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EX Lupi in Quiescence

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EX Lupi in Quiescence A&A 507, 881–889 (2009) Astronomy DOI: 10.1051/0004-6361/200911641 & c ESO 2009 Astrophysics EX Lupi in quiescence N. Sipos1, P. Ábrahám1, J. Acosta-Pulido2, A. Juhász3, Á. Kóspál4,M.Kun1, A. Moór1, and J. Setiawan3 1 Konkoly Observatory of the Hungarian Academy of Sciences, PO Box 67, 1525 Budapest, Hungary e-mail: [email protected] 2 Instituto de Astrofísica de Canarias, E-38200 La Laguna, Tenerife, Canary Islands, Spain 3 Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany 4 Leiden Observatory, Leiden University, PO Box 9513, 2300RA Leiden, The Netherlands Received 9 January 2009 / Accepted 4 June 2009 ABSTRACT Aims. EX Lupi is the prototype of EXors, a subclass of low-mass pre-main sequence stars whose episodic eruptions are attributed to temporarily increased accretion. In quiescence the optical and near-infrared properties of EX Lup cannot be distinguished from those of normal T Tau stars. Here we investigate whether it is the circumstellar disk structure that makes EX Lup an atypical Class II object. During outburst the disk might undergo structural changes. Our characterization of the quiescent disk is intended to serve as a reference for studying the physical changes related to one of EX Lupi’s strongest known eruptions in 2008 Jan–Sep. Methods. We searched the literature for photometric and spectroscopic observations including ground-based, IRAS, ISO, and Spitzer data. After constructing the optical–infrared spectral energy distribution (SED), we compared it with the typical SEDs of other young stellar objects and modeled it using the Monte Carlo radiative transfer code RADMC. We determined the mineralogical composition of the 10 μm silicate emission feature and also gave a description of the optical and near-infrared spectra. Results. The SED is similar to that of a typical T Tauri star in most aspects, though EX Lup emits higher flux above 7 μm. The quiescent phase data suggest low-level variability in the optical–mid-infrared domain. By integrating the optical and infrared fluxes, we derived a bolometric luminosity of 0.7 L. The 10 μm silicate profile could be fitted by a mixture consisting of amorphous silicates, but no crystalline silicates were found. A modestly flaring disk model with a total mass of 0.025 M and an outer radius of 150 AU was able to reproduce the observed SED. The derived inner radius of 0.2 AU is larger than the sublimation radius, and this inner gap sets EX Lup apart from typical T Tauri stars. Key words. stars: formation – stars: circumstellar matter – stars: individual: EX Lup – infrared: stars 1. Introduction to a circumstellar disk (Gras-Velázquez & Ray 2005), no de- tailed analysis or modeling of the disk structure has been per- In 2008 January the amateur astronomer A. Jones announced formed so far. Such an analysis could contribute to clarifying that EX Lupi, the prototype of the EXor class of pre-main se- the eruption mechanism and also answering the longstanding quence eruptive variables, had brightened dramatically (Jones open question of what distinguishes EXors from normal T Tau 2008). The quiescent phase brightness of EX Lup, an M0 V star ≈ stars. Herbig (2008) found no optical spectroscopic features that in the Lupus 3 star-forming region, is V 13 mag. During its un- would uniquely define EXors, and he also concluded that their predictable flare-ups, its brightness may increase by 1−5mag location in the (J−H)vs.(H−Ks) color−color diagram coincides for a period of several months (Herbig 1977). In past decades with the domain occupied by T Tau stars. EX Lup produced a number of eruptions, the last one in 2002 In this work we search the literature and construct a complete (Herbig 1977; Herbig et al. 2001; Herbig 2007). During the optical–infrared spectral energy distribution (SED) of EX Lup present outburst, which lasted until 2008 September (AAVSO 1 that is representative of the quiescent phase. The SED is mod- International Database ), EX Lup reached a peak brightness of eled using a radiative transfer code and will be compared with 8 mag in 2008 January, brighter than ever before. typical SEDs of T Tau stars, in order to reveal differences possi- According to the current paradigm, eruptive phenomena in bly defining the EXor class. During the recent extreme eruption, pre-main sequence stars (FU Ori and EX Lup-type outbursts) are EX Lupi was observed with a wide range of instruments. Our caused by enhanced accretion onto the central star. Intense build- results on the physical parameters of the system can be used as up of the stellar mass takes place during these phases of the a reference for evaluating the changes related to this outburst. star formation process (Hartmann & Kenyon 1996). In this pic- ture circumstellar matter must play a crucial role in the eruption mechanism. Surprisingly little is known about the circumstel- 2. Optical / infrared data lar environment of the EXor prototype, EX Lup. While its in- frared excess emission detected by IRAS and ISO was attributed 2.1. Optical and near-infrared photometry from the literature Full Table 3 is only available in electronic form at the CDS via We searched the literature and collected available optical and anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via near-infrared photometric observations of EX Lup obtained dur- http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/507/881 ing its quiescent phase. We defined quiescence periods as when 1 www.aavso.org the source was fainter than 12.5 mag according to the visual Article published by EDP Sciences 882 N. Sipos et al.: EX Lupi in quiescence Table 1. Optical and infrared observations of EX Lup collected from System implemented on the telescope, which produced 39 one the literature, obtained in its quiescent periods. dimensional spectra of echelle orders and a merged spectrum of the entire wavelength range. λ [μm] Julian date Magnitude Reference With a cross-correlation method, we computed the projected 0.36 (U) 2 443 349.545 14.21 1 rotational velocity and radial velocity of EX Lup in quiescence 2 443 349.572 14.21 from the FEROS spectra. We cross-correlated the spectra with 2 443 349.681 14.23 1 . a numerical template mask (Baranne et al. 1996). The mean 2 443 349.717 14 25 1 radial velocity is −0.948 ± 0.1kms−1.FromtheB − V color 2 443 349.728 14.28 1 . of EX Lup and by using a projected rotational velocity calibra- 2 443 350.653 14 22 1 v = 2 443 353.659 14.13 1 tion for FEROS (Setiawan et al. 2004), we derived a sin i . ± −1 2 443 895.85 14.52 2 4 4 2kms . Sections of the spectrum showing the most promi- 0.44 (B) 2 443 349.545 14.33 1 nent emission lines are plotted in Fig. 2. 2 443 349.572 14.36 1 2 443 349.681 14.24 1 2 443 349.717 14.36 1 2.3. NTT/SOFI near-infrared spectroscopy 2 443 349.728 14.33 1 . We found near-IR spectra in the ESO Data Archive, obtained on 2 443 350.653 14 20 1 2001 May 4 using SOFI at the ESO 3.5 m NTT telescope, un- 2 443 353.659 14.07 1 . der the program 67.C-0221(A) (PI:D. Folha). Two spectra were 2 443 894.85 14 52 2 ∼ 2 443 895.85 14.42 2 taken using the blue and red grisms (R 1000), which cover the 2 448 896 14.21 3 ranges 0.95−1.64 and 1.53−2.52 μm, respectively. In both cases, 0.55 (V) 2 443 349.545 13.13 1 the total exposure time was 500 s, divided into 4 exposures fol- 2 443 349.572 13.19 1 lowing an ABBA nodding pattern. The data were reduced using 2 443 349.681 13.27 1 our dedicated routines developed in the IRAF environment. The 2 443 349.717 13.27 1 data reduction included sky-subtraction, wavelength-calibration, 2 443 349.728 13.21 1 and frame combination. Spectra of the G3V star HIP 78466 were 2 443 350.653 13.14 1 . used to correct for telluric absorption. A modified version of the 2 443 353.659 13 22 1 Xtellcor software (Vacca et al. 2003) was used in this step. The 2 443 894.85 13.20 2 2 443 895.85 13.22 2 spectrum is presented in Fig. 3. 2 448 896 13.03 3 . 0.64 (R) 2 448 896 12 15 3 2.4. InfraRed Astronomical Satellite (IRAS) 0.79 (I) 2 451 295.81 11.109 4 1.25 (J) 2 441 474 9.76 5 EX Lup is included in the IRAS Point Source Catalog with 1.25 (J) 2 445 075 9.92 6 detections at 12, 25 and 60 μm, and with an upper limit at 1.235(J) 2 451 314.781 9.728 7 100 μm. The measurements took place during a quiescent phase 1.221(J) 2 451 295.81 9.92 4 of EX Lup. In order to refine these data and also try to ob- 1.65 (H) 2 441 474 9.04 5 μ . tain a 100 m flux density value, we analyzed the IRAS raw 1.65 (H) 2 445 075 9 11 6 scans again using the IRAS Scan Processing and Integration tool 1.662(H) 2 451 314.781 8.958 7 2 . (SCANPI) available at IPAC .
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