Table Of ContentAstronomy&Astrophysicsmanuscriptno.9058 (cid:13)c ESO2008
February5,2008
α
Chromospheric features of LQ Hydrae from H line profiles
8 A.Frasca1,Zs.Ko˝va´ri2,K.G.Strassmeier3,andK.Biazzo1
0
0
2 1 INAF–CataniaAstrophysicalObservatory,viaS.Sofia78,I-95123,Catania,Italy
n e-mail:[email protected], [email protected]
a 2 KonkolyObservatory,H-1525Budapest,P.O.Box67,Hungary
J e-mail:[email protected]
5 3 AstrophysicalInstitutePotsdam,AnderSternwarte16,D-14482Potsdam,Germany
2 e-mail:[email protected]
]
h Received14Nov2007/Accepted3Jan2008
p
- ABSTRACT
o
r
t WeanalyzetheHαspectralvariabilityoftherapidly-rotatingK1-dwarfLQHyausinghigh-resolutionHαspectrarecordedduringApril-May
s
2000.ChromosphericparameterswerecomputedfromtheHαprofileasafunctionofrotationalphase.Wefindthatalltheseparametersvary
a
[ inphase,withahigherchromosphericelectrondensitycoincidingwiththemaximumHαemission.Wefindaclearrotationalmodulationof
theHαemissionthatisbetteremphasizedbysubtractingareferencephotospheric templatebuiltupwithaspectrumofanon-activestarof
1 thesamespectraltype.AgeometricalplagemodelappliedtotheHαvariationcurveallowsustoderivethelocationoftheactiveregionsthat
v
comeouttobecloseinlongitudetothemostpronouncedphotosphericspotsfoundwithDopplerimagingappliedtothephotosphericlinesin
8
thesamespectra.OuranalysissuggeststhattheHαfeaturesobservedinLQHyain2000areascaled-upversionofthesolarplagesasregards
9
dimensionsand/orfluxcontrast.Noclearindicationofchromosphericmassmotionsemerges.
8
3
. Keywords.stars:activity–stars:late-type–stars:chromospheres–stars:starspots–stars:individual:LQHydrae
1
0
8
0 1. Introduction investigated by Strassmeieretal. (1993, hereafter Paper II),
: who attributed it to the presence of chromospheric velocity
v According to the solar-stellar analogy, chromospheric stellar
i fields in the Hα forming layer probably surrounding photo-
X activity can be established by the presence of emission in the
sphericspots.
coreoftheBalmerHαline.AsintheSun,Hαemissioninten-
r
a sification hasoftenbeenobservedin surfacefeatures(plages)
InourspectroscopicstudyinPaperI,wepresentedDoppler
spatially connected with the photospheric starspots (see, e.g.,
images using the Fe-6411, Fe-6430, and Ca-6439 lines
Catalanoetal. 2000; Biazzoetal. 2006, 2007, and references
for both the late April data and the early May 2000 dataset.
therein).Thus,the timevariabilityofthe Hαspectralfeatures
Dopplerimagingwas supportedby simultaneousphotometric
can be used to estimate the basic properties of the emitting
measurementsin Johnson-CousinsVI bands. Doppler images
sources,allowingthe geometryofthestellarchromosphereto
showed spot activity uniformly at latitudes between −20◦and
bemapped.
+50◦, sometimes with high-latitude appendages, but without
In this paper we analyze the Hα spectral variability of
a polar spot. Comparingthe respectivemaps from two weeks
the young, rapidly-rotating (P ≈ 1.6d) single K2-dwarf
rot
apart,rapidspotevolutionwasdetected,whichwasattributed
LQHydrae (HD82558 = Gl355) using 15 high-resolution
tostrongcross-talksbetweentheneighboringsurfacefeatures
spectrarecordedduringApril-May2000.AllHαspectrawere
throughmagneticreconnections.InTable1wegiveasummary
acquired simultaneously with the mapping lines used for the
ofthestellarparametersastheyemergedfromPaperI.
year-2000Dopplerimagingstudypresentedinourpreviouspa-
per (Ko˝va´rietal. 2004, hereafter Paper I). LQHya was first
recognized as a chromospherically active star through Ca TheobservationsareagainbrieflypresentedinSect.2and
H&Kemission(Bidelman1981;Heintz1981).TheHαabsorp- the method used for the Hα spectral study is described in
tionfilled in bychromosphericemissionwas reportedfirst by Sect.3.TheresultsarepresentedinSect.4.SincetheHαob-
Fekeletal. (1986), and LQHya was classified as a BYDra- servationsofthispaperwereincludedinthespectroscopicdata
type spotted star. Variable Hα emission-peak asymmetry was used in Paper I to reconstruct the year-2000Doppler images,
inSect.5wetaketheopportunitytocomparethephotospheric
Sendoffprintrequeststo:A.Frasca featureswiththecontemporaneousHα-emittingregions.
2 A.Frascaetal.:ChromosphericfeaturesofLQHydraefromHαlineprofiles
Table 1. Astrophysical data for LQHya (adopted from Table2.Observinglogandradialvelocities
Ko˝va´rietal.2004)
Parameter Value HJD Phase vr σvr
(kms−1) (kms−1)
Classification K2V
2451639.681 0.187 7.1 0.6
Distance(Hipparcos) 18.35±0.35pc
2451640.647 0.791 7.8 0.6
(B−V) 0.933±0.021mag
Hipparcos 2451641.648 1.416 8.6 0.5
(V−I) 1.04±0.02mag
Hipparcos 2451642.662 2.050 7.4 0.6
Luminosity,L 0.270±0.009L⊙ 2451643.744 2.726 7.6a 2.4
logg 4.0±0.5
2451644.677 3.309 8.3 0.6
T 5070±100K
eff 2451660.683 13.308 8.8 0.5
vsini 28.0±1.0km/s
2451661.668 13.924 8.7 0.6
Inclination,i 65◦±10◦
2451662.652 14.538 9.5 0.6
Period,P 1.60066±0.00013days
rot 2451663.654 15.165 7.5 0.5
Radius,R 0.97±0.07R
⊙ 2451664.667 15.798 9.0 0.6
Mass ≈0.8M
⊙ 2451665.653 16.413 9.8 0.5
Age ≈ZAMS
2451666.661 17.043 8.0 0.5
2451667.665 17.670 7.4 0.6
2451668.670 18.298 8.1 0.7
2. Spectroscopicobservations
The series of 15 Hα spectra was collected, one spectrum awith16VirasRVstandard
per night, during two observing runs (April 4–9, 2000, and
April25–May3,2000)attheKittPeakNationalObservatory
(KPNO)withthe0.9mcoude´-feedtelescope.The3096×1024 emission, which can be integrated to estimate the total radia-
tivechromosphericlossintheline.
F3KB CCD detector was employed, together with grating A,
camera 5, and the long collimator. The spectra were centered We point out that one can obtain the true chromospheric
at6500Åwithawavelengthrangeof300Å.Theeffectiveres- Hαemissionwiththismethodonlyifthechromosphereisalso
olutionwas≈28000(11 kms−1).Weachievedsignal-to-noise opticallythin inside the activeregionsorif the plagesare not
ratios(S/N)ofabout250in 45-minintegrationtime,with the extended enough to appreciably affect the photospheric spec-
onlyexceptionbeingthespectrumat2.726phaseforwhichan trum. In the opposite case, the subtraction of the underlying
S/Nofjust50couldbereachedduetobadskyconditions. photosphericprofilewouldoverestimatetheHαchromospheric
ThemeanHJDsoftheobservations,thephases,aswellas emission.
theradialvelocities,aresummarizedinTable2.Forphasingthe For this reason we have also measured the Hα equivalent
spectra,HJD=2448270.0+1.600656×Ewasused,wherethe width in the observed spectra (EWHα), fixing a constant in-
arbitrarilychosenzeropointisthesameasusedforourformer tegration window of 5.0Å, i.e. integrating the emission core
Dopplerimagingstudies(Strassmeieretal.1993;Ko˝va´rietal. inside the absorption wings (see Fig. 1). The net Hα equiva-
2004). As radial velocity (RV) standard star, βGem (vr = lentwidth,∆EWHα,wasinsteadmeasuredintheresidualspec-
3.23kms−1)wasmeasured,exceptforHJD2451643.744when tra obtained from the originalones with the “spectral synthe-
16Vir (v = 36.48kms−1) was observed (Scarfeetal. 1990). sis” method. The non-active template was built using a high-
r
FormoredetailswerefertoPaperI. resolution(R=42000)spectrumoftheK2VstarHD3765re-
trievedfromtheELODIEarchive.WechoseHD3765because
itsspectraltypeandcolorindexB−V =0.942(Weis1996)are
3. Dataanalysis
nearlyidenticalwiththoseofLQHya,becauseitsrotationve-
TheBalmerHαlineisausefulandeasilyaccessibleindicator locityisverylow(vsini=2.5kms−1,Strassmeieretal.2000),
of chromosphericactivity in the opticalspectrum. Ithas been and because it has a low level of magnetic activity compared
proventobeveryeffectivefordetectingchromosphericplages to LQHya. The spectrum was first degradedto the resolution
both in the Sun and in active stars, due to its high contrast of the KPNO spectraby convolvingitwith a Gaussiankernel
against the surrounding chromosphere (see, e.g., Frascaetal. and subsequently broadened by convolution with a rotational
1998;Biazzoetal.2006,2007). profilecorrespondingtothevsiniof28kms−1ofLQHya.
However, the Hα line is formed in a wide depth range in The error of the equivalentwidth, σEW, was evaluated by
thestellaratmosphere,rangingfromthetemperatureminimum multiplying the integration range by the photometricerror on
totheupperchromosphere.Toextractthechromosphericcon- eachpoint.Thiswasestimatedbythestandarddeviationofthe
tributionfromthelinecore,weapplyamethodusuallycalled observed flux values on the difference spectra in two spectral
“spectralsynthesis”, which consists in the subtractionof syn- regions near the Hα line. The values of EWHα, ∆EWHα, and
thetic spectra or of observed spectra of non-active standard σEW arereportedinTable3.
stars(referencespectra)(see,e.g.,Herbig1985;Barden1985; We also measuredthecentralwavelengthoftheHαemis-
Frasca&Catalano 1994; Montesetal. 1995). The difference sionbothintheraw(observed)andresidualspectrabyevaluat-
between observed and reference spectra provides the net Hα ingthecentroidofthecoreemission.Thisquantityprovidedus
A.Frascaetal.:ChromosphericfeaturesofLQHydraefromHαlineprofiles 3
We estimated the chromospheric electron density in
LQHyaadoptingtheassumptionsfromPaperII,i.e.anisother-
malchromospherewithT = 10000Kforwhichtheopti-
chrom
caldepthintheHαcentercanbederivedbytheformulationof
Cram&Mullan(1979):
∆λ 2
peak
lnτ = , (1)
chrom 2∆λ !
D
where ∆λ is the wavelength separation of the blue and
peak
red emission peaks, and ∆λ = 0.28Å is the chromospheric
D
Doppler width. We measured ∆λ on the observed spectra
peak
by fitting two Lorentzian functions to all the double-peaked
Hαprofiles.Obviously,thisanalysiscouldnotbeappliedtothe
single-peakedspectraclosetotheminimumemissionatphase
0.2–0.5. Combining equations 8 and 14 in Cram&Mullan
(1979),theelectrondensityn incm−3canbeevaluatedas
e
F B(T )
n =1.67·1014 max eff τ−1 , (2)
e F B(T ) chrom
cont chrom
where Fmax is the ratio of the Hα peak to the continuumflux,
Fcont
asmeasuredonourspectra,andBisthePlanckfunctioncalcu-
lated for the effectivetemperatureof LQHya (T = 5100K)
eff
and at T = 10000K. The optical depth at the Hα cen-
chrom
ter changesfrom about 30 to 4 and the electron density from
5·1011to4·1012cm−3fromphase≈0.2(neartheminimumHα
emission)tothephasesofmaximumemission(φ=0.6−0.8,see
Fig.2).Inthesamefigure,thefluxratiobetweentheredandthe
blue peaks FR and the peak separation∆λ are plotted ver-
FV peak
sustherotationalphase.Allthesequantitiesappearmodulated
Fig.1. Time series of Hα profiles for LQ Hya in April-May bythestellarrotation.Symmetricprofiles(FR ≃ 1)tendtobe
FV
2000 sorted in phase order. The non-active template built up observedwhenthemostactiveregions,judgingfromthepho-
with a spectrum of the non-active, slowly-rotating K2V star tosphericspotcontrast,areinthevisiblehemisphere,similarto
HD3765 is shown by a dash-dotted line at the bottom. The whatwasfoundbyStrassmeieretal.inPaperII.
vertical dotted lines mark the integration interval for the raw TheHαequivalentwidthmeasuredintheobserved(EWHα)
Hαequivalentwidth(EWHα). andintheresidualspectra(∆EWHα),theradialvelocityshifts
of the Hα emission (∆V and ∆Vres), and the contempora-
em em
neous light curve in the V band (from Paper I) are plotted in
Fig.3asfunctionsoftherotationalphase.Aremarkableanti-
withtheradialvelocityshiftsbetweentheHαemissionandthe
correlationofV brightnessandHαemissionisapparent.
photospheremeasuredbothintheraw(∆V )andintheresid-
em
ualspectra(∆Vres).Thevaluesof∆V and∆Vres arereported ThehighestelectrondensityisobservedwhentheHαline
em em em
isstrongest,indicatingadenserchromosphereabovemoreex-
inTable3aswell.
tended active regions, the opposite of what was observed in
1991(PaperII).We also foundanaveragevalueforthe chro-
4. Results mospheric electron density that was higher than the one re-
portedinPaperII(2·1011cm−3).Moreover,therotationalmod-
IntheLQHyaspectra,theHαlineisastronglyvariablefeature ulationofallthequantitiesdeducedbytheHαlineanalysisis
displayingabsorptionwingsandacorechangingfromacom- much more evidentin the presentdata. Presumably, in April-
pletely filled-in configuration to a moderate emission above May 2000, LQHya was more active than in 1991, and it dis-
the continuum (see Fig. 1). A faint single peak just filling in playedmoreandlargeractiveregions.
the line core, without reaching the continuum, is observed at
the phases of minimum Hα emission (φ=0.2−0.4), whereas
5. Discussionandconclusions
double-peakedemissionprofilesareobservedwithacentralre-
versal at the other phases. The “blue” emission peak is often To obtain information about the surface location of the ac-
stronger than the “red” one, although at some phases, nearly tive regions in the chromosphere of LQ Hya, we applied a
equal peak heights and even reversed intensity ratios are also simple geometric plage model to the rotationally modulated
observed.The Hα profilesare shown in Fig. 1 along with the chromosphericemission.Thismethodhadbeendescribedand
photospherictemplatebuiltupwithanHD3765spectrum(see successfully applied to the Hα modulation curves of sev-
Sect.3). eral other active stars (e.g., Frascaetal. 2000; Biazzoetal.
4 A.Frascaetal.:ChromosphericfeaturesofLQHydraefromHαlineprofiles
Fig.2. From top to bottom.The computedelectrondensity n Fig.3.Toppanel:V lightcurvecontemporaneoustothespec-
e
(cm−3),thefluxratiobetweentheredandthebluepeaks,and tra.Middlepanels:RotationalmodulationoftherawHαequiv-
thepeakseparationinÅagainsttherotationalphase. alent width (EW , open circles) and of the residual Hα EW
Hα
(∆EW , filled circles). Bottom panel:Velocity shift between
Hα
Hα emission and photospheric lines both for observed (open
2006; Frascaetal. 2007). On LQ Hya, two circular bright
circles) and residual spectra (filled circles). The continuous
spots (plages) are normally sufficient for reproducingthe ob-
linesin themiddle andbottompanelsrepresentthe bestfit of
servedvariationswithinthedataerrors(cf.Frascaetal.2000;
our3-plagemodel(cf.Fig.4),whilethedottedlinesrepresent
Ko˝va´ri&Bartus 2001). In our model the flux ratio between
theresultofour2-plagemodel.
plages and the surrounding chromosphere (F /F ) is
plage chrom
a free parameter. Solar values of F /F ≈ 2, as de-
plage chrom
duced from averaging many plages in Hα (e.g., Ellison stronglydependentontheassumedfluxcontrastF /F .
plage chrom
1952;LaBonte1986;Ayresetal. 1986),are toolowto model Thus, only the combined information between plage dimen-
the high amplitudes of Hα ∆EW curves in very active stars sions and flux contrast, i.e. some kind of plage “luminosity”
(Biazzoetal.2006;Frascaetal.2007).Infact,extremelylarge in units of the quiet chromosphere (L /L ), can be de-
plage quiet
plages, covering a significant fraction of the stellar surface, duced as a meaningful parameter. Note also that we cannot
would be required with such a low flux ratio, and they could estimate the true quietchromosphericcontribution(network),
not reproduce the observed modulation.In order to achieve a since the Hα minimum value, ∆EW = 0.84Å, could still
quiet
goodfit,afluxratioof5,whichisalsotypicalofthebrightest be affected by a homogeneousdistribution of smaller plages.
parts of solar plages or of flare regions (e.g., Sˇvestka 1976; However,such anapproximationseemsvalid forLQ Hya be-
Zirin1988),wasadopted. cause our aim was only to comparethe spatial distribution of
The solutions essentially provide the longitude of the themainsurfaceinhomogeneitiesatchromosphericandphoto-
plagesandgiveonlyroughestimatesoftheirlatitudeandsize. sphericlevels.
The information about the latitude of surface features can be We also searched for solutions with three active regions,
recoveredfromtheanalysisofspectral-lineprofilesbroadened finding a small but possibly significant increase in the good-
bythestellarrotation,aswedidforthespotsofLQHyawith ness of the fit, with the χ2 passing from 5.77 to 4.45 (see
DopplerimaginginPaperI.Asimilartechniquecannotreach Fig. 3). The model with only two plages requires features at
the same level of accuracy when applied to the Hα profile rather high latitudes (Table 4) to explain the nearly flat and
whosebroadeningis dominatedbychromosphericheatingef- long-lasting maximum,while a 3-plage modelallows placing
fects,whichareparticularlyefficientintheactiveregions(e.g., plagesatlowerlatitudes,inbetteragreementwiththespotlo-
Lanzafameetal.2000). cationsfromDopplerimaging.However,thelongitudesofthe
We searched for the best solution by varying the longi- two largestplagesdo notchangeverymuch(less than 15 de-
tudes, latitudes, and radii of the active regions. The radii are grees).
A.Frascaetal.:ChromosphericfeaturesofLQHydraefromHαlineprofiles 5
Table 3. Equivalentwidths of the raw (EW ) and the resid- Table4.PlageparametersforLQHyainApril-May2000.
Hα
ual spectra (∆EW ), errors of the equivalent widths (σ ),
Hα EW
andvelocityshifts∆V and∆Vresfortherawandtheresidual
em em
spectra,respectively. Radius Lon. Lat. Fplage
Fchrom
(◦) (◦) (◦)
Phase EW ∆EW σ ∆V ∆Vres
Hα Hα EW em em 2PlagesSolution
(Å) (Å) (Å) (kms−1) (kms−1) 15.5 215 35 5
0.187 0.172 0.934 0.041 −27.46 −1.78 20.0 353 66 5
0.791 0.117 1.003 0.038 −14.05 −0.13 3PlagesSolution
1.416 0.183 0.935 0.036 −29.11 −0.96
13.0 210 20 5
2.050 0.107 1.013 0.043 7.40 −1.29
13.0 8 15 5
2.726 0.116 1.045 0.213 −17.00 −5.80
9.5 280 15 5
3.309 0.239 0.879 0.037 −32.70 −2.92
13.308 0.248 0.858 0.044 −28.68 −2.19
13.924 0.058 1.045 0.048 −10.77 1.38
14.538 0.025 1.050 0.041 −20.01 −1.38
Asymmetryinthepeaksofanemissionlinewithacentral
15.165 0.099 0.998 0.041 −0.69 −0.56 reversal has been frequentlyobserved for Ca K line both in
15.798 0.073 1.050 0.036 −1.87 1.09
the Sun (e.g., Zirin 1988; Ding&Schleicher 1998, and refer-
16.413 0.215 0.919 0.035 −29.60 −0.49
ence therein) and in cool stars (e.g., Montesetal. 2000, and
17.043 0.094 1.024 0.034 −0.18 0.65
17.670 0.038 1.074 0.051 −13.38 0.36 referencetherein).Theblueasymmetryismorefrequentlyob-
18.298 0.325 0.834 0.038 −43.07 0.29 served in the solar chromosphereand is commonlyattributed
tothepropagationofacousticwaves(e.g.,Cram1976)withan
upwardvelocityontheorderof10 kms−1inthelayerinwhich
the K emission peaks are formed or a downward motion in
2
thelayerproducingthecentralreversalK (e.g.,Durrantetal.
3
Theresultingplagelongitudesareingoodagreementwith 1976).
theDopplerresultsinPaperI;i.e.,photosphericminimaat0.6 Oranje(1983)hasshownthattheaverageCaKemission
and0.9withthelargest/coolestphotosphericspotscorrespond profileinsolarplageshasacompletelydifferentshapefromthe
to the most luminous chromospheric phases, and vice versa, surroundingchromosphere,withtheredpeakslightlystronger
the brightest photometric phase at 0.3 overlap with the low- thantheblueone.Thiscouldreflectthedifferentphysicalcon-
estchromosphericHαemission,againsupportingtheparadigm ditions and velocity fields in active regions compared to the
thatphotosphericspotsarephysicallyconnectedwithchromo- quietchromosphere.Asimilaranalysiscannotbemadeforthe
sphericplages(Fig.4). Sun as a star in Hα, because thisline is an absorptionfeature
ThesyntheticHαEWcurveforthemodelwiththreeplages in the quietchromosphereand only a filling in is observedin
isplottedinthemiddlepanelofFig.3asacontinuouslinesu- plages.However,similarchangesoftheHαlineprofilecanbe
perimposed on the data (dots), while the 2-plage solution is expectedintheplagesofanystarsthataremoreactivethanthe
displayed by a dotted line. Both of them reproduce the ob- Sun. Thus, the rotational modulationof FR is consistent with
FV
servedrotationalmodulationquitewell.Withthesamemodels thepresenceofplagesinthechromosphereofLQHya.
wewereabletocalculatetheradialvelocityshiftbetweenthe These results suggest scaled-up versions (regarding size
Hαsyntheticemissionprofile,resultingfromthequietchromo- and brightness) of solar-type plages for the Hα features ob-
sphereplusthevisibleplages,andthephotosphere.Itisclearin servedinLQHyain2000,withoutanyclearevidenceofstrong
thebottompanelofFig.3thattheamplitudeofthetheoretical massmotionsinitschromosphere.
∆V curves, both for a 2-plage and a 3-plage model, is very
em
low, consistent with the valuesderived from the residualpro- Acknowledgements. We are grateful to an anonymous referee for
files (∆Vres) andin totaldisagreementwith the velocityshifts helpfulcommentsandsuggestions.Thisworkhasbeenpartiallysup-
em
ported by the Italian Ministero dell’Universita` e Ricerca (MUR),
derivedfromtherawspectra.Thisstronglysupportsthespec-
which is gratefully acknowledged. Zs. K. is a grantee of the Bolyai
tral synthesis method we used to evaluate the chromospheric
Ja´nosScholarshipoftheHungarianAcademyofSciencesandisalso
emissionintheHαline.
gratefultotheHungarianScienceResearchProgram(OTKA)forsup-
Wewouldliketooutlinetherotationalmodulationofother
portundergrantsT-048961andK68626.K.G.S.thankstheU.S.Kitt
features observed in the Hα profile, such as the peak separa-
PeakNationalObservatoryforthepossibilitytorecordlongtimese-
tion, which is related to the chromospheric electron density riesofstellardatawiththelateCoude´-feedtelescope,nowretired.
(Sect.4)andtheasymmetryofblue/redpeakintensity(Fig.2).
Thechromosphereof LQHya displaysa higherelectronden-
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