Modeling of PMS Ae/Fe Stars Using UV Spectra ,
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A&A 456, 1045–1068 (2006) Astronomy DOI: 10.1051/0004-6361:20040269 & c ESO 2006 Astrophysics Modeling of PMS Ae/Fe stars using UV spectra, P. F. C. Blondel1,2 andH.R.E.TjinADjie1 1 Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands e-mail: [email protected] 2 SARA, Kruislaan 415, 1098 SJ Amsterdam, The Netherlands Received 13 February 2004 / Accepted 13 October 2005 ABSTRACT Context. Spectral classification of PMS Ae/Fe stars, based on visual observations, may lead to ambiguous conclusions. Aims. We aim to reduce these ambiguities by using UV spectra for the classification of these stars, because the rise of the continuum in the UV is highly sensitive to the stellar spectral type of A/F-type stars. Methods. We analyse the low-resolution UV spectra in terms of a 3-component model, that consists of spectra of a central star, of an optically-thick accretion disc, and of a boundary-layer between the disc and star. The disc-component was calculated as a juxtaposition of Planck spectra, while the 2 other components were simulated by the low-resolution UV spectra of well-classified standard stars (taken from the IUE spectral atlases). The hot boundary-layer shows strong similarities to the spectra of late-B type supergiants (see Appendix A). Results. We modeled the low-resolution UV spectra of 37 PMS Ae/Fe stars. Each spectral match provides 8 model parameters: spectral type and luminosity-class of photosphere and boundary-layer, temperature and width of the boundary-layer, disc-inclination and circumstellar extinction. From the results of these analyses, combined with available theoretical PMS evolutionary tracks, we could estimate their masses and ages and derive their mass-accretion rates. For a number of analysed PMS stars we calculated the corresponding SEDs and compared these with the observed SEDs. Conclusions. All stars (except β Pic) show indications of accretion, that affect the resulting spectral type of the stellar photosphere. Formerly this led to ambiguities in classificaton of PMS stars as the boundary-layer was not taken into consideration. We give evidence for an increase of the mass-accretion rate with stellar mass and for a decreases of this rate with stellar age. Key words. stars: pre-main sequence – accretion, accretion disks 1. Introduction Valenti et al. 2000). Hillenbrand et al. (1992) have successfully fitted the visual and near-infrared (NIR) spectral energy distri- During the past decades it has become clear that low-mass stars butions (SEDs) of a large sample of Herbig Ae/Be stars with a ( M < 1.0 M) in their PMS evolutionary stage, known as “clas- model consisting of a stellar photosphere and a flat, optically- sical” T Tauri stars (cTTs), are generally surrounded by accre- thick accretion disc with an optically-thin circular central hole. tion discs, which the contraction models of a rotating protostar These authors did not include the contribution of an accreting predict. This has raised the question of whether more massive boundary-layer to the calculated SED. stars (1.0 M < M < 4.0 M) that radiate strongly in the UV and thus are able to ionise the infalling material and even reverse the The larger brightness of the PMS Ae/Fe stars makes it infall of material, have a similar PMS evolutionary phase, in par- easier to obtain well-exposed (UV) spectra than for T Tauri ticular, if Herbig Ae/Be stars also feature an accretion disc. stars. We analysed the low-resolution UV spectra, obtained with Herbig stars are usually intrinsically brighter than T Tauri IUE in both the short (SW) and long (LW) wavelength range stars, which makes them observable at larger distances and larger (Appendix F), of a sample of PMS Be/Ae/Fe stars (TWP94) in foreground extinction. However, because of their higher masses terms of a 3-component model (UV3C hereafter) that consists and therefore faster evolution, they are less numerous than the of a star, an accretion disc, and a boundary-layer (BL hereafter), T Tauri stars. through which the disc interacts with the star. The formalism of So far, various indications of matter falling towards a the model is given in Sect. 2, and in Sect. 3 we discuss the nor- Herbig Ae/Be star have been found from high-resolution spec- malisation and the extinction corrections needed before we can troscopy in the visual (Hamann & Persson 1992; Graham 1992; compare the model spectrum to the observed spectrum. Tambotseva et al. 2001) and in the UV (Blondel et al. 1993; Pérez et al. 1993; Brown et al. 1997; Tjin A Djie et al. 1998; In Sect. 4 we describe the procedure for matching the model spectrum to the observed spectrum and thus of determining the model parameters. Input values for these parameters were ob- Based on observations by the International Ultraviolet Explorer col- − lected at the Villafranca Satellite Tracking Station of the European tained from Appendices B D and from Table 1. In this section Space Agency, on observations made by the Hipparcos Astrometry we also show the sensitivity of the model spectrum to variations Satellite, and on ground-based photometric and spectroscopic observa- in the various model parameters. An important result is that in tions made at the European Southern Observatory, La Silla, Chile. the IUE range the model spectrum receives no significant direct Figures 4–9, 33–40 and 45–110 and Appendices A−G are only contributions from the disc component, so that our analysis only available in electronic form at http://www.edpsciences.org models the photosphere and the BL. Indirectly, however, the disc Article published by EDP Sciences and available at http://www.edpsciences.org/aa or http://dx.doi.org/10.1051/0004-6361:20040269 1046 P. F. C. Blondel and H. R. E. Tjin A Djie: Modeling of PMS Ae/Fe stars using UV spectra plays a role by shadowing the photospheric contribution, which Comparison with spectra from computed model atmospheres justifies the use of the name UV3C. (e.g. Kurucz models, Kurucz 1979, 1991) would be more elab- Section 5 presents the results of the UV spectral analysis of orate and less accurate (because these models lack peculiar ab- 35 PMS Be/Ae/Fe stars and 2 cTTs. All stars (with the possi- sorption lines that may still be unexplained). After we know the ble exception of β Pic and BD+47◦4206) are well-known young spectral type (ST) and luminosity class (LC) of the photosphere emission-line stars. Our choice of stars is restricted by the avail- from the UV classification, we use Kurucz model fluxes to cal- ability of low-resolution UV spectra (both IUE SW and LW). A culate the photospheric contributions in the visual and NIR pho- second restriction was made based on the quality of the observed tometric bands. spectra, which eliminates the following 6 stars from Table 1: WW Vul, BD+24◦676, V350 Ori, R CrA, RR Tau, and RY Tau. The main conclusion from our analysis is that the BL contribu- 2.3. The accretion disc tion to the spectra in our sample can be simulated by spectra of i i i Following BBB88 we assume that the disc is a flat, optically- type B5 -A0 , most frequently B9 ab. In several cases the BL thick, region with a continuum spectrum that can be simulated component in the UV spectrum is so strong that it influences the by the summation of a sequence of concentric, isothermal rings photometric bands in the near-UV and, therefore, the spectral T r + ◦ with Planck spectra. The local temperature d( ) of each ring classification based on photometry (e.g. NX Pup, BD 46 3471, can be derived from the sum of the local rate of viscous heating HK Ori, V380 Ori, and KK Oph). We also calculated the SEDs from accretion Fv(r) and the local rate of absorption of radiation of the program stars in the visual and NIR. Some of these stars from the stellar photosphere F (r): – NX Pup, V380 Ori, HK Ori, and HD 139614 – are observed a at low inclinations (almost pole-on), which implies that their BL σT 4(r) = F (r) + F (r, T ). (1) may also contribute significantly to the UBV-fluxes. The pres- d v a ence of this BL contribution may be an important reason why F r, T spectral classifications of the stars based on different distinct fea- The absorption from the stellar photosphere a( ) has been tures of the spectrum have led to different spectral types. evaluated by Adams & Shu (1986): In Sect. 6 we use the UV3C-model parameters to determine T 4 R the position of the PMS stars in the HRD and to look for corre- F (r, T ) = σ (ϕ − sin ϕ) with sin ϕ = · (2) lations between the evolutionary status of the star-disc-system, a 2π r and its mass and c.s. extinction. And finally in Appendix A we discuss alternative interpretations of the spectral features of If in addition the disc is in Kepler rotation and the star rotates PMS Ae/Fe stars in the UV and compare our results with those slowly, then of a few theoretical models. 3GM M˙ √ R F (r) = 1 − y with y = · (3) v 8πr3 r 2. The UV3C-model 2.1. General description In this case the integrated dissipated energy in the disc is half of the total accretion luminosity GM R M˙ ,where M and R are Our way of analysing the low-resolution UV spectra of PMS the stellar mass and radius and M˙ is the accretion rate through Ae/Fe stars is similar to that developed by Bertout et al.