Rotational modulation of hydrogen Lyman alpha flux from 44i Bootis

ROTATIONAL MODULATION OF HYDROGEN LYMAN ALPHA FLUXFROM 44i BOOTIS*O. VILHU l, J.E. NEFF 2, and T. RAHUNEN 31Observatory, Umverslty of Helsinki, SF-00130 Helsinki, Finland2JILA, University of Colorado, Boulder, CO 80309-0440, LISA3Sarkanniemi Oy, Tampere, Finland* Based on observations with the Intemational Ultraviolet Explorer (1UE)obtained at the Vlllafranca satellite tracking station of the European SpaceAgency and at NASA/GoddardSpace Flight Center./ABSTRACTWe present IUE observations that cover the entire 6.4 hourorbital cycle of the late-type contact binary 44i Bootis. Theintrinsic stellar hydrogen Lyman alpha emission flux wasdetermined from low-resolution IUE spectra, compensating forgeocoronal emission and for interstellar absorption. Thevariation of the stellar Lyman alpha emission flux correlates wellwith the variation of the CII and CIV emission fluxes, and itshows orbital modulation in phase with the visual hght curve.The ratio of Lyman alpha to CII flux (15 to 20) is similar to thatobserved in solar active regions. Hydrogen Lyman alphaemission is thus one of the most important cooling channels inthe outer atmosphere of 44i Boo. We obtained a high-resolutionspectrum of the Lyman alpha line between orbital phases 0.0and 0.6. The integrated flux in the observed high-resolutionLyman alpha profile is consistent with the fluxes determinedusing low-resolution spectra, and the composite profile indicatesthat both components of th_s binary have equally activechromospheres and transition regions. The uncertainty in theinterstellar hydrogen column density cannot mimic the observedvariation in the integrated Lyman alpha flux, because the stellarline is very much broader than the interstellar absorption.Key Words: magnetic activity-chromospheres-Lyman alphaemission- ultraviolet spectroscopy -contact binaries1, INTRODUCTIONThe atmospheres of late-type stars consist of relatively coolphotospheres, warm chromospheres, and hot coronae. Thechromosphere and corona are separated by a thin transitionregion, in which the temperature changes abruptly from about7000 K to several million K.The Lyman alpha line at 1216 Angstroms plays a crucial role inthe energetic relationship between the chromospheres andcoronae of cool stars. Unfortunately, the possible correlationbetween the the Lyman alpha line flux and other chromosphericand transition region line fluxes has not yet been investigated,mainly because the very strong Lyman alpha line is generallyoverexposed on all IUE SWP spectra taken with exposure timeslong enough to make other interesting spectral features, such asCIV 1550 and CII 1335, ws_ble. In addition, the stellaremission in low-resolution spectra is severely contaminated byscattered solar Lyman alpha emission from the geocoronalenvironment of the IUE satellite. Further, interstellar absorptioncan remove a significant fraction of the intrinsic stellar flux fromthe line.Nearby contact binaries are good targets to study the Lymanalpha emission, because they are expected to have strongemission and because thexr emission lines are rotationallybroaded so that interstellar absorption does not remove a largefraction of the total flux. In addition, the short periods ofcontact binaries allow an orbital modulation study to be made. Itis possible to perform a simultaneous study of the Lyman alphaline and many chromospheric and transition region emi.ssionlines from the contact binary system 44i Bootis with short SWPexposures (which would not be saturated at Lyman alpha). Inaddiuon, 44i Boo is bright enough that a high resolution Lyrnanalpha spectrum can be obtained (although over a large spread inorbital phases), allowing an important check on the techniquesused to remove the effects of geocoronal emission andinterstellar absorption from the low-resolution spectra.2. OBSERVATIONS AND CORRECFION FOR THEGEOCORONAL EMISSION AND INTERSTELLARABSORFlqON44i Bootis (HD 133640; SAO 45357) is a contact binary at adistance of 12 pc. It has been well studied with the IUE(Rucinski and Vilhu 1983) and with the EINSTEIN (Cruddaceand Dupree 1984) and EXOSAT (Vilhu and Heise 1986)observatories.The amplitudes of the radial velocity variations are KI=115 kms1 and K2=231 km s-1 (Batten, Fletcher, and Mann 1978).The mass ratio is M2/Ml=0.50, and the orbital inclination i=77 °(Rucinski 1973). The ephemeris of light minimum given inVilhu and Heise (1986) needs to be corrected by 0.015 days tobe consistent with the July 1986 photometry of A1-Naimiy et at.(1986). We used the orbital ephemeris JD(min)=2439852.5053and an orbital period of 0.2678159 days.The low-resolution SWP observations were performed on 28June 1986 during a contiguous US2+ESA double shift (16hours), covering roughly one orbital cycle. A high-resolutionspectrum was obtained on 6 July 1986, with an exposure begunat about primary minimum lasting until secondary minimum(one half of the orbital cycle).A Decade of UV Astronomy with IUE. Proc. Celebratory Symposium, GSFC, Greenbelt, USA, 12-15 April 1988, ESA SP-281, Vol. 1 (June 1988)