Near‐Ultraviolet Spectra of Nine M Dwarf Stars, or a Second Effort to Find Optical CoronalLines in M Dwarf StarsAuthor(s): George Wallerstein and Sudhi TyagiSource: Publications of the Astronomical Society of the Pacific, Vol. 116, No. 820 (June 2004),pp. 554-557Published by: The University of Chicago Press on behalf of the Astronomical Society of the PacificStable URL: http://www.jstor.org/stable/10.1086/420981 .

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554

Publications of the Astronomical Society of the Pacific, 116:554–557, 2004 June� 2004. The Astronomical Society of the Pacific. All rights reserved. Printed in U.S.A.

Near-Ultraviolet Spectra of Nine M Dwarf Stars, or a Second Effort to FindOptical Coronal Lines in M Dwarf Stars

George Wallerstein1 and Sudhi Tyagi

Department of Astronomy, University of Washington, Seattle, WA 98195; wall@astro.washington.edu, styagi@u.washington.edu

Received 2004 January 23; accepted 2004 March 9; published 2004 May 19

ABSTRACT. We have searched for optical coronal lines in the 3100–3700 A˚ region of eight M dwarf starswith rather low levels of activity. This brief survey supplements a similar search in 15 active stars published in1991. No coronal lines could be identified. However, the emission spectra including lines of Hi, He i, Ca ii, Ca i,Si i, and Fei are described and illustrated. Radial velocities of the emission lines show no systematic differencesfrom the stellar absorption lines. Coronae with temperatures similar to those in the solar corona seem to be rareamong the M dwarfs, although at least one example has been found by Schmitt & Wichmann.

1. INTRODUCTION

Hydrogen emission lines in the spectra of M dwarf (hence-forth dM) stars have been known for many years (Joy & Hu-manson 1941). Joy & Humason (1949) also found emissionlines of He i in the flare star L726-8. Since solar flares areknown to be one of the heating sources of the solar corona, asearch for coronal lines in the ground-based accessible regionof dM stars was initiated about 15 years ago (Wallerstein etal. 1991, henceforth WBO). That search was concentrated onknown active stars, but no coronal lines were found. Soft X-ray observations were later found to show that active dM starsusually have a two-temperature corona, with most of the fluxoriginating at temperatures that are too high to excite the linesthat might have been visible between 3100 and 3700 A˚ . In themeantime, Schmitt & Wichmann (2001) found the [Fexiii]line at l3388 in the M5.5 V star CN Leo. This demonstratedthat coronal lines in dM stars could indeed be found from theground. Shortly before that, one of us (G. W.) took advantageof an opportunity to observe eight additional dM stars with theCerro Tololo Inter-American Observatory (CTIO) 4 m echellespectrograph. The short camera with the 79 lines mm�1 echellegrating, and the KPGL1 cross-disperser in the first order, whoseresolving power was about 20,000, were used to record thespectra. This arrangement provided usable spectra from 3100to 4950 A. The S/N varied from about 30 at the short wave-length end, increasing rapidly to about 200 at 3600 A˚ , andseveral hundred at Hb. The spectra were reduced with standardIRAF programs by S. T. to yield the intensity as a function ofwavelength over the entire interval. The wavelength intervalwas fully covered, with significant overlap at the ends of each

1 Visiting Observer, Cerro Tololo Inter-American Observatory, managed bythe Associated Universities for Research in Astronomy under a cooperativeagreement with the National Science Foundation.

order. The individual orders were measured separately usingthesplot routine. By measuring them separately rather than firstcombining the orders, spurious lines could be recognized bytheir appearance in one order only.

The observed stars are described in Table 1 and are largelyfrom the SIMBAD database. In Table 2 we show theROSATall-sky counts for both the 2001 targets and the stars observedin 1988. Our measured velocities of both emission and ab-sorption lines are shown in Table 3.

2. THE EMISSION LINES

No coronal lines were recognized. However, our spectrashow a number of emission lines of lower ionization presum-ably formed in the stellar chromospheres. Absorption velocitieswere derived from the measurement of about 12 lines of neutralFe, Mn, Cr, and Ca. For two stars, our measurements differfrom catalog values in SIMBAD, for reasons that are unclear.It is known that the CTIO Cassegrain echelle suffers fromflexure, so frequent ThAr spectra were obtained during theobserving session. However, it is still possible that some spectrawere not properly calibrated for wavelength.

Emission in the Caii H and K lines was seen in every star.Our resolution was not sufficient to measure their intrinsicwidths. We show two Caii line profiles in Figure 1. Note thepresence of emission by He in V577 Mon. Emission by Feiimultiplets 1, 6, and 7, which is often seen in M giants andsupergiants, was not seen in any of our stars (Bidelman &Pyper 1963). None of our stars showed the Tiii emission linesreported by Fuhrmeister et al. (2004)

Four of our stars show Hi emission, and V577 Mon showsweak emission by Hei at ll4026 and 4471. V1216 Sgr alsoshows emission at 4026 A˚ . He ii emission was searched forunsuccessfully at both thell3203 and 4686 lines. The spectrumof V1216 Sgr includes emission by Fei, Si i, and Cai. Such a

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NEAR-UV SPECTRA OF NINE M DWARF STARS 555

2004 PASP,116:554–557

Fig. 1.—Spectrum of V577 Mon. Note the presence of Caii H and K linesin emission as well as He in emission. Fig. 3.—Emission lines of Fei and H i are shown in V1216 Sgr.

Fig. 2.—Spectrum of V1216 Sgr. A weak emission line of Hei is presentat l4026 along with a number of absorption lines.

Fig. 4.—Region around the resonance line of Cai at l4226 in V1216 Sgr.Note the likely presence of the Fei line atl4216.19, and the possible emissionat l4232.72 also of Fei.

cool chromosphere in a dM star is probably rare. It was firstseen in the well-known flare star EV Lac (Wilson 1961). Weshow some line profiles in V1216 Sgr in Figure 2. In Figure 3we show emission by one of several Fei lines, as well as theH i line at l3889. The Hei line at l3888.64 does not seemto be present. Figure 4 shows the resonance line of Cai atl4226 in V1216 Sgr. As shown in Table 3, the velocities ofthe emission lines in our target stars show very small anduncertain differences from the measured absorption velocities.Hence, we have no evidence that supports an expanding chro-mosphere around these stars.

3. DISCUSSION

As discussed by WBO, coronal lines might be expected tobe present in dM stars if their coronas were of similar tem-

peratures as the solar corona, and they should be visible in thenear-UV region. The discovery by Schmitt & Wichmann (2001)of one star with a coronal line shows that coronal lines in dMstars can be found from the ground. However, our unsuccessfulsurveys of 1988 and 2001 show that the phenomenon is rare.The reason for this is likely to be the temperature structure ofdM coronas. Most of the optical coronal lines in the Sun areformed in regions with temperatures of K. If the62–3# 10coronas of dM stars are either hotter or cooler than this, theywill show different lines. In fact, there is some evidence thatdM stars have two temperature peaks in their coronas (Redfieldet al. 2003; Sanz-Forcada et al. 2003). Detailed high-resolutionX-ray spectra covering a sufficient energy range will be veryuseful in restricting model coronas of dM stars.

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556 WALLERSTEIN & TYAGI

2004 PASP,116:554–557

TABLE 1Stars Observed in 2001

Star Name Other NameR.A.

(2000)Decl.(2000) V B–V Spectral Type

HD 225213 . . . . . . G1-1 00 05 22.4 �37 21 26 8.58 1.45 M4 VYZ Cet . . . . . . . . . . L725-32 01 12 30.6 �16 59 56 11.60 1.20 M4.5 VHD 33793 . . . . . . . Kapteyn’s 05 11 40.6 �45 01 05 8.89 1.55 sdM1V577 Mon . . . . . . . Ross 614 06 29 03.4 �02 48 50 11.10 2.00 M4.5 VV2500 Oph . . . . . . Barnard’s 17 57 48.5 �04 41 36 9.54 1.74 M4 VV1216 Sgr. . . . . . . Ross 154 18 49 49.4 �23 50 10 10.93 1.60 M3.5 VHD 202560 . . . . . . Lac8760 21 17 15.3 �38 52 03 6.68 1.40 M0 VHD 217987 . . . . . . Lac9352 23 05 52.0 �35 55 11 7.34 1.48 M2 V

Note.—Units of right ascension are hours, minutes, and seconds, and units of declination are degrees,arcminutes, and arcseconds.

TABLE 2X-Ray Fluxes from the ROSAT Survey for Stars Observed in this Program and

the 1989Survey

Part A, Stars Observed in 2001 Part B, Stars Observed in 1989

Star Counts s�1 � Star Counts s�1 �

HD 225213 . . . . . . 0.06 0.02 GX And . . . . . . . . . . 0.26 0.03YZ Cet . . . . . . . . . . . 0.25 0.03 CR Dra . . . . . . . . . . 0.92 0.03HD 33793 . . . . . . . 0.05 0.02 Wolf 630 . . . . . . . . . 3.97 0.24V577 Mon . . . . . . . 0.77 … BY Dra . . . . . . . . . . 2.41 0.05V 1216 Sgr . . . . . . 0.14 0.08 V1285 Aql . . . . . . . 1.39 0.06HD 202560 . . . . . . 0.14 0.02 HU Del . . . . . . . . . . 0.14 0.02HD 217987 . . . . . . 0.02 0.01 EV Lac . . . . . . . . . . 5.38 0.10

DO Cep . . . . . . . . . . 0.45 0.03EQ Peg . . . . . . . . . . 0.99 0.25

TABLE 3Measured Radial Velocities (km s�1)

Star Vabs � (others)Vabs H i em Hei em Caii em Neutral em

HD 225213 . . . . . . �25.7 2.5 �22.1a … … �25.7 …YZ Cet . . . . . . . . . . �34.3 0.8 �37.0b �27.6 … �27.2 …HD 33793 . . . . . . . �217.3 1.3 �245.5a … … �214.6 …V577 Mon . . . . . . . �8.9 2.4 �16.3c �2.8 �3.1 �3.9 …V2500 Oph . . . . . . �101.3 0.6 �106.4b �99.1 … �101.8 …V1216 Sgr. . . . . . . �4.3 1.1 �7.0b �3.3 … �3.9 �3.0 � �1.4HD 202560 . . . . . . �26.3 2.3 �20.0a … … �28.1 …HD 217987 . . . . . . �10.9 1.4 �9.6a … … �10.3 …

a SIMBAD database.b Reid et al. (2002).c Spectroscopic and astrometric binary (Segransan et al. 2000; Gatewood et al. 2003). A hot continuum

is present, indicating a flare in progress at the time of observation.

S. T. is pleased to acknowledge the Washington NASA SpaceGrant Consortium for support during the summer of 2002. ChrisLaws provided useful help on the use of IRAF to reduce echellespectra, Suzanne Hawley made helpful suggestions on target

selection, and Marcel Agueros looked up theROSAT data thatappear in Table 2. We have made frequent use of the SIMBADdatabase and the ADS Abstracts.

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