar activity. Our results suggest that rotation and activity in late-type
M dwarfs may not always be linked, and open several additional possibilities including a rotation
dependant activity threshold, or a Maunder-minimumphenomenon in fully convective stars.
Keywords: stars: low mass — stars: low-mass, brown dwarfs — stars: rotation — stars: activity
PACS: 97.10.Jb, 97.10.Kc, 97.20.Jg
INTRODUCTION
Many M dwarfs, which are the most abundant stars in the Milky Way, have strong mag-
netic dynamos that give rise to chromospheric and coronal heating, producing emission
from the x-ray to the radio. Although this magnetic heating (or activity) has been ob-
served for decades, the exact mechanisms that control magnetic activity in M dwarfs are
still not well-understood.
In the Sun, activity is strongly linked to rotation. The rotation in solar type stars slows
with time due to angular momentum loss from magnetized stellar winds; as a result,
magnetic activity decreases. There is strong evidence that the rotation-activity relation
extends from stars more massive than the Sun to smaller dwarfs [1, 2]. However, at a
spectral type of ∼M3 [0.35 M⊙; 3, 4], stars become fully convective. This transition
marks an important change in the stellar interior that has been thought to affect the
production and storage of internal magnetic fields.
A few studies have uncovered evidence of a possible rotation-activity (using Hα)
relation extending past the M3 convective transition and into the brown dwarf regime
[5, 6, 7]. However, the lack of an unbiased sample of high resolution spectra of late-type
M dwarfs complicates the situation.
Using over 30,000 spectra from the Sloan Digital Sky Survey [SDSS; 8], West et al.
[9, 10] showed that the activity fraction of M dwarfs varies as a function of stellar age
(using Galactic height as a proxy for age) and that the Hα activity lifetime for M6-M7.5
stars is 7-8 Gyr. Nearby samples of late-type M dwarfs are therefore biased towards
young populations with high levels of activity; until recently every known M7 dwarf
was observed to be magnetically active [11, 12, 13].

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We present results from our study of the vsini rotation velocities for a small sample
of M6-M7 dwarfs, most of which were selected to be inactive or weakly active from the
SDSS low-mass star spectroscopic sample.
DATA
Our sample was selected from the West et al. [10] Sloan Digital Sky Survey (SDSS)
M dwarf catalog, a spectroscopic sample of almost 40,000 M and L-type dwarfs. We
selected the brightest M7 and M6 stars which were either inactive or weakly active
(as measured by their Hα emission). 12 stars were selected using these criteria. Two
additional active M7 dwarfs were added to the sample for comparison to previous
studies. While our sample is not a complete unbiased sample, representative of the
underlying M dwarf population, it does consist of late-type M dwarfs with activity
properties selectively different than previously observed.
ANALYSIS
To measure the vsini rotation velocities for our sample, we used a cross-correlation
techniquesimilartothat ofpreviousstudies[e.g. 5, 6]: wecross-correlated each program
spectrum with the spectrum of a slowly rotating comparison star. The width of the
resulting cross correlation function is a direct probe of the rotational broadening.
To measure the rotational broadening of each spectrum, we compared the result-
ing cross-correlation function to that of a rotationally broadened template. The GL
406 template was rotationally broadened to larger rotation velocities using the tech-
nique of Gray [14] and cross-correlated with the original (unbroadened) template. A
vsini was determined based on the best fit spun-up template. Figure 1 shows the cross-
correlation function of SDSS094738.45+371016.5 with GL406 in the 7080-7140Å re-
gion (solid) compared with the cross-correlation function of GL406 with the best-fit
rotationally broadened GL406 spectrum (dotted; 6 kms−1), and the auto-correlation
function of GL406 (dashed; 0 kms−1broadening). The cross-correlation reveals that
SDSS094738.45+371016.5 appears to be rotating with a velocity ≥ 6 kms−1
All of the spectra were spectral typed by eye using the Hammer spectral analysis
package [15] on the SDSS spectra. We measured the equivalent widths (EW) of the
Hα emission lines in both the SDSS and Keck spectra. The Keck spectra are more
sensitiveto low levels of emission(they can be distinguishedbetter against the pervasive
molecular features); it is also true in general that equivalent widths tend to be smaller
when measured from high resolution spectra. Almost all of our targets chosen to be
inactive at low resolution proved inactive even at high resolution, and the EWs when
detected were similar.

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RESULTS
Six of the fourteen M6-M7 dwarfs in our sample have detectable rotation. 3 of the
rotating stars have measurable activity but the other 3 show no signs of activity in any
of the emission lines in either the SDSS or Keck spectra. The cross-correlation shown in
Figure 1 (SDSS094738.45+371016.5) is an example of one of the inactive M7 dwarfs
that appears to be rotating despite not being magnetically active.
-10-505 10
Pixels
0
20
40
60
80
Cross Correlation Function
FIGURE 1.
Å region (solid) compared with the cross-correlation function of GL406 with the best-fit rotationally
broadened GL406 spectrum (dotted; 6 kms−1), and the auto-correlation function of GL406 (dashed; 0
kms−1broadening). The cross-correlation reveals that SDSS094738.45+371016.5 appears to be rotating
with a velocity ≥ 6 kms−1despite not having any signs of activity in either the SDSS or Keck spectra.
Cross correlation function of SDSS094738.45+371016.5 with GL406 in the 7080-7140
Figure 2 shows LHα/Lbol(activity) as a function of vsini for the M6-M7.5 dwarfs
from this paper. Lower limits in both velocity and activity denote the levels to which
our sample could probe. All previous M6-M7.5 dwarfs were found to be active, while 9
of the 14 stars in our sample show no activity in either the SDSS or Keck spectra. The
lack of activity is not surprising since that was our main selection criterion, however
it highlights the fact that we are probing a sample with very different properties than
previously studied.
DISCUSSION
We conducted high resolution spectral observations of 14 M6-M7 dwarfs and found
vsini rotation velocities for 6 for the stars. Three of the stars showed both activity and
rotation, 6 of the stars showed neither rotation nor activity, 2 of the stars showed activity
but no rotation and 3 stars showed rotation but no activity. These results are in contrast
with previous studies that found a strong connection between rotation and activity in all
(active) M6-M7 dwarfs [5, 6, 7]. Our sample is the first rotation study to include M6-M7
dwarfs that are inactive.
ACKNOWLEDGMENTS
We gratefully acknowledge the support of the AAS International Travel Grant.

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10
vsini (kms-1)
10-7
10-6
10-5
10-4
LHα/Lbol
6
FIGURE 2.
study. LHα/Lbolvalues were calculated from Equivalent Width measurements using the χ conversions
of Walkowicz et al. [16]. Our sample includes M6-M7 dwarfs with both measured rotation as well as
activity from the SDSS and Keck spectra (filled circles), measured rotation and activity from the Keck
spectra (CaII K detected in SDSS; filled diamonds), activity from the Keck spectra but no rotation
(filled triangles), activity from the SDSS spectra but no rotation (filled squares), no rotation or activity
(hexagonallyalligned dots; number denotes number of stars) and measured rotation but no activity (filled
stars). Lower limits in both velocity and activity denote the levels to which our sample (and previous
studies) could probe. All previous M6-M7.5 dwarfs were found to be active, while 9 of the 14 stars in our
sample show no activity in either the SDSS or Keck spectra. The dearth of inactive late-type M dwarfs
in previous studies is due to a selection effect that biases nearby samples to younger, more active stars
[9, 10]. 3 of our stars show strong evidence for rotation despite having no activity.
LHα/Lbol(activity) as a function of vsini (rotation) for the M6-M7.5 dwarfs from this
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