Supergiant HD 62623 with the VLTI/MIDI

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 circumstellar 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 instrument 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 constrained. 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. using typical Kurucz models (Kurucz 1979) for supergiant stars; / In this paper, we present new spectrally resolved VLTI MIDI i.e., with 1.5 < log g<2.5 and 8000 K < Teff < 10 000 K. None interferometric observations of HD 62623 that allow us to char- of these models were able to simultaneously fit all the SED from acterize its circumstellar envelope geometry and determine its UV wavelengths up to 1 μm, and the observed SED seems to physical parameters. be slightly reddened. To explain this reddening, two hypotheses The paper is organized as follows. In Sect. 2 we start our can be formulated: (1) if the circumstellar dust becomes opti- modeling of HD 62623 by reconstructing its SED to constrain cally thick at short wavelengths, it can absorb a part of the stellar its stellar parameters and distance. The VLTI/MIDI observations UV emission, assuming this dust is located in the line of sight, and the data reduction process are briefly introduced in Sect. 3, or (2) the reddening can be due to interstellar matter. and a first analysis of the calibrated visibilities using simple geo- It is hard to distinguish between these two hypotheses metrical models is presented in Sect. 4. The result of this study is without fully modeling HD 62623’s circumstellar environment. used as a starting point for a three-dimensional Monte-Carlo However, it is still possible to estimate the reddening without

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Table 1. Stellar parameters from the ﬁt of HD 62623 SED.

Parameter Value Teﬀ 8250 ± 250 K log g 2.0 ± 0.5 R 65 ± 5 R d 650 ± 100 pc E(B − V)0.17± 0.031 solving the issue concerning its origin. Using the Meynet & Hauck (1985) intrinsic Geneva colors laws for A-F supergiants, Plets et al. (1995) estimated a reddening E(B−V)of0.17± 0.03.

2.3. Stellar parameters Assuming that the extinction originates in a standard interstellar medium (RV = 3.1) and that it follows Cardelli et al.’s (1989) law from UV to far-IR wavelengths, the SED can be successfully fitted with reddened Kurucz models with Teff ranging from Fig. 2. UV plane coverage obtained for HD 62623. The dotted line rep- 8000 K to 8500 K and log g from 1.5 to 2.5. Our best model, resents the polarization angle measured by Yudin & Evans (1998). The using T ff = 8250 K and log g = 2.0, is overplotted in Fig. 1.The circle colors represent the two triplets of aligned baselines discussed in e ∼ ◦ ∼ ◦ good agreement between the observed SED and the modeled one Sect. 3: red for PA 40 and green for PA 85 . is obtained for an d/R ratio of 10 ± 1, where the distance d is expressed in parsecs and the stellar radius R in R.Sinceatyp- ical value of R for an A supergiant is 65 ± 5 (Allen 2000), we 4. Results can estimate a distance for the star of 650 ± 100 pc, which is 4.1. Visibilty and Gaussian equivalent FWHM compatible with the previous estimation by Bittar et al. (2001). The resulting parameters used in this paper to model HD 62623’s The calibrated visibilities for the nine projected baselines are central star are presented in Table 1. plotted as a function of wavelength in Fig. 3. All these spectrally resolved visibilities, except the ones from the two longest baselines (i.e., B7 and B8), exhibit a small drop with a minimum 3. VLTI/MIDI observations and data reduction around 9.6 ± 0.1 μm. Such an effect, often found in N-band in- VLTI/MIDI observations of HD 62623 were carried out at terferometric observations of dusty circumstellar environments, Paranal Observatory between October 2006 and January 2008 comes from an opacity increase between the continuum and a (quaranteed time observing runs 078.D-0511 and 080.D.0181) silicate band. This greater opacity in the silicate band causes the with the 1.8 m Auxiliary Telescopes (ATs). We obtained circumstellar envelope to appear more extended than in the con- ff nine visibility measurements with projected baselines ranging tinuum. That such an e ect is not visible in the B7 and B8 data from 13.4 to 71.4 m and with various orientations on the sky probably means that both the silicate and the continuum emis- plane. Three stars were used as calibrators during this observ- sion are over-resolved with these baselines. We note that, for ing campaign: Procyon, Sirius, and Alphard. The log of these this A[e] supergiant star, the silicate band is clearly in emission observations is presented in Table 2. (see Fig. 1). All observations were made with the SCI-PHOT mode, The object is partially resolved for all baselines, even for the = = which enables a better visibility calibration since the photom- shortest ones (i.e., V < 0.9forB6 13.3mandB9 14.0m). etry and the interferometric fringes are recorded simultaneously. This implies that the circumstellar envelope is quite extended. Thanks to the PRISM low spectral dispersion mode, we also ob- To roughly estimate its size, we calculated the Gaussian equiv- tained spectrally resolved visibility with R = 30 in the N band alent FWHM for each baseline and at three wavelengths (i.e., 8, (7.5–13.5 μm). Two different reduction packages were used to 10, and 12 μm). The results, presented in Table 3, clearly show reduce these data: MIA developed at the Max-Planck Institut that the envelope extension is growing by about 45% between 8 für Astronomie and EWS developed at the Leiden Observatory and 10 μm, whereas it remains nearly constant between 10 and (MIA + EWS, ver. 1.5.1). Since the two methods provide the 12 μm. As already mentioned, the envelope size variation be- same results within the error bars, we decided to use the MIA tween 8 and 10 μm is mainly due to the change in opacity be- reduced visibilities in this work. tween the continuum and the silicate band, but another effect HD 62623’s (u, v) plane coverage is plotted in Fig. 2.The has to be considered to explain why the size does not drop back wide spread of baseline length (between 13.4 and 71.4) and PA to its 8 μmvaluefrom10to12μm. Remembering that longer (between –4.1◦ and 85.3◦) will allow us to put strong constraints wavelengths can probe a cooler region, it implies that the size on the envelope geometry in the N band (i.e. extension, flatten- of the emission is growing with the wavelength for an untrun- ing, radial intensity profile). Moreover, two triplets of baselines cated gaseous or dusty envelope heated by a central source, i.e., ◦ are almost aligned: B2,B3,andB6 with PA ∼ 40 ,andB4,B5, a single star, as the cooler regions are located farther from the ◦ and B9 with PA ∼ 85 . This will simplify our modeling by al- heating source. If the size in the continuum were not increas- lowing us to directly determine the object’s projected intensity ing with the wavelength, this would have implied that the disk profile at these two orientations. was somehow truncated, as was discovered for the classical Be star α Arae (Meilland et al. 2007), or that the disk was mainly 1 Value from Plets et al. (1995). isothermal (Jones et al. 2004).

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Table 2. VLTI/MIDI observations log of HD 62623 and its calibrators.

Name Obs. Date Obs. Time Stations Projected baseline Calibrators (UT) Length(m) PA(◦) B1 2006/10/17 06:42 D0-H0 49.5 27.2 γ Eri, Procyon, β Gru B2 2006/10/17 08:10 D0-H0 56.8 48.0 γ Eri, Procyon, β Gru B3 2006/12/16 04:00 G0-H0 27.8 45.8 Procyon, 4 Eri B4 2006/12/19 08:38 G0-K0 56.6 84.8 Alphard, Procyon B5 2006/12/21 08:19 G0-H0 29.0 84.0 γ Eri, Procyon, Alphard B6 2007/10/04 08:22 E0-G0 13.3 39.4 Sirius B7 2007/12/10 05:22 G1-H0 71.4 −4.1 Sirius B8 2007/12/14 08:13 G1-H0 70.9 15.4 Sirius B9 2008/01/12 07:08 E0-G0 14.0 85.3 Sirius, Alphard

Table 3. Gaussian-disk equivalent FWHM for all baselines.

Gaussian Disk FWHM (mas) PA(◦) 8 μm10μm12μm B4 17.6 ± 0.3 24.1 ± 0.3 24.4 ± 1.2 ◦ B5 21.5 ± 0.7 30.0 ± 1.5 32.3 ± 1.0 PA ∼ 85 B9 31.7 ± 4.0 40.7 ± 4.9 43.4 ± 6.3 B2 16.6 ± 2.6 26.4 ± 1.0 26.1 ± 1.5 ◦ B3 22.7 ± 0.7 32.8 ± 1.7 31.6 ± 3.5 PA ∼ 40 B6 43.8 ± 3.8 58.7 ± 4.8 59.2 ± 5.9 ◦ B1 21.2 ± 2.6 32.4 ± 1.1 32.4 ± 1.6 PA = 27.2 ◦ B7 16.8 ± 0.7 23.8 ± 0.7 27.7 ± 1.0 PA = –4.1 ◦ B8 15.4 ± 0.6 22.4 ± 0.6 24.9 ± 0.7 PA = 15.4

finally, the Gaussian equivalent size measured for the shortest baseline is about twice the one measured for the longest baseline. This result is also valid if we model the object as a single uniform ellipse or elliptical ring. This clearly shows that the object geometry is more complicated and should be modeled using at least two components: a compact one that is resolved only by the longest baselines and a more extended one that may even be fully resolved by the smallest baselines. This assumption becomes obvious when plotting the visibility for all baselines as a function of the spatial frequency |B|/λ (Fig. 4). This plot also exhibits all the previously described char- acteristics of the object in a very synthetic way. The wavelength dependence is shown in each of the seven curves representing the visibility for each baseline. These curves are inverted compared to Fig. 3 (i.e., the wavelength increases from right to left), but both the silicate band and the continuum effects on the visibility are shown. Clues to the flattening of the envelope, at least in the silicate band, are also shown since the visibility depends not only on the baseline length but also on its orientation. The ◦ object is larger in the B2,B3,andB9 orientation (PA ∼ 40 ) ◦ than in the B4,B5,andB9 one (PA ∼ 85 ),andisevenlarger ◦ along B1 (PA = 27.2 ). These visibility variations as a function of the PA are consistent with an orientation of the major axis / Fig. 3. HD 62623 VLTI MIDI calibrated visibilities for the nine base- of 5◦ derived from Yudin & Evans (1998) polarization measure- lines from Fig. 2 plotted as a function of wavelength. The dotted line ments. Nevertheless, the flattening cannot be accurately deter- corresponds to the central wavelength of the silicate band, i.e., 9.6 μm. mined without knowing the radial intensity profile of the object. We note that B7 and B8 are almost aligned with the putative en- If the intensity distribution of the object was close to an el- velope major-axis. liptical Gaussian, the estimated FWMH should only depend on the baseline orientation and not on its length. As already men- 4.2. A two-component model tioned in the previous section, two groups of three baselines are almost aligned. Consequently, the Gaussian equivalent FWHM It is obvious from Table 3 and Fig. 4 that HD 62623 MIDI vis- should be nearly constant within each one. As shown in Table 3, ibilities cannot be fitted by a single component model. Thus, this is not the case for HD 62623 and for a given wavelength we try to fit them with three different kinds of two-component and PA The estimated size shrinks with the baseline length, and geometrical models. For all of these models, the most compact

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– G3: in this last model, the minor axis FWHM (b2)oftheex- tended component is also a free parameter. Thus, this model has 5 free parameters: a1, a2, b1, b2,andF1.

Since the envelope extension varies within the N band, we assumed that the free-parameters are all wavelength-dependent. For each wavelength, we tried to minimize the model reduced χ2 using a Levenberg-Marquardt 2D algorithm with 103 random starting positions. We finally calculated the global χ2 for all wavelengths to compare the best-fitted models of the three geometries (and the reference G0). The value of the global χ2 are 9.14 for G0, 2.62 for G1, 2.62 for G2, and 3.26 for G3. Thus, this modeling clearly shows the lack of data to constrain the geometry of the large component of our model but the consistency of a two-component modeling approach. More proof of the difficulty constraining the large component geometry is that the wavelength dependence of a2 and b2 for the best G2 and G3 models is erratic, with values randomly distributed between 100 mas and 1000 mas. Finally, it is obvious that we need more measurements with lower spatial frequencies to constrain the geometry of this extended component. Nevertheless, we can still determine lower and upper limits to its size, considering that it Fig. 4. HD 62623 VLTI/MIDI calibrated visibilities plotted as a func- is fully resolved at 8 μm with the 13 m baseline (∼150 mas for a tion of the spatial frequency B/λ. The colors correspond to the orienta- uniform disk model) and smaller than the VLTI/MIDI interfero- tion of the projected baselines: green for 85◦, blue for 40◦, and red for ∼ ◦ metric field of view ( 250 mas at 10 μm). 27.2 . The 17 mas (dashed line) and 60 mas (dotted line) Gaussian disks On the other hand, the relative flux of the two components are overplotted to illustrate the inconsistency of modeling HD 62623’s and the minor and major axes of the compact component are circumstellar environment as a single Gaussian disk or ellipse. well-constrained and do not depend on the geometry used to model the extended component in the 8–12.5 μm range. We have shown that we cannot constrain the geometry of the extended component is an elliptical Gaussian defined by three parameters: component; consequently, in the following, we will only discuss the major-axis FWHM (a1), the minor-axis FWHM (b1), and the results concerning the best-fitted G1 model. The wavelength de- 2 orientation of the major-axis into the sky plane (θ1). Since the pendence of F1, a1, a1/b1, and the reduced χ for the best-fitted polarization angle measurement seems to provide the orienta- G1 model are plotted in Fig. 5. tion of the minor axis with a good accuracy, we can set the value This figure clearly exhibits the effect of the 9.6 μm silicate ◦ of θ1 to 5 . We see in Sect. 5 that this component probably cor- band on the object geometry. The compact component extension responds to the inner rim of the circumstellar dusty disk and that and flattening strongly varies between 8 and 10 μm. At 8 μm it can be modeled as an elliptical ring. However, the envelope is this structure does not show any flattening, and a1 and b1 are not fully resolved, and this component can be successfully fitted about 15–16 mas. Their respective size increases to about 22 and by several other flattened models including Gaussian or uniform 28 mas at 10 μm and then remains almost constant up to 13 μm. ellipse. On the other hand, the relative flux of the compact component The last parameter concerning this compact component is the F1/Ftot is almost constant between 8 and 10 μm and about 0.75± ± flux ratio between this Gaussian (F1) and the total normalized 0.02. Beyond 10 μm, it starts rising up to 0.82 0.03 around flux at each wavelength. The flux of the extended component 13 μm. is consequently (1-F1). Since we want to determine if it can be As already mentioned in the previous sections, the envelope fully resolved by the smallest baselines, we model it following extension variation in the N band can be explained easily by the three different ways of increasing complexity to test if we can put silicate band and continuum effects. However, the apparent vari- some constraints on its geometry. To check the consistency of ation of its flattening remains hard to explain in the scheme of our two-component modeling, we also add a “reference” model a classical dusty disk. If the disk is Keplerian and stratified, we assuming only one elliptical Gaussian. The parameters of the would expect it to be flared. Remembering that longer wave- four different geometries tested (G0, G1, G2, and G3) are the lengths are probing cooler regions, hence farther from the cen- following: tral star, we would expect that the apparent flattening of a flared disk decreases with wavelength. Another possibility is that the – G0: this is the “reference” model. The geometry is defined disk might be created by an outflowing radiative wind. In this by a single elliptical Gaussian. This model has only 2 free case, if the terminal velocity of the wind is high enough to avoid parameters: a1 and b1; any stratification of the disk, we would expect that the flattening – G1: here, we consider that the extended component is fully does not depend on the wavelength. Additional effects have to be resolved, and we model it as a homogeneous constant con- considered to allow the disk to appear more flattened at greater tribution. This first model has 3 free parameters: a1, b1,and distances from the central star. One solution might be the pres- ff F1; ence of a pu ed-up inner rim, as already proposed for Herbig – G2: we try to model the extended component as a Gaussian stars by Natta et al. (2001). However more measurements are with the same flattening as the compact one so that it is only needed to solve these issues. defined by its major axis FWHM a2. This model has 4 free If we consider that the 13 μm envelope is a geometrically parameters: a1, a2, b1,andF1; thin equatorial disk, the flattening would only come from its

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Fig. 5. Best-fit model parameters and χ2 for a fully resolved component + elliptical Gaussian geometry (G1). From left to right: normalized flux 2 of the most compact component F1, FWHM of the major and minor axes of the compact component, and corresponding reduced χ . The vertical dotted line indicates the position of the center of the silicate band, i.e. 9.6 μm. projection onto the sky plane, and we can estimate HD 62623’s method and solves the radiative transfer equations self- inclination angle to be 38 ± 8◦ (using the 13 μm value for the flat- consistently. Starting with a spatial density distribution of scat- tening). Unfortunately, the disk may not be geometrically thin terers and absorbers and some primary radiative source(s), it and may also be flared, as already discussed. Consequently, we can be used to determine the temperature in this medium by can only estimate a lower limit on this parameter of 30◦. an iterative process of heating, absorption, and reemission by dust grains. When the self-consistent temperature distribution is reached, the software can compute the resulting observables: 4.3. Comparison with KECK measurements SED, wavelength-dependent images, and polarization maps. Using Keck I aperture-masking interferometry, Bittar et al. The software allows various geometries for the density dis- (2001) partially resolved HD 62623’s circumstellar environment tribution: one-dimensional (i.e., dust shells), two-dimensional in the near-infrared. They estimated the circumstellar environ- (i.e., axi-symmetric disk or torus), or fully three-dimensional in ment extension using Gaussian disk models and found that its spherical, cylindrical, or Cartesian coordinates (for example, to size was 6.8 ± 1 mas at 1.65 μm, 24.8 ± 1 mas at 2.26, and model inhomogeneities in a circumstellar disk). 27.3±3 mas at 3.08 μm. Even if their estimation at 3.08 is almost two times more than our compact component at 8 μm, one must 5.2. Dust disk model keep in mind that they only obtained measurements for spatial frequencies less than 0.02 cycles/mas and that we should com- We decided to model HD 62623 with a two-dimensional dust pare it to a “single” Gaussian estimation for our small and inter- density distribution and a radial and vertical structure, as de- mediate baselines (i.e., B3,B5,B6,andB9 from Table 3). Using scribed in Shakura & Sunyaev (1973) and Wood et al. (2002): these measurements, we obtained an average size of 29.9 mas at ⎛ ⎞α ⎛ ⎞ 8 μm. This value is clearly compatible with the one determined ⎜ ⎟ ⎜ 2 ⎟ = ⎜R ⎟ ⎜− z ⎟· μ ρ(r, z) ρ0⎝⎜ ⎠⎟ exp⎝⎜ ⎠⎟ (1) by Bittar et al. (2001) at 2.26 and 3.08 m. Moreover, the esti- r 2h(r)2 mated size does not strongly vary between 2.26 and 8 μm(i.e., only 1.21 times larger), which might indicate that the measured Here, r is the radial distance in the midplane of the disk, α the extension in the K band is close to the diameter of the inner rim density parameter in the midplane, and h(r) is given by: of the dusty disk. ⎛ ⎞ The existence of the two components in our HD 62623 mod- ⎜ ⎟β ⎜ r ⎟ eling probably comes from the presence of a bright inner rim in h(r) = h0⎝⎜ ⎠⎟ (2) the dusty disk that probably lies within the compact component R of our model and is “blurred” because we do not fully resolved it. Thus, the inner radius of the dusty disc should be smaller than where β is the vertical density parameter, and h0 the theoretical the Gaussian equivalent size measurement of the compact com- disk scale height at R. In order to be more understandable, the ponent at 8 μm. scale height at 100 AU (h100) is used as an MC3D input parameter instead of h . The three geometrical parameters α, β,andh We obtained 15 mas ∼ 16 R at 8 μm for the compact compo- 0 0 nent Gaussian equivalent FWHM, and the extension at 2.26 μm are related to the physical nature of the dusty disk. Porter (2003) is 1.21 times smaller than the one at 8 μm. Moreover, when the shows that both the viscous disk and the equatorial wind model envelope is not fully resolved, estimating its size with a ring di- can theoretically allow the formation of dust around supergiant = = ameter corresponds to a value 1.24 times less than if we were B[e] stars. A Keplerian√ viscous disk will imply α 3.5, β 1.5, = using a Gaussian FWHM. Thus, we can roughly estimate the and h0 cs/ GM,wherecs is the sound speed. On the other hand, α = 2andβ = 1 for a wind model with a constant expan- dusty disk inner rim radius to be Rin ∼ 10 R or 3 AU. sion velocity. In this case, h0 roughly defines the opening angle of the dusty disk. A typical density distribution for an equatorial 5. MC3D simulations of HD 62623 wind and a Keplerian viscous disk are plotted in Fig. 6. 5.1. The Monte-Carlo code Details of the spatial distribution of chemical compositions, shapes, sizes, and other parameters of real dust grains in this MC3D (Wolf et al. 1999;Wolf2003) is a three-dimensional con- object are poorly known. Large uncertainties of the real proper- tinuum radiative transfer code. It is based on the Monte-Carlo ties of dust enormously increase the free parameter space of the

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Table 4. Parameters of the best-ﬁtted MC3D models.

Parameter Best Wind Model Best Keplerian disk model

Teff 8250 K 8250 K L 17 000 L 17 000 L distance 650 pc 650 pc Rin 4 ± 0.5 AU 3.7 ± 0.5 AU Rout 150 AU 150 AU −7 −7 Menv 2 × 10 M 2 × 10 M Tdust 1250 K 1250 K α 23.5 Fig. 6. Typical radial and vertical density distribution for an equatorial β 11.5 wind (left) and a Keplerian viscous disk (right). h100 AU 24 AU 28 AU Inclination angle 60 ± 10◦ 60 ± 10◦ Major-axis PA 10 ± 10◦ 15 ± 10◦ models to be explored. Since our purpose is to reconstruct global properties of the disk, we decided to fix the dust grain properties. We have only treated the transfer of dust radiation in this work. 5.4. The best models The gas component, present in the model only implicitly, is described by a dust-to-gas ratio of 1% within the dense disk. The parameters of the best equatorial wind and Keplerian viscous disk models are presented in Table 8, and the correspond- We assumed the standard interstellar size distribution from ing 8.2, 9.9, and 13.1 μm images for both geometries are plotted Mathis et al. (1977): in Fig. 7. The fit of the visibilities is plotted in Fig. 8.Thesefig- ures show that both scenarios are able to reproduce the general ∂n(a) α a−3.5 (3) trends of the seven shorter baseline visibility variations, but the ∂a ones measured for B7 and B8 are significantly underestimated for wavelengths shorter than 11 μm. Such a residual visibil- where a is the dust grain radius extending from 0.005 to 0.25 μm ity might suggest the presence of structures not fully resolved (Draine & Lee 1984). We considered the dust grains to be homo- with the 70 m baselines, which are not taken into account in our geneous spheres. MC3D model. Some hypotheses on the nature of these structures are discussed in Sect. 6.1. 2 5.3. Model fitting The reduced χ on the interferometric data of the best equatorial wind and Keplerian disk are 14.9 and 11.5, respectively. We calculated several hundred models to explore the free- These high values are mainly due to the poor fit of the 70 m parameters space and tried to constrain their values and uncer- baselines. Thus, skipping B7 and B8 measurements, the reduced tainties. We focused on both the viscous disk and the equatorial χ2 reach values of 2.8 and 2.4, respectively. The main qualitative wind models defined in the previous section to test if we can difference between the two scenarios is the silicate-band visibil- clearly distinguish between them. Finally, we also tried many ity drop scale. This favors the hypothesis of the stratification of intermediate models which imply that the disk is slowly expand- the disk. ing and not perfectly stratified. However, the differences are small, and a large set of inter- We considered the following outputs from our simulations: mediate values of α and β are also possible so that the disk stratification cannot be inferred accurately. Actually, as can be seen 1. the total SED computed from 1 to 20 μm that can be directly in Fig. 7, the geometrical differences between these scenarios compared to our reconstructed SED, including our MIDI are too small to be properly constrained by our nine VLTI/MIDI data presented in Sect. 2; measurements with uncertainties of 5%. 2. twenty 301 × 301 pixels intensity maps in the MIDI wave- Nevertheless, despite our inability to deduce accurate values length range, i.e., 8–13.5 μm, with a pixel size of 0.7 mas. of the radial and vertical disk structure from the visibility fits, Each image allows us to fit the spectrally dispersed visibil- we clearly constrain the inner disk radius, Rin = 3.85 ± 0.7(or ities at one wavelength. The transformation of the images Rin = 4 ± 0.5 AU for the equatorial wind and Rin = 3.7 ± 0.5AU into a visibility signal is straightforward: the images are ro- for the stratified disk). This represents the first measurement of tated by the PA angle of the object onto the sky and then the extension of the inner rim of a B[e] supergiant dusty disk. collapsed as a 1D flux distribution in the direction of each The mass of the envelope does not strongly depend on the disk −7 of the nine baselines. These vectors are Fourier-transformed stratification and is also well constrained, i.e., 2 ± 110 M,so and normalized. is the inclination angle, i.e., 60 ± 10◦. Finally, the position of the major axis is roughly perpendicular to Yudin & Evans (1998) For each model we computed the SED and visibilities χ2 sepa- polarization angle measurement 95◦. rately. The use of a single global χ2 would have implied solv- The SED fit for each scenario is presented in Fig. 8.For ing the problem of the relative weight of the SED and visibility both scenarios, the fit of the SED is not good enough, especially in the computation. Finally, we decided to concentrate our ef- the 9.6 μm silicate peak, which is too strong for both models, forts on the visibility fit since it contains more information on whereas the IR-excess is overestimated for the equatorial wind HD 62623’s circumstellar environment geometry than the SED. and underestimated for the Keplerian disk scenario. However, Nevertheless, for each model we verified that the fit of the SED the general agreement is better for the Keplerian disk, and we is accurate enough, both in term of IR-excess and silicate band note that both models managed to reproduce the 13–20 μmSED emission intensity. well.

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Fig. 7. From left to right: 8.2, 9.9, and 13.1 μm MC3D intensity maps for the best-ﬁtted equatorial wind model (top) and Keplerian disk (bottom) of HD 62623’s circumstellar environment.

In fact, an accurate fit of the silicate feature is very often dif- measurements simultaneously to the other ones. Actually, with ficult to obtain. The strength of the feature depends on the size these 70 m baselines, the dusty disk inner rim (i.e., Rin = and composition of the silicate grains, but also on the level of the 3.85 AU = 12.8 R) is almost fully resolved. Thus, the visibil- underlying continuum, thus, on the quality of the model density ity should be close to zero. Taking a smaller Rin would lead to structure. The near-IR region of the SED is also strongly influ- a better fit of B7 and B8 visibilities, but to an overestimation for enced by the detailed structure of the inner rim of the disk which the other ones. Moreover, the fit of the largest baselines would is assumed to be a wall in this study. Recently, many studies have not be totally satisfying since the silicate band effect would be been devoted to this point in the field of young stellar objects, in- clearly visible in the modeling. ff volving a pu ed-up inner rim and dust settling (Dullemond et al. Thus, in the following, we investigate two different hypothe- 2001, 2004; Tannirkulam et al. 2007). ses to explain the residual visibility measured with the 70 m For the Keplerian disk, the near-IR flux is underestimated baselines. because the inner rim is probably not large enough. On the con- trary, the equatorial wind model overestimates the near-IR as a ffi direct consequence of the smaller density coe cient α,making 6.1.1. Clumping of the dust disk the inner rim almost as opaque as a true wall. Moreover, the flaring of the density structure is also greater; thus, more of the Effects of clumping on the visibility are not trivial and depend surface of the dust disk is directly exposed to the stellar radiation mainly on the number of clumps and their extension. If they and the vertical extent of the structure is probably overestimated. are unresolved, i.e. Dclump B/λ, they will significantly increase the visibility level. However, if their number is small, they 6. Discussions will act like multiple objects and produce oscillations structures in the visibility function as a function of the spatial frequency. 6.1. Nature of the residual visibility Consequently, since a clear increase in the visibility level but no As already mentioned in Sect. 4.1, that the two largest baselines, oscillation are evident in our data, we can assume that the clumps are unresolved and numerous. i.e. B7 and B8, do not exhibit the same wavelength dependence as the shorter ones, in particular the lack of a clear drop in the To simulate the effect of clumping on our visibility mea- visibility in the 9.6 μm silicate band, might be a clue that both surements, we used a simple and ad-hoc method to produce the continuum and the silicate emissions are over-resolved with a clumped, dusty disk emission map. We computed a random these baselines. Since the visibility does not reach zero with clumps map in which each clump is a normalized Gaussian. We these baselines, but 0.17 ± 0.05 at 8 μm and 0.12 ± 0.03 at 13 μm, then multiplied this map by an MC3D emission map. The result- this might advocate for the presence of a unresolved structure ing map consists of a clumped version of our previous MC3D that contributes to the total flux at least to 17% at 8 μm and 12% emission map. Figure 9 (left) presents the visibility plotted as a at 13 μm, assuming it is fully unresolved. function of the spatial frequency for several random clump dis- This assumption is confirmed by our MC3D models that tributions using a 13.1 μm intensity map from our best MC3D clearly show the impossibility reproducing the B7 and B8 Keplerian disk model as an input and with Nclump = 10 000 and

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Fig. 8. Fit of the nine visibility measurements (top)andSED(bottom) of the best equatorial wind model (left) and Keplerian viscous disk model (right). The dotted lines correspond to the models and the errors bars to the data.

FWHMclump = 3 mas. An illustration of a randomly clumped of active hot stars. This code is based on a latitudinal-dependent map is also plotted in Fig. 9 (right). CAK wind model (Castor et al. 1975) and large velocity gra- This figure exhibits the random clump distribution effect dient radiative transfer (Sobolev 1960) and is able to compute on the visibility. For the selected clump parameters, it clearly SED, hydrogen emission line profiles, and intensity maps in the shows that, when the global disk structure is fully resolved (i.e. continuum and in the emission lines. However, in our case we B/λ > 0.05), the visibility can take any value between 0 and 0.2, only need to compute radiative transfer in the continuum to ob- depending on the clump distribution. Such random effects due to tain 8 and 13 μm intensity maps and 0.1–100 μmSED.Theaim clumpiness have already been studied using fully radiative trans- of this modeling is not to constrain the physical properties of fer clumped-disk models by Hönig et al. (2006) for AGN tori. HD 62623’s gaseous inner envelope, which is not possible be- Finally, it seems that random clump distributions with the cause of our small number of measurements, but rather to deter- previous parameters can produce visibilities compatible with our mine whether this component can be responsible for the 70 m measurements. We note that this result strongly depends on the N-band residual visibility. baseline PA, and two baselines separated by a few degrees can In the SIMECA code, m1 and m2 are exponents used to set produce totally different visibility profiles. Consequently, know- the latitudinal dependence of the mass-flux (Φ) and terminal ing that our two largest baselines are separated by nearly 20◦, velocity (v∞), respectively: that they exhibit nearly the same trend can be considered an ar- gument against the dust disk clumping hypothesis. However, we Φ(θ) =Φ(pole) + Φ(eq) − Φ(pole) sinm1 θ (4) cannot totally exclude that a random distribution of clumps may m reproduce the observed visibilities. v∞(θ) = v∞(pole) + (v∞(eq) − v∞(pole)) sin 2 θ. (5)

6.1.2. Inner gaseous disk Typical values of the terminal velocity of an A supergiant wind are on the order of 500 km s−1 (Achmad et al. 1997), although To test if the inner gaseous disk of HD 62623 can be responsible Lamer et al. (1995) derived a terminal velocity of 150±50 km s−1 for the residual visibility, we used the SIMECA code developed for HD 62623 from IUE observations. However, HD 62623 may by Stee (1996) to model the circumstellar gaseous environment be rotating at a signiﬁcant fraction of its breakup velocity; i.e.,

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Fig. 9. Left: visibility plotted as a function of the spatial frequency for several random clump distributions (colored solid lines) with Nclump = 10 000 and FWHMclump = 3mas,usingthe8μm intensity map from our best MC3D wind model as an input. The visibility corresponding to this map is also plotted as a dashed line for comparison. The vertical dotted lines represent the position of the spatial frequencies for a 70 m baseline at 8 and 13 μm. Right: example of a clumped map with a random distribution for the same parameters.

−1 −1 −1 V sin i ∼ 70 km s and 160 km s

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Fig. 10. Left: visibility plotted as a function of the spatial frequency for the SIMECA + MC3D model with parameters from Table 5. Right: corresponding 8 μm SIMECA intensity map.

These two conditions are hard to find at the same location 6.3. Comparison with other A-type supergiants in the circumstellar environment of these stars since, for most models, the density quickly falls with the distance. However, the HD 62623 is one of the coolest stars that exhibit the B[e] phe- stellar radiation might be shielded by the gaseous inner envelope. nomenon. Actually, it is a member of the very rare A[e] super- This would allow the temperature to decrease faster than in a giant class. A similar object has recently been discovered in the normal reprocessing disk, and thus, the sublimation temperature Small Magelenic Cloud by Kraus M. et al. (2008). Apart from could be reached closer to the central star. the spectral features relative to the B[e] phenomenon exhibiting the presence of a highly illuminated gas and dust region, these Since we managed to significantly constrain the inner rim stars seem to share the same properties as their “normal” coun- extension of a supergiant dust disk for the first time, we can test terparts. this hypothesis by inferring the temperature variation in the inner Normal-A-Type supergiants are known to exhibit radiation- gaseous envelope. We assume that the temperature distribution driven stellar wind with a terminal velocity of several hundred follows a power law as a function of the distance to the central −1 −8 −6 km s and mass-loss between 10 M and 10 M (Achmad star: et al. 1997). However, no dust has been found in the circum- ⎛ ⎞γ stellar environment of these standard A-Type supergiants. As ⎜ ⎟ ⎜R ⎟ already discussed in the previous section, this implies that the T(r) = T ff.⎝⎜ ⎠⎟ · (7) e r critical density of chemical elements that can be involved in the dust formation (i.e., mainly Si, O, and C) is reached too close where Teff and R are the stellar effective temperature and ra- to the star, where the temperature is still higher than the subli- dius. In the vacuum, the only effect on the stellar radiation is the mation limit. This is probably because the relatively high wind geometrical dilution of the energy, so γ = 0.5. For a simple re- terminal velocity is reached close to the central star. Aufdenberg processing disk, i.e. stellar radiations absorbed by the disk and et al. (2002) studied the mass-loss and radiative stellar wind of re-emitted at the LTE temperature, γ = 3/4 (Porter 2003). If the the A-supergiant prototype Deneb. They approximated the wind stellar radiation is highly shielded by the gaseous environment, velocity assuming: then γ 3/4. = − β Using Rin = 3.85±0.7 AU (or 12.8 ± 2.5)andTin = 1250 K, v(r) v∞ 1 R/r (9) we can determine the value of γ for HD 62623’s circumstellar −1 disk. Using Eq. (4) with r = Rin leads to: and found that v∞ = 225 km s and β = 3. However, even if Deneb and HD 62623 have the same spectral class (i.e., A2Ia), their physical parameters can be different ln Tin Teff enough to affect the wind properties. For instance, Schiller & γ = = 0.74 ± 0.1. (8) 5 Przybilla (2008) derive a luminosity of 1.96 × 10 L for Deneb, ln R Rin thus about 12 times our estimation for HD 62623. Remembering that the mass loss rate scales the luminosity, this should imply This value of γ is very close to the value of the reprocessing disk. a stronger wind for Deneb. But, in their estimation the authors If we use the estimation of the inner radius from our SIMECA use a distance of 802 pc, while the one derived from “Hipparcos + MC3D model, i.e. Rin = 4.5 AU, we find γ ∼ 0.79, again the New Reduction” (van Leeuwen 2007) is 1.86 smaller (i.e., close to the reprocessing disk. Consequently, no strong shielding 432 ± 61 pc). Finally, using this latest estimation of the distance, 4 effect of the stellar radiation by the gaseous envelope is needed we derived a luminosity of 5.5 ± 1 × 10 L, closer to HD 62623 to obtain a temperature low enough to form dust at Rin.Ifthestar luminosity. is rotating close to its critical velocity, the equator can even be Thus, we can still use Deneb’s wind parameters as a rough cooler thanks to the Von Zeipel effect. This would even decrease estimate for HD 62623. Consequently, at 12.8 R, i.e. dust the value of the γ parameter. sublimation radius of HD 62623 determined by our MIDI

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Fig. 11. A composite map of HD 62623’s circumstellar environment in the N band from our MC3D + SIMECA model. observation and MC3D modeling, the expansion velocity should is 1.6 AU < a sin i < 2.4 AU. Considering that the companion be on the order of 176 km s−1. orbits in the equatorial plan of HD 62623’s primary, and using ◦ It is difficult to explain why HD 62623 is forming dust in our estimation for the inclination angle i = 60 ± 10 , we obtain its circumstellar environment, while Deneb exhibits a purely a semi-major axis of a = 2.3 ± 0.6AU. gaseous stellar wind. Among the putative explanations for such Finally, Plets et al. (1995) proposed a general scheme for adifference is a possible effect from to the stellar rotation. In HD 62623. The matter is probably ejected by the evolved the bi-stability model developed by Lamers & Pauldrach (1991), massive primary stellar wind with typical terminal velocity of the fast rotation induces a strong opacity change in the wind be- 80 km s−1. However, the matter located in the equatorial plane tween the pole and the equator resulting in two distinct regions cannot reach this value since it is decelerated by the companion, in the circumstellar environment, i.e., a quickly expanding, low finally forming a circumbinary disk. Thanks to this deceleration, density polar wind and a dense equatorial outflow with a lower the density in the equatorial plane could be 10 times higher than terminal velocity. However, they conclude that this mechanism the typical density for a single A2I star stellar wind. is more efficient for stars with 15 000 K < Teff < 30 000 K. Binarity can be detected with interferometric techniques, and For lower temperature, the mass-loss rate is usually too low to recent VLTI/AMBER observations of the classical Be star δ Cen reach the optically thick wind near the equator unless the stars by Meilland et al. (2008), and VLTI/AMBER + VLTI/MIDI ob- are almost critical rotators. servations of the unclassified B[e] HD 87643 by Millour et al. In the case of HD 62623, considering that V sin i ∼ (2009) resulted in the discovery of companions around these two −1 ◦ −1 −1 70 km s , i = 60 ± 10 , and 160 km s

Page 12 of 13 A. Meilland et al.: VLTI/MIDI observations of the A[e] supergiant HD 62623 geometry determined using the MC3D code and our interfero- Acknowledgements. The Programme National de Physique Stellaire (PNPS) and metric observations is thus compatible with Plets et al.’s (1995) the Institut National en Sciences de l’Univers (INSU) are acknowledged for their scheme for HD 62623. financial support. F. Millour & A. Meilland acknowledge financial support from the Max Planck Institut für Radioastronomy. 7. Conclusion Thanks to these first VLTI/MIDI observations of HD 62623, we References were not only able to determine the extension and flattening of a Allen, C. W. 2000, Allen’s Astrophysical Quantities, 15.3.1, 388 supergiant star showing the B[e] phenomenon, as already done Allen, D. A. 1973, MNRAS, 161, 145 by Domiciano de Souza et al. (2007) on CPD –57◦ 2874, but Bittar, J., Tuthill, P., Monnier, J. D., et al. 2001, A&A, 368, 197 Achmad, L., Lamers, H. J. G. L. 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