Mem. S.A.It. Vol. 77, 362c© SAIt 2006 Memorie della

Seismology and rotation of the Herbig Ae starHD 104237

T. Bohm1, M.A Dupret.2, and H. Aynedjian1

1 Observatoire Midi-Pyrenees, LATT, 14 ave E. Belin, F - 31400 Toulousee-mail: boehm@obs-mip.fr

2 Observatoire de Paris-Meudon, LESIA, F - 92195 Meudon

Abstract. HD 104237 is the first pulsating Herbig Ae star for which very recently a sig-nificant number of pulsation frequencies has been detected by means of high-resolutionspectroscopy. The high quality radial velocity curve based on 1888 individual echelle spec-tra obtained in 42 nights in 1999 and 2000 revealed for the first time by spectroscopic meansmultiperiodic oscillations in a pre-main sequence star: a total of 8 frequencies have been de-tected so far, 5 amongst them were detected at a very high confidence level Bohm et al.(2004). This result encouraged us to develop asterosismological models for such young pre-main sequence pulsator (Dupret et al. (2005)), but the dramatic lack of a precise knowledgeof the fundamental stellar parameters appear still to be critical. A summary of the mainresults of the pulsation study are presented and first indications for a stellar rotational mod-ulation are exhibited.

Key words. Stars: pre-main-sequence – Stars: oscillations – Stars:individual: HD104237 –Binaries:spectroscopic

1. Introduction

The Herbig Ae/Be stars are pre-main sequence(PMS) objects of intermediate mass (2-8 M�)(Herbig (1960)). They all show signs of in-tense stellar activity and strong stellar windswhich are most likely linked to the pres-ence of stellar magnetic fields and, in somecases, interacting circumstellar accretion disks.However, their position in the HR diagram in-dicates that they are in the radiative phase oftheir contraction towards the main sequence(Iben (1965)) and should in principle not pos-sess any outer convective zone; therefore, if theyoung stellar evolutionary theory is correct, theclassical solar-type magnetic dynamo mecha-

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nism could not be responsible for the observedphenomena. Finding the origin of this para-doxical activity is a major concern for youngstellar evolution. Marconi & Palla (1998) pre-dicted the existence of a pre-main-sequence in-stability strip, which is being crossed by mostof the intermediate mass PMS objects for a sig-nificant fraction of their evolution to the mainsequence. This strip covers approximately thesame area in the HR diagram as the δ Scutivariables and until now about 20 PMS pul-sators have been detected, HD 104237 beingamong the brightest PMS pulsators detected sofar. Asteroseismological studies of these youngpulsators will provide new insights about theirinternal structure and help understanding theorigin of activity and winds in these objects.

Bohm: Seismology and rotation of the Herbig Ae star HD 104237 363

A recent review by Catala (2003) presentsthe status of observational studies of pulsationsin these stars, but the field is actually in veryquick evolution.

The aim of our study (Bohm et al. (2004))was to provide a first set of asteroseismicconstraints for forthcoming non-radial pulsa-tion models by determining unambiguouslya higher number of periodicities with theircorresponding amplitude and phase values:to achieve this goal, we decided to performhigh resolution spectroscopic observations ona large time basis and with optimized time cov-erage.

2. Detection of multiperiodic stellaroscillations

On the basis of two years of spectroscopicechelle observations providing a high precisionradial velocity curve (the resolving power ofthe spectrographs employed in the bi-site cam-paign was higher than R = 35000), we clearlyestablish for the first time that HD 104237 Ais a multiperiodic pre-main sequence pulsator.Five oscillation frequencies between 32.37 and35.60 d−1 have clearly been observed in 1999and in 2000, but with different amplitudes andamplitude ratios. Their existence is thereforefirmly established. An additional 3 frequenciesare reported based on the 1999 observations,but have not been found in the year 2000 ob-servations. The numerical results of the periodanalysis are given in Table 1.

3. Clues for rotational modulation

Detecting the largest possible amount of pul-sation frequencies is obviously the essentialfactor concerning the development of reliableasteroseismological models. Of similar impor-tance is however the precise determination ofall fundamental stellar parameters which en-ter the oscillation code either directly (suchas vsin i, sin i, Prot) or indirectly via the stel-lar structure model (eg. Teff , log g, metallic-ity). HD 104237 shows a fairly small vsini of12±2 km s−1 (Donati et al. (1997)), whichcould be due to i) an intrinsically large pe-riod of stellar rotation or ii) a star seen more

or less pole on. Rotational splitting of oscilla-tion modes being obviously sensitive only to(i), it was important to search for clues of rota-tional modulation of lines forming nearby thestellar photosphere providing us with an in-dependent measure of the stellar rotation pe-riod. We produced therefore two-dimensionaltime series based on our extensive data setshowing the evolution of highly resolved lineprofiles of the most active lines in the visi-ble spectrum. Fig. 1 shows the variation ofthe Hα profile during more than 350 hrs. It isclearly seen that around 50 hrs, 150 hrs and250 hrs the blueshifted absorption componentat approximately - 300 km s−1 becomes signif-icantly deeper and broader. If interpretated ascorotating clouds close to the stellar surface,this would yield a Prot of 100±5 hrs. These re-sults are still preliminary and need further con-firmation. The 2D periodogram (right of Fig.1) shows much more energy in the redshiftedemission, which does not seem to be concen-trated around a particular frequency. These re-sults are difficult to interpret, but the resid-ual profile 2D spectrum (obtained after sub-tracting an average profile from all individualprofiles) represented in Fig. 2 shows interleav-ing emission and absorption features (relativeto the average profile) transiting from the redto the blue part of the P Cygni emission pro-file. Typical transition timescales appear to bearound 100 hrs. If confirmed, and by taking Teff

= 8500 K, R = 2.5± 0.2 R� (calculated fromthe stellar luminosities announced in the litter-ature) and vsini = 12±2 km s−1, the inclinationof the stellar rotation axis would be i = 23+9

−8 de-grees, corresponding to an intermediate posi-tion, but closer to pole-on (i=0o) than to edge-on (i=90o).

4. First asteroseismological models

The numerous pulsation frequencies detectedin HD 104237 give a unique opportunityto probe the internal structure of a youngpre-main sequence star. Further work is re-quired to understand the driving of the modes,which cannot be the classical κ-mechanism ((2005)). HD 104237 is thus not a δ Sct-typepulsator. The equidistance found in the fre-

364 Bohm: Seismology and rotation of the Herbig Ae star HD 104237

Table 1. Frequencies and amplitudes derived from the Fourier analysis of the 1999 (left) and2000 (right) data. The uncertainty of the frequency is less than 0.04 c/d (Bohm et al. (2004)).

1999 2000Frequency Amplitude Conf. Frequency Amplitude Conf.(c/d) (km s−1) ¿% (c/d) (km s−1) ¿%f1: 33.289 1.320 99.9 33.283 0.258 99.9f2: 35.606 0.474 99.9 35.609 0.328 99.9f3: 28.503 0.195 99.9 28.521 0.165 99.9f4: 30.954 0.139 99.0 31.012 0.177 99.9f5: 33.862 0.099 99.0f6: 32.616 0.105 99.0 32.375 0.113 99.9f7: 34.88 0.1f8: 35.28 0.05

Fig. 1. Dynamical spectrum of the P Cygni Hα profile during the bi-site campaign SAAO/MtStromlo in April 2000. The left figure shows the periodically varying blueshifted absorptioncomponent. The redshifted emission reaches up to more than 7 times the continuum and showsalso strong variations, which are more difficult to interpretate. The right figure shows the cor-responding 2D time serie analysis (frequencies being expressed in cycles/days). Each velocitybin was analyzed making use of the ”SigSpec” periodicity analysis software (P. Reegen, Vienna,private communication). The 2D periodogram reveals the presence of the periodically varyingblueshifted absorption component at approximately - 300 km s−1 and 0.01c/d.

Bohm: Seismology and rotation of the Herbig Ae star HD 104237 365

0

1

2

3

27 28 29 30 31 32 33 34 35 36 37

Frequency (c/d)

M/M0 = 2.50 Teff = 7657.3 K Log (L/L0) = 1.6242

R/R0 = 3.6935 age(My) = 2.7561

p11 p12 p13 p14

p10 p11 p12 p13

p10 p11 p12 p13

p10 p11 p12 p13

Fig. 3. Theoretical and observed frequenciesfor the actual ”best fit” model of HD 104237.The observed frequencies correspond, if con-firmed, to high radial order p11 to p15 modes.

Fig. 2. Dynamical spectrum of the residual Hαprofile of HD 104237.

quency spectrum could be the large separa-tion. Luminosities higher than the ones cur-rently found in the literature (Van den Anckeret al. (1998)) are required to fit this large sep-aration. Better spectroscopic determinations ofTeff and log g will be preformed soon by Bohmet al., which will allow us to go further in ourseismic study and better constrain the internalphysics of this star. The actual ”best fit” modelis represented in Fig. 3.

5. Conclusion

The work presented in this paper is still in apreliminary state. However, it can clearly beseen how promising is the interwork betweenhigh-resolution spectroscopic campaigns andtheir results and in-depth asteroseismologicalmodeling. We intend to eliminate uncertaintieson fundamental stellar parameters in the nextstep and to improve the precision of the modelin the very near future.

Acknowledgements. We would like to thank PeterReegen for providing us with the periodicity analy-sis tool ”SigSpec”. We are also especially gratefulto C. Catala and M.-J. Goupil for the fruitful collab-oration on this topic.

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