A&A 512, A73 (2010) Astronomy DOI: 10.1051/0004-6361/200913640 & c ESO 2010 Astrophysics Resolving the dusty circumstellar environment of the A[e] supergiant HD 62623 with the VLTI/MIDI A. Meilland1, S. Kanaan2, M. Borges Fernandes2, O. Chesneau2, F. Millour1,Ph.Stee2, and B. Lopez2 1 Max Planck Intitut für Radioastronomie, Auf dem Hugel 69, 53121 Bonn, Germany e-mail: [email protected] 2 UMR 6525 CNRS H. FIZEAU UNS, OCA, Campus Valrose, 06108 Nice Cedex 2, France CNRS, Avenue Copernic, Grasse, France Received 11 November 2009 / Accepted 9 December 2009 ABSTRACT Context. B[e] stars are hot stars surrounded by circumstellar gas and dust which is responsible for the presence of emission lines and IR-excess in their spectra. How dust can be formed in this highly illuminated and diluted environment remains an open issue. Aims. HD 62623 is one of the very few A-type supergiants showing the B[e] phenomenon. We studied the geometry of its circum- stellar envelope in the mid-infrared using long-baseline interferometry, which is the only observing technique able to spatially resolve objects smaller than a few tens of milliarcseconds. Methods. We obtained nine calibrated visibility measurements between October 2006 and January 2008 using the VLTI/MIDI instru- ment in SCI-PHOT mode and PRISM spectral dispersion mode with projected baselines ranging from 13 to 71 m and with various position angles (PA). We used geometrical models and physical modeling with a radiative transfer code to analyze these data. Results. The dusty circumstellar environment of HD 62623 is partially resolved by the VLTI/MIDI, even with the shortest baselines. The environment is flattened (a/b ∼ 1.3 ± 0.1) and can be separated into two components: a compact one whose extension grows from 17 mas at 8 μm to 30 mas at 9.6 μm and stays almost constant up to 13 μm, and a more extended one that is over-resolved even with the shortest baselines. Using the radiative transfer code MC3D, we managed to model HD 62623’s circumstellar environment as ◦ −7 a dusty disk with an inner radius of 3.85 ± 0.6 AU, an inclination angle of 60 ± 10 ,andamassof2× 10 M. Conclusions. It is the first time that the dusty disk inner rim of a supergiant star exhibiting the B[e] phenomenon is significantly con- strained. The inner gaseous envelope likely contributes up to 20% to the total N band flux and acts like a reprocessing disk. Finally, the hypothesis of a stellar wind deceleration by the companion’s gravitational effects remains the most probable case since the bi-stability mechanism does not seem to be efficient for this star. Key words. techniques: high angular resolution – techniques: interferometric – stars: emission-line, Be – stars: winds, outflows – stars: individual: HD 62623 – circumstellar matter 1. Introduction In this context, HD 62623 (3 Puppis, HR 2996) was classi- fiedasanA2IabeintheYale Bright Stars Catalogue (Hoffeit & The B[e] phenomenon is defined by strong Balmer lines in emis- Jaschek 1982) making it one of the latest stars showing the B[e] sion, low-excitation emission lines (especially of ionized metals phenomenon. Other classifications range from A2Ia to A3IIpe. such as FeII), forbidden emission lines of [FeII] and [OI], and Its distance was estimated to be 700 ± 150 pc, and its radius strong infrared excess due to hot circumstellar dust (Lamers et al. R = 55 ± 10 R (Bittar et al. 2001). It exhibits radial veloc- 1998). However, these objects are not a homogeneous group of ity variations with a period of 137.7 days, interpreted as direct stars in terms of stellar evolution, as such features have been de- evidence of an unseen companion (Lambert 1988). Plets et al. tected for both pre-main sequence and evolved stars. Thus, this (1995) found that a 161.1-day period can also fit these varia- group of stars has been divided into four subclasses: B[e] su- tions. In both hypotheses, the system seems to be composed of a pergiants, Herbig AeB[e] stars, compact planetary nebulae B[e]- massive primary star with 31 M < M < 39 M and a low mass type stars, and symbiotic B[e]-type stars. Moreover, since the ratio of 0.03 < M2/M1 < 0.15, while the projected semi-major spectral class of some of these stars was still very difficult to axis is 1.6 AU < a sin i < 2.4 AU. determine accurately, a fifth class was added: unclassified B[e] Using color indices (H − K)and(K − L), Allen (1973) stars. Recently, in an effort to investigate this mysterious sub- found that HD 62623’s infrared excess is too large to only orig- group, Miroshnichenko (2007) has proposed to classify most of inate in the free-free and free-bound emissions of a gaseous en- their members as a new homogeneous group of non-supergiant velope and is probably due to the presence of optically thick and non-pre-main sequence stars. He proposes to name this dust in the circumstellar environment of this star. Observations group after its precursor, i.e. FS CMa’s group. Finally, the bi- with the low-resolution spectrograph (LRS) of the InfraRed narity detected for 30% of these stars could play a key role in Astronomical Satellite (IRAS) clearly exhibit a 9.6 μm silicate the origin of the B[e] phenomenon for these FS CMa stars. feature in the object spectra, confirming the presence of dust in the envelope. Since the star also shows evidence of Hα and Based on observations made with ESO Telescopes at Paranal [OI] emission lines (Swings & Allen 1976), it was classified as Observatory under programs 078.D-O511 and 080.D.0181. a B[e] supergiant, i.e. sgB[e] (Miroshnichenko 2007). Article published by EDP Sciences Page 1 of 13 A&A 512, A73 (2010) Fig. 1. Left: HD 62623 SED from various sources of literature: red circles are flux measurements listed in Bittar et al. (2001); green lines, IRAS LRS spectra; dark blue lines, IUE spectra; light blue lines with errors bars, our VLTI/MIDI measurements; and the dark line is the best reddened Kurucz model, whose parameters are presented in Table 1. Right: zoom of the N band of the SED (8–13 μm) to exhibit differences between the various measurements obtained in this spectral band and the VLTI/MIDI spectra. The formation mechanisms responsible for sgB[e] stars’ cir- radiative transfer modeling presented in Sect. 5. Finally, the cumstellar environment are still an open issue. Spectroscopic physical properties of HD 62623 are discussed in Sect. 6, while and polarimetric observations suggest that their circumstellar Sect. 7 draws the main conclusions of this study. envelopes are nonspherical (Magalhães 1992). Zickgraf et al. (1985) proposed a general scheme for these stars consisting of a hot and fast line-driven polar wind responsible for the pres- 2. Preliminary study ence of forbidden lines and a slowly expanding dense equatorial region where permitted-emission lines and dust can form. The 2.1. The spectral energy distribution deviation from the spherical geometry for this model can be pro- To accurately determine the stellar parameters of HD 62623, we duced by a nearly critically rotating star. started our study by reconstructing the object SED using photo- Using aperture masking interferometry with the Keck I tele- metric and low-resolution spectral measurements ranging from scope, Bittar et al. (2001) partially resolved HD 62623’s circum- ultraviolet to far-infrared. We used data from various sources stellar envelope between 1.65 and 3.08 μm and show that the in the literature already described in Bittar et al. (2001). These envelope extension was increasing from 6–7 mas at 1.65 μm data include IRAS fluxes and low resolution spectra (LRS), IUE to 25–30 mas at 3.08 μm. Stee et al. (2004) have successfully spectra, and other photometric measurements in the UV, the vis- modeled both these observations and the spectral energy distri- ible, and the near-IR. We also included our N band VLTI/MIDI bution (SED) using a gas + dust model with an inner dust radius spectra in this study. The resulting reconstructed SED is plot- of 15–20 R and a temperature at this boundary of 1500 K. In tedinFig.1. The strong IR-excess due to circumstellar dust is their best model, the gas emission originates in the inner part of shown by the inflection point in the SED around 1.5 μmand the envelope and not in a polar wind. Nevertheless, the issue of by the 9.6 μm silicate emission band visible on IRAS LRS and HD 62623’s envelope flattening remains unsolved, even if Yudin VLTI/MIDI spectra between 8 and 13 μm. The VLTI/MIDI flux &Evans(1998) measured a polarization in the visible of 1.5% ◦ is close to the other mid-IR measurements. This clearly shows with a polarization angle of 95 ± 5 , which seems to favor a non- that most of the N-band emission comes from the 250 mas inter- spherical object. ferometric field of view of this instrument. We note that a sec- By putting strong constraints on the envelope geometry, ond silicate emission band is also visible in IRAS LRS spectra long-baseline infrared interferometry can be a key technique around 20 μm. for studying the physics of active hot stars (Stee et al. 2005). Using both VLTI/MIDI and VLTI/AMBER data, Domiciano de Souza et al. (2007) resolved the circumstellar environment of the 2.2. Circumstellar or interstellar extinction ◦ sgB[e] CPD –57 2874 and modeled it as a flattened compact en- We tried to fit the UV and visible parts of the reconstructed SED velope with an extension increasing with wavelength.