Mem. S.A.It. Vol. 84, 1147c© SAIt 2013 Memorie della

Spots and white light ares in an L dwarf

J. E. Gizis1, A. J. Burgasser2, E. Berger3, P. K. G. Williams3, F. J. Vrba4,

K. L. Cruz5,6, and S. Metchev7

1 Department of Physics and Astronomy, University of Delaware, Newark, DE 19716, USA2 Center for Astrophysics and Space Science, University of California San Diego, La Jolla,

CA 92093, USA3 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138,

USA4 US Naval Observatory, Flagstaff Station, 10391 West Naval Observatory Road, Flagstaff,

AZ 86001, USA5 Department of Physics and Astronomy, Hunter College, City University of New York,

695 Park Avenue, New York, NY 10065, USA6 Department of Astrophysics, American Museum of Natural History, Central Park West at

79th Street, New York, NY 10025, USA7 Department of Physics and Astronomy, State University of New York, Stony Brook, NY

11794, USA

Abstract. We present a summary of Kepler and supporting observations of a nearby L1dwarf. We find periodic (8.9 hour) variations with an amplitude of 1.5%. The surface fea-tures have remained consistent for 15 months. Magnetic starspots are a plausible explana-tion, but a cloud-related feature is also possible. We also detect white light flares as powerfulas the strongest solar flares, despite the low temperature of the photosphere.

Key words. Stars: atmospheres

1. Introduction

The Kepler Mission (Koch et al. 2010) pho-tometrically monitored more than one hun-dred thousand main sequence stars in the op-tical for four years in order to detect plane-tary transits. Bandwidth limitations restrictedthe regular monitoring to a limited subsetof pixels corresponding to desired targets.Unfortunately, at the time of mission launch,no ultracool L or T dwarfs were knownin the field of view. In 2011, Gizis et al.(2011) discovered a nearby L1 dwarf, WISEP

Send offprint requests to: J. E. Gizis

J190648.47+401106.8 (hereafter W1906+40).Although considerably fainter than the pri-mary G dwarf targets, it is easily detected inthe Full Frame Images. It was added to thetarget for Quarters 10-13 through Director’sDiscretionary Time (Program GO30101) andQuarters 14-17 through an Guest Observerproposal (Program GO40004). The resultingdataset comprises the longest and best sam-pled light curve for an early L dwarf. Here wepresent an overview of results on W1906+40.A full account and analysis is given in Gizis etal. (2013).

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14. ABSTRACT We present a summary of Kepler and supporting observations of a nearby L1 dwarf. We find periodic (8.9hour) variations with an amplitude of 1.5%. The surface features have remained consistent for 15 months.Magnetic starspots are a plausible explanation,but a cloud-related feature is also possible. We also detectwhite light flares as powerful as the strongest solar flares, despite the low temperature of the photosphere.

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1148 Gizis et al.: Spots and white light flares in an L dwarf

Fig. 1. Kepler image of W1906+40 from the FullFrame Images. Each pixel is four arcseconds on aside. The arrow marks the direction of motion.

W1906+40 is a ordinary L1 dwarf in boththe optical and the near-infrared. There is noevidence of lithium absorption, and it is mostlikely a hydrogen-burning star. It does showevidence of magnetic activity in the form of ra-dio emission and quiescent Hα emission (ob-served on five different nights with Gemini-North and MMT), which places it in the moreactive half of L1 dwarfs. Our preliminarytrigonometric parallax places it at 16.4 parsecs.It is likely that W1906+40 is single, but thereis not yet any high resolution imaging.

2. Photometric period and evidenceof spots

The first five quarters (1.25 years) of data areshown in Figure 2. These “long cadence” ob-servations consist of thirty minute exposures.The data are phased to a period of 0.37015days, which we identify as the rotation pe-riod. This is consistent with our measurementof v sin i. The most striking aspect of the plot is

that, while some variations are apparent fromquarter to quarter, the main variation of ∼ 1.5%remains in phase for the entire period of ob-servations. The uncertainty of each measure-ment is 0.5%. There is no evidence of ape-riodic variations. This is quite different thancooler brown dwarfs, which do not remain inphase, and can change in a single rotation pe-riod (Radigan et al. 2012, also see the contri-bution by Burgasser in this volume). It is alsoin contrast to ground-based results for otherearly L dwarfs which suggest surface featuresevolve on the timescale of hours (Bailer-Jones& Mundt 2001) or are aperiodic (Gelino etal. 2002). To be sure, the W1906+40 periodicvariations are too low amplitude to be detectedin most ground-based studies of L dwarfs. Oneplausible explanation is that this is a long-livedmagnetic starspot. As the Kepler photometryprovides only a single filter, there are no con-strains on the color or temperature of the sur-face feature. Indeed, it may even be brighterthan the surrounding photosphere.

3. Evidence of flares

During Quarter 14, we also collected short-cadence data for W1906+40. This providessampling every minute, but because there arefewer counts, the uncertainty per measure-ment is 2%. A number of white light flaresare detected, with the rapid rise and slow de-cay typical of stellar flares. A few such can-didate flares are also seen in the long ca-dence data for earlier quarters. Kepler is sen-sitive to flares because the broad filter allowsthe blue flare to dominate over the red photo-sphere (Figure 3). In some cases, the missionpipeline flags these sharp rises as possible cos-mic ray strikes. However, we can confirm thatthese are genuine flares on W1906+40. On 29July 2012, we were observing W1906+40 withthe Gemini-North (Program GN-2012A-Q-37)GMOS spectrograph (Hook et al. 2004) whena series of flares occurred. A spectrum of themost powerful flare in its impulsive phase isshown in Figure 4. The time series is shownin Figure 5. A smaller flare preceded the mainflare.

Gizis et al.: Spots and white light flares in an L dwarf 1149

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Fig. 2. Kepler photometry for five quarter years.The data is phased to an 8.9 hour period.

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L1 Photosphere with 8000K Flare

Fig. 3. Expected count rate for an L1 dwarf and ahypothetic 8000K blackbody flare that provides thesame number of counts. The spectra are weighted bythe Kepler filter response. Counts for the quiescentL dwarf photosphere are dominated by the red, butthe wide filter is sensitive to flares.

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Fig. 4. Gemini-North GMOS spectra ofW1906+40 in quiescence (below) and in flare.Note the broad Hα, atomic emission lines, andwhite light continuum during the flare.

The flare observations demonstrate that theflare is similar to those seen in hotter stars.The white light suggests that the photosphereis heated to ∼ 8000K, presumably by high en-ergy particles accelerated after a magnetic re-connection event. A large volume of chromo-sphere is also heated, which cools more slowlythan the photosphere.

4. Conclusions

Kepler monitoring of W1906+40 has revealedwhite light flares in an L dwarf for the firsttime. These flares are much less frequent than

1150 Gizis et al.: Spots and white light flares in an L dwarf

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Fig. 5. Kepler photometry (top panel) and spectro-scopic line strengths for the 29 July 2012 flares.

in M dwarfs with comparable rotation peri-ods, but the frequency is comparable to thesolar rate. There are also long-lived surfacefeature(s) which are likely due to magneticstarspot. The features seen in cooler browndwarfs, associated with clouds, have muchshorter lifetimes. The rotation period and v sin iare consistent with the expected radius.

Acknowledgements. We are grateful to Martin Stillfor approving the Kepler observations. This pa-per includes data collected by the Kepler mission.Funding for the Kepler mission is provided by theNASA Science Mission directorate. The materialis based upon work supported by NASA underaward No. NNX13AC18G. Based on observationsobtained at the Gemini Observatory, which is oper-ated by the Association of Universities for Researchin Astronomy, Inc., under a cooperative agreementwith the NSF on behalf of the Gemini partnership.

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