Magnetic Activity in M Dwarfs with Binary

CompanionsWe present a study of close white dwarf + M dwarf (WD+dM) binary systems and examine the effect that a close companion has on the magnetic field generation in M dwarfs (Morgan et al. 2012, preprints available).  From the SDSS DR8 spectroscopic database, we construct a sample of 1756 WD+dM high-quality pairs from our color cuts and previous catalogs (Silvestri et al. 2006, Rebassa-Mansergas et al. 2010). We separate the individual WD and dM from each spectrum using an iterative technique that compares the WD and dM components to best-fit  templates.   Using the height above the Galactic plane (as a  proxy for age), magnetic activity (as measured by Hα), projected linear binary separation (measured from radial velocity shifts in the spectra), and WD cooling ages (Garcés et al. 2011; Catalán et al. 2008b),  we investigate the age-activity relation for our  sample and the effects of a close binary companion on magnetic activity properties of the dM. We show that early-type WD+dMs (≤ M4) are more likely to be active, young, and have longer activity lifetimes compared to their field counterparts. However, beginning at a spectral type of  M5 (just past the onset of full convection in dMs), the activity fraction and lifetimes of WD+dM binary systems are consistent with field dMs. Also, the WD+dM active and inact ive populations become indistinguishable. Magnetic strength, as  measured by Hα, is noticeably higher between paired and field M dwarfs until a spectral type of M6/M7 where activity strength becomes much stronger in WD+dM compared to field dMs.  Also, the closer the separation between the WD+dM, the more likely the dM is to be active.

Abstract

Close WD+dM: What effect does a close companion have on the activity of the M dwarf?

Wide WD+dM: The key to unlocking an dM age-activity relation?

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We use spectroscopic observations of wide WD+dM binary systems in

order to constrain an M dwarf age-activity relation. Previous studies

(Silvestri et al. 2006; West et al. 2008; Morgan et al. 2012) have

investigated age-activity relationships for isolated dMs and

close WD+dM binary pairs, respectively. These studies utilized

Galactic height as a proxy for the age of the system and WD cooling

ages for the close WD+dM studies. Unfortunately, these relationships

have significant scatter due to using Galactic height as a proxy for age as well as inaccurate WD cooling ages (due to potential mass transfer and

lack of progenitor ages).

Wide WD+dM pairs present a more accurate dating method; because these systems are widely separated and have co-evolved independently, the ages of WDs can be accurately (within a few 100 Myr) measured using white dwarf cooling models and detailed stellar evolution models (Garcés et al. 2011; Catalán et al. 2008b). Wide WD+dM present tidally unaffected coeval laboratories from which the dM age-activity relationship can be further constrained. We present preliminary data from our sample of wide WD+dM pairs.

Dylan P. Morgan1 (dpmorg@bu.edu), Andrew A. West1, Ane Garcés2, Silvia Catalan3, Saurav Dhital1,4, Miriam Fuchs5, Nicole M. Silvestri61Boston Univ., 2Institut de Ciències de lʼEspai (IEEC-CSIC), 3Univ. of Hertfordshire, 4Vanderbilt Univ., 5Haverford College, 6Univ. of Washington

0.000.250.500.751.00 M0+M1

M2

0.000.250.500.751.00 M3

M4

0 200 400 600 800 0.000.250.500.751.00 M5

200 400 600 800 1000 M6+M7

Galactic Height (pc)

Act

ivity

Fra

ctio

n

WD+dMdM (W11)

Are M dwarfs with close WD companions more likely to be active?

WD+dMs are more likely to be active than field dMs at every spectral type.

Is the activity stronger in WD+dMs than in field dMs?

Activity strength (LHα/Lbol) is noticeably stronger in WD+dM pairs in comparison to field dMs, especially in the M6+M7 bin.

What does the activity fraction in WD+dM pairs look like when analyzed in the proper Galactic context?

Close WD+dMs have elevated activity fractions close to the Galactic plane with activity decreasing farther from the plane (age), indicating extended yet finite lifetimes in early-type WD+dMs. Whereas, the lifetimes in late-type WD+dMs are comparable to the late-type field population.M0 M1 M2 M3 M4 M5 M6+M7

-5.5

-5.0

-4.5

-4.0

-3.5

-3.0

dM Spectral Type

log(

L H/L

bol)

dM (W11)WD+dM

M0 M1 M2 M3 M4 M5 M6+M7dM Spectral Type

0.0

0.2

0.4

0.6

0.8

1.0

Act

ivity

Fra

ctio

n

dM (W11)WD+dM

The closer the proximity of the WD, the more likely the dM is to be active. We postulate that this is

due to the increased rotation in the dM, which is induced by the companion.

Is there a correlation between activity and binary separation?

M0 M1 M2 M3 M4 M5 M6+M7dM Spectral Type

0.0

0.2

0.4

0.6

0.8

1.0

Act

ivity

Fra

ctio

n

<0.1 AU0.1 1 AU1 100 AUW11

Fraction of active stars vs. spectral type for close WD+dM pairs (filled circles) and isolated field dMs (open diamonds, West et al. 2011, hereafter W11), error bars computed from binomial distribution.

Activity strength (LHα/Lbol) vs. dM spectral type, binned by spectral type and median value of LHα/Lbol is plotted with error bars representing the 25th and 75th quartiles; WD+dM are given by red filled circles isolated dMs by blue open triangles.

Activity fraction as a function of Galactic height (pc) for each spectral type; filled circles are bins of WD+dM (≥ 2 stars) and open triangles are field dMs (W11).

Activity fraction for WD+dM vs. spectral

type grouped by projected linear

separation: ≤ 0.1 AU (solid purple), from

0.1-1 AU (dotted blue), 1-100 AU (red dashed line), and the

field dM population (black dash-dotted,

W11)

What can we infer about the activity in WD+dM pairs using the cooling age of

the WD?

There is a dearth of active WD+dMs at early spectral types while the late-type active and

inactive populations are indistinguishable from one another.

dM Spectral type vs. WD cooling age (Gyr) as a

function of activity state.

Silvestri et al. 2006, AJ, 131, 1674West et al. 2008, PASP, 120, 1161

West et al. 2011, AJ, 141, 97 (W11)Images of interacting binaries credit to P. Marenfeld

and NOAO/ AURA/NSF

REFERENCESCatalán et al. 2008b, A&A, 477, 213Garcés et al. 2011, A&A, 531, A7+Morgan et al. 2012, SubmittedRebassa-Mansergas et al. 2010, MNRAS, 402, 62

M0+M1

M2M3M4M5

M6+M7 Active

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4M0+M1

M2M3M4M5

M6+M7 Inactive

WD Cooling Age (Gyr)

dM S

pect

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ype

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Age=1.1(0.2) GyrMass=0.92(0.07) M☀

Age=3.0(1.4) GyrMass=0.58(0.07) M☀

Age=1.6(0.3) GyrMass=0.61(0.02) M☀

Age=1.5(0.3) Gyr

Mass=0.61(0.04) M☀