arXiv:astro-ph/0503548v1 24 Mar 2005 bevtr,L il,Chile Silla, La Observatory, as o xdrtto aetednm cini togri 1987). stronger Vilhu, is e.g, action (see, dynamo zone convective the thicker rate a be rotation secondary, fixed con- active a magnetically Shallower for less review.) cause a his mean in would AML-models vection does the (2003) Webbink include that way, not the by (1988), e. (Note, see Maceroni (2003). (TRO), & Webbink models Veer van’t Oscillation Relaxation mod- and Thermal the (1982) (AML) and Vilhu braking e.g. magnetic see particula via els, in loss equilibrium), momentum thermal i angular of of (out because spots secondary, status the shal physical for predict cool zones models convective of evolutionary outer the presence lower of the Some primary. to the discrepancy on this of origin observations but (by companion, opposite. the hotter the than show slightly degrees) i.e be hundred ( should few primary a component the en- masssive) 1968) constant more (Lucy, theory of the the envelope each to convective touch According common tropy. components a Their W-subtype. inside of other ) UMa W ( Y r,GV P G5V, Eri, (YY EPoncs(EPe 0,P G0V, Phe, (AE Phoenicis AE Introduction 1. edo Send ⋆ later) hand by inserted be will Astrophysics (DOI: & Astronomy ae nosrain olce tteErpa Southern European the at collected observations on based uln(95 n uisi(95 srbdtepossible the ascribed (1985) Rucinski and (1975) Mullan ff opr h nrni qiaetwdh fbt components both of widths equivalent intrinsic CAT the the with compare 1995 and 1990 1989, in obtaineded 1 2 epeetaddsusteH the discuss and present We Abstract. accepted ; Received httepiayi h oemgeial ciecmoeta component active magnetically more the is primary the Th that activity. chromospheric enhanced indicating emission, n htteaeaeH average the that find e words. Key plages. chromospheric rn eussto requests print ufc mgn flt-yecnatbnre I H II: binaries contact late-type of imaging Surface bevtr,Bx1,FN004Uiest fHlik,Fin Helsinki, e-mail: of University FIN-00014 14, Box Observatory, NF sevtroAtooiod oa i rsai3,I- 33, Frascati e-mail: via Roma, di Astronomico Osservatorio INAF- [email protected] [email protected] orb otc iais–H – binaries contact = msini EPoncsadY Eridani YY and Phoenicis AE in emission .Vilhu O. : .1 )aelt yecnatbinaries contact type late are d) 0.312 α qiaetwdh r ls otestrto odradtha and border saturation the to close are widths equivalent aucitn.aepheyyeri-c5 no. manuscript orb α = ( λ α .6 )adY Eridani YY and d) 0.362 = eiso antcactivity magnetic – -emission 53)osrain ftecnat( M ye iaisA P AE binaries type) UMa (W contact the of observations 6563Å) .Vilhu O. / E eecp fteSuhr uoenOsraoy(S) I (ESO). Observatory European Southern the of telescope CES the r 1 si optbewt ohtertcladosrainlsu observational and theoretical both with compatible is is n .Maceroni C. and g. ts n - - ihteNxGntertclmdl n h auainlimi saturation the and models theoretical NextGen the with di le ih(otyurslal)dr pt n associ and spots dark unresolvable) (mostly with filled is nd land 04 otpri R)Italy (RM) Monteporzio 00040 andmsl nxlrdatog aeoie l (1994), al. H and et photometry Maceroni both although using unexplored mostly mained ope mgn ehius on h rmr fA Phe indicati AE spots. further of dark a unresolved primary provided and the cooler, found spectroscopically techniques, imaging Doppler activity. generated) (dynamo gene magnetic are the spots to Dark related surface. ally primary the phot on large spots and dark component metric secondary the than cooler slightly .Osrain n H and Observations 2. 4. Section in given Sec conclusions in the discussed and 3 are results chromospheri The t the filling absorption). regions to photospheric spot penumbral (due from or limit plages from saturation emission the with and tions H Sec the in explained and are 2 observations The 2004). techniq al., imaging interest et Doppler (Barnes rejuvenated the to a due is mostly binaries, motivation contact observat The unpublished 1995. similar here- in with (1994, collected al. together et I), Maceroni paper by performed after Eri YY and Phe AE CueAxlayTlsoe fteErpa Southern European 20-25, November the during Chile) CAT-telescope Silla, of La at the Telescope) (ESO Observatory with Auxiliary performed (Coude were observations The h bevtoa ofimto fti rdcinhsre- has prediction this of confirmation observational The ntepeetpprw eaayeteH the re-analyse we paper present the In resolution high using (2004), al. et Barnes recently, More α 2 qiaetwdh oprdwt oe predic- model with compared widths equivalent h rmr opnnshv xesH excess have components primary the t α α qiaetwidths equivalent pcrsoy on primary a found spectroscopy, oncsadY Eridani, YY and hoenicis ⋆ α atclr we particular, n 6563 α osrain of -observations ggestions .We t. uy2,2018 23, July ated Å α - nof on ions tion tion ues he o- in r- c 2 O. Vilhu and C. Maceroni: Hα 6563Å emission in AE Phe and YY Eri

Fig. 1. Hα 6563Å dynamic spectra of AE Phe and YY Eri from the 1995 observations. The grey scale is linear, the white corresponding to the continuum and the darkest colour (at phase 0.5) to 60 per cent of the continuum level.

1989, November 17-23, 1990, and October 26-31, 1995. The resulting equivalent widths are, however, relative to the total Coude Echelle Spectrometer (CES), with the short camera in continuum. Since we are interested in the intrinsic equivalent the red andresolutionof 60 000( 5 kms−1)atHα 6563 Å, was widths of the components, the measured values were further used. The exposure times were 15 and 20 minutes for AE Phe scaled with the component . If (m = 7.9) and YY Eri (m = 8.4), respectively. This guaran- V V Lp = L/aand Ls = L(a-1)/a, teed orbital smearing of less than 0.05 in phase for both stars. where L, L and L are the total, primary and secondary Complementary photometric observations in B, V, and I filters p s luminosities, respectively, then were obtained with the ESO 50 cm telescope. The sky con- 0.92 4 ditions during the 1995 observations, however, allowed to get a = 1 + q (Ts/Tp) complete, but low quality, light curves for AE Phe only. The which follows directly from the contact condition and photometric observations were useful, at any rate, to check the the definition of effective temperatures Tp and Ts (see e.g. correct orbital phasing of the spectra. Webbink, 2003). Here q is the mass ratio Ms/Mp. A sample of line profiles for the 1989 and 1990 were Using the photometric effective temperatures and mass ra- shown in Maceroni et al. (1994) and are not repeated here for tios found by Maceroni et al. (1994): ((Ts, Tp, q) = (6140 K, the sake of brevity.In Fig. 1 the grey-scaledynamiclight curves 6000 K, 0.39) for AE Phe and = (5590 K, 5390 K, 0.44) for (phase vs. λ) for the 1995 are shown. The grey-scaleis lin- YY Eri), we find a = 1.46 and = 1.54 for AE Phe and YY ear, the white corresponding to the continuum and the darkest Eri, respectively. These values are very close to those of paper colour (at phase 0.5) to 60 per cent of the continuum level. I from photometric solutions. The intrinsic equivalent widths The curves of the broad Hα-absorption in both can be computed from the measured ones by components are clearly seen, the curves with larger amplitudes EWp = axEWp(measured) and corresponding to the secondary (less massive) component. EWs = a/(a-1) x EWs(measured). The equivalent widths were measured at elongations, as The equivalent widths are listed in Table 1 and their mean mean values between phases 0.15 - 0.35 and 0.65 - 0.85. The values plotted in Fig.2. We also used NextGen photospheric O. Vilhu and C. Maceroni: Hα 6563Å emission in AE Phe and YY Eri 3

Fig. 2. Mean Hα equivalent widths for the primary and secondary components of AE Phe and YY Eri (from Table 1). The values from theoretical NextGen-models (Hauschildt et al., 1999) are also shown (solid line) together with the saturation limit found by Herbst & Miller (1989) (dotted line). models1, with solar abundances and gravity log(g) = 4.5 Table 1. Hα equivalent widths (EW, in units of Ångstr¨oms) of (Hauschildt et al., 1999; Short & Hauschildt, 2005) and com- the more massive (p, primary) and the secondary (s) compo- puted the Hα equivalent widths for several temperatures, as nents of AE Phe and YY Eri. The values are average values shown in Fig.2. from the observations of 1989, 1990 and 1995. The observed These theoretical models match quite well with the solar values are direct measurements, with the total as value marked in Fig.2 (EW = 3.2 Å), as observed with the continuum, over the orbital phases 0.15 - 0.35 (marked as 0.25) CAT/CES telescope by exposing the twilight sky before the ob- and 0.65 - 0.85 (0.75).The intrinsic values are scaled with the servations. The (absorption) equivalent widths of AE Phe and components’ individual luminosities (see text).These intrinsic YY Eri are clearly below the theoretical predictions. values are shown in Fig.2. The errors include the differences An explanation (which we adopt here) for this defi- from to epoch. ciency is that chromospheric emission fills-in the photospheric absorption, thus lowering the measured equivalent widths. comp EW(0.25) EW(0.75) EW mean EW intrinsic ± . ± . ± . ± . Herbst & Miller (1989) have estimated this emission for a large AE Phe p 1.25 0 1 1.05 0 05 1.15 0 07 1.68 0 10 AE Phe s 0.90 ±0.07 0.95 ±0.07 0.92 ±0.07 2.92 ±0.20 sample of K-M stars. They found an upper bound (the satura- YYErip 1.00 ±0.10 0.90 ±0.07 0.95 ±0.10 1.45 ±0.15 tion limit) to the fraction of star’s bolometric luminosity that ± . ± . ± . ± . −3.9 YYEris 0.70 0 07 0.75 0 07 0.74 0 07 2.00 0 20 can appear as Hα emission: LHα/Lbol = 10 . Using FHα/Fbol- values at different effective temperatures, as computed from the NextGen - models, this relation can be easily converted to the 3. Discussion saturation line in Fig.2. Both AE Phe and YY Eri clearly have equivalent widths of the 1 The NextGen uses the model atmosphere code. The Hα-absorptionsmaller than the Sun and as well than those, pre- code is available from http://dilbert.physast.uga.edu/yeti dicted by NextGen-models for normal main sequence stars of 4 O. Vilhu and C. Maceroni: Hα 6563Å emission in AE Phe and YY Eri similar effective temperatures. This can be interpreted as being References due to extra chromospheric Hα emission, that partly fills the Barnes, J. R., Lister, T. A., Hilditch, R. W., & Collier Cameron, photospheric absorption. The average values of both stars lie A. 2004, MNRAS, 348, 1321 close to the saturation limit. This behaviour is similar to other Hauschildt, P. H., Allard, F., & Baron, E. 1999, ApJ, 512, 377 chromosphericemission diagnostics (see e.g. Vilhu, 1987), giv- Herbst, W., & Miller, J. R. 1989, AJ, 97, 891 ing additional support for this interpretation. Lucy, L. B. 1968, ApJ, 151, 1123 The components of YY Eri are not very different from Maceroni, C., Vilhu, O., van’tVeer, F., & van Hamme, W. 1994, each other, but in AE Phe the primary has clearly much more A&A, 288, 529 (paper I) Hα-emission than the secondary. This is presumably due to a Mullan, D.J., 1975, ApJ198, 563. weaker dynamo-generated magnetic activity of the secondary. Rucinski,S.M. 1985, in Pringle J.E. and Wade R.A. (eds.), Since both components rotate with the same rate and have Interacting Binary Stars, Cambridge Univ. Press, p. 85. almost the same spectral types (effective temperatures) they Short, C. I., & Hauschildt, P. H. 2005, ApJ, 618, 926 probably differ with respect to another crucial parameter of dy- Webbink R.F. 2003, in Turcotte S., Keller S., Keller S.C., namo theories, the thickness of the convective zone; the shal- Carallo R.M. (eds.), ASP Conf.Ser.Vol.293, 3D Stellar lower this zone, the weaker dynamo action (see e.g. Vilhu, Evolution Contact Binaries, Astron.Soc.Pac., San Francisco, 1987). Theoretical contact binary models predict shallower p.76. convective zones for the secondaries, as well, due to their ther- van’t Veer, F., & Maceroni, C. 1988, A&A, 199, 183 mal non-equilibrium condition (AML- or TRO-models, (see Vilhu, O. 1982, A&A, 109, 17 e.g. Vilhu, 1982; Webbink, 2003)). Vilhu, O. 1987, in Linsky J.L. and Stencel R.E. (eds.), The equivalent widths remained practically the same over Cool Stars, Stellar Systems, and the Sun, Lecture Notes all our observing runs, from 1989 to 1990 and 1995. In particu- in Physics No.291, Springer-Verlag, Berlin-Heidelberg-New lar, the 1989 and 1990 observationsshowed that the larger pho- York, p.110. tometric spots are found on the primary star (Maceroni et al., 1994), as well as weaker Hα-absorption, compatible with the present results . Barnes et al. (2004) interpreted their spectro- scopic observations (analysed by Doppler-imaging) by intro- ducing unresolved dark spots on the primary. Since the appear- ance of active chromospheres (plages) and cool spots correlate and are the results of the same physical phenomenon,our inter- pretation sounds valid. The phase 0.75 side of the AE Phe primary is chromospher- ically more active than the 0.25 side (see Table 1). This is com- patible with the larger spots found on this side during the first two observing runs by Maceroni et al. (1994) (paper I).

4. Conclusions We have shown that the contact binaries AE Phe and YY Eri have a weaker photospheric Hα 6563 Å absorption than normal slowly rotating main sequence stars of the same spectral type have. This can be interpreted as due to the enhanced chromo- spheric emission in the rapidly rotating components of these contact binaries. This emission is close to the saturation limit (see Fig.2) and the behaviour is similar to many other chromo- spheric diagnostics found earlier. In AE Phe the primary (moremassive component)is clearly more active in this respect (smaller absorption as compared with the secondary or the saturation limit). This is apparently a result from the larger depth of its outer convective zone supporting a stronger dynamo-action, compatible with some structural and evolutionary models of contact binaries: AML- models, (see e.g. Vilhu, 1982; van’t Veer & Maceroni, 1988) and TRO-models, (see e.g. Webbink, 2003, for references). Acknowledgements. We are grateful to the 6.3 magnitude earthquake during the 1995 observations, whose only consequence was that we will remember that night forever. CM acknowledeges funding of this research project by MIUR/Cofin and F-INAF programs.