Rotation and Chromospheric Activity in Field M Dwarfs*
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Astron. Astrophys. 331, 581–595 (1998) ASTRONOMY AND ASTROPHYSICS Rotation and chromospheric activity in field M dwarfs? X. Delfosse1, T. Forveille1, C. Perrier1, and M. Mayor2 1 Observatoire de Grenoble, 414 rue de la Piscine, Domaine Universitaire de St Martin d’Heres,` F-38041 Grenoble, France 2 Observatoire de Geneve,` CH-1290 Sauverny, Switzerland Received 29 July 1997 / Accepted 7 November 1997 Abstract. We have obtained high resolution spectra for a younger or more massive stars: LX /Lbol and LHα /Lbol both cor- volume-limited sample of 118 field M dwarfs. From these ob- relate with v sin i for v sin i < 4 5km.s−1 and then saturate servations we derive projected rotational velocities and fluxes at respectively 10−2.5 and 10−∼3.5. − 1 in the Hα and Hβ lines . 8 stars are double-lined spectroscopic binaries with measured or probable periods short enough for ro- Key words: stars: activity – stars: rotation – stars: chromo- tation to be tidally synchronized with the orbit, and another 11 spheres – stars: coronae – stars: low-mass, brown dwarfs are visual binaries where we cannot yet separate the lines of the two stars. Of the remaining 99 stars, 24 have rotational velocities above our detection limit of 2km.s−1, and some are quite fast rotators, including two with∼ v sin i 30 km.s−1 and one with v sin i 50 km.s−1. Given the small' radii of M dwarfs, these moderate' rotational velocities correspond to rather short maxi- 1. Introduction mum rotational periods, of only 7-8 hours. These three stars are Surface rotation is a key observational parameter for stellar evo- good candidates for Doppler imaging. lution, as a diagnostic of the mechanisms responsible for stellar We find that rotation is strongly correlated with both spec- angular momentum loss and internal angular momentum trans- tral type and kinematic population: all stars with measurable port. The latter are responsible for chemical mixing and they rotation are later than M3.5, and all but one have kinematic modify the stellar thermal structure (e.g. Martin & Claret 1996), properties typical of the young disk, or intermediate between and hence affect the overall evolution. Rotation is also the driv- the young disk and the the old disk. We interpret this correla- ing force behind stellar activity (coronae, chromospheres, spots, tion as evidence for a spin-down timescale that increases with flares), through dynamo production of magnetic fields. decreasing mass. At the age of the old disk or halo, all stars ear- Pre-main-sequence rotational evolution of low mass stars is lier than M5-M6 (0.1-0.15M ) have spun-down to below our generaly presumed to be dominated by magnetic coupling with detection limit, while at the age of the young disk this has only the parent protostellar disk (Cameron & Campbell 1993, Shu et happened for stars earlier than M3.5. The one star with measur- al. 1994): as long as a star is surrounded by a substantial accre- able rotation and a kinematics intermediate between old disk tion disk, its equatorial velocity remains approximately constant and population II has spectral type M6. It is probably an old − at 20 km.s 1 (e.g. Bouvier et al. 1993). Once accretion stops, star whose mass is low enough that it has retained significant disk∼ braking disappears, and angular momentum conservation rotation up to present, still consistently with longer spin-down then becomes the dominant factor. Stars therefore spin up dur- times for lower mass stars. We observe, on the other hand, no ing their contraction along an Hayashi track, as their moment of conspicuous change in the v sin i distribution or activity pat- inertia decreases. Since the time at which this happens depends tern at the mass (M 0.35 M ) below which stars remain fully on details of their circumstellar environment, stars arrive on the convective down to∼ the main sequence. ZAMS with a broad distribution of rotational velocities (e.g. These new data are consistent with a saturated correlation Bouvier et al. 1997). Once on the main sequence the moment between rotation and activity, similar to the one observed for of inertia no longer changes significantly. As first suggested by Schatzman (1962), magnetic winds then brake down the stars Send offprint requests to: Xavier Delfosse −1/2 ([email protected]) to low final equatorial velocities, approximately with a t ? Based on observations made at the Observatoire de Haute-Provence Skumanich (1972) law. The time scale for this angular momen- (CNRS), France tum dissipation is mass-dependent: rapid rotators are found at 1 Tables 2 and 4 are also available in electronic form at the all spectral types (G-M) at the age of the α Persei cluster ( 50 ∼ CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via Myr) (Prosser 1991), in the Pleiades ( 70 Myr) only later than http://cdsweb.u-strasbg.fr/Abstract.html mid-K (Stauffer & Hartmann, 1987), and∼ in the Hyades ( 600 ∼ 582 X. Delfosse et al.: Rotation and chromospheric activity in field M dwarfs Myr) only within the M dwarfs (Stauffer et al. 1987; Stauf- Table 1. Stars with d ≤ 9pc and δ ≥−16◦ too faint to be measured fer et al. 1997). The situation in the field is slightly less clear, with ELODIE since several age groups are represented, but all G dwarfs rotate name α (2000.) δ (2000.) spectral type mV slowly, and there are good spectroscopic or dynamic arguments LHS 1326 02:02:16.1 +10:20:17 M6 15.61 to attribute a (very) young age to all K-early M rapid rotators. LHS 1375 02:16:29.8 +13:35:17 M5.5 15.79 Over the years, a large observational effort has established Gl 105C 02:35:58.7 +06:52:00 ≥M7 ≥ 16 this picture of rotation along low mass stellar tracks, and mod- LHS 2065 08:53:36.3 −03:29:33 M9 18.8 els incorporating the above general elements (e.g. Bouvier & LHS 292 10:48:12.7 −11:20:13 M6.5 15.6 Forestini 1995, Krishnamurthi et al. 1997) successfully repro- Gl 644C(VB8) 16:55:34.9 −08:23:43 M7 16.7 duce the rotational velocity distribution of solar mass dwarfs Gl 752B(VB10) 19:16:57.5 +05:09:02 M8 17.5 at all ages: pre-main-sequence T Tauri and post-T Tauri star, young main sequence stars in open clusters, and field stars. In the course of a radial velocity survey for low mass com- Some aspects of the models however remain largely phe- panions and planets around nearby M dwarfs (Delfosse et al. nomenological, mostly because the complex physics of accre- 1997), we have obtained high resolution optical spectra for a tion and stellar dynamos is only partly understood. A number volume-limited sample of 118 K5 to M6.5 dwarfs. Here we of competing models therefore exist, that differ on, for instance, analyse the accurate v sin i measurements (or significant up- radiative core/convective envelope decoupling, or parameterisa- per limits, < 2km.s−1) that we derive from those data, using tion of the angular momentum loss law. The uncertain physical digital correlation∼ techniques. They fill the spectral type gap be- mechanisms are expected to have a strong mass dependence, tween the generally earlier slow rotators measured by Marcy & and it is therefore useful to extend the observational database Chen (1992) and the later than M6 very low mass rapid rotators to the lower mass M dwarfs (e.g. Krishnamurthi et al. 1997). observed by Basri & Marcy (1995) and Martin et al. (1996). The [M0V,M6V]range is also interesting because it contains the This brings important new constraints on angular momentum mass ( 0.35 M , M2.5V) below which main sequence stars no dissipation in mid-M dwarfs at a few Gyr. longer∼ develop a radiative core. The radiative/convective bound- ary layer is essential (e.g. Spiegel & Weiss 1980; Spruit & van Ballegooijen 1982) to the operation of the standard α Ω shell 2. Observations and data processing − dynamo which is generally believed to generate the large scale 2.1. Sample selection solar magnetic field, and a break in both the activity level and the rotation properties could, at least naively, be expected at this As we are interested in a fair sampling of M spectral subtypes, spectral type. we decided to observe a volume-limited sample and initially se- A number of authors have discussed the rotation behaviour lected all M dwarfs in the third edition of the nearby star catalog of M dwarfs in young open clusters (most recently Stauffer et al. (Gliese and Jahreiss 1991) with a distance closer than 9 pc and (1997) for the Hyades, Stauffer et al. (1994) for the Pleiades, Pat- a declination above -16 degrees. 136 stars fullfill these criteria. ten & Simon (1996) for IC 2391), but there are fewer studies for Of these, 7 (listed in Table 1) have apparent magnitudes fainter field M dwarfs. Stauffer & Hartmann (1986) measured v sin i than V=15 and had to be dropped because they are beyond the for approximately 200 field M dwarfs with a 10 km.s−1 sensi- sensitivity limit of the instrument we used. Gl 53B and Gl 451B tivity. Only 11 have significant broadening, a few of which may are close companions to the much brighter Gl 53A (G5 VI) and actually be unrecognized double-lined spectroscopic binaries. Gl 451A (G8 VI) from which they cannot be separated by the Marcy & Chen (1992) observed 47 field M dwarfs with v sin i spectrograph input fiber, and they were thus not observed.