This is the author accepted manuscript. The final version is available from ASP via the link in this record.We present the first spectro-interferometric observations of the circumstellar envelope (CSE) of a B[e] supergiant (CPD−57°2874), performed with the Very Large Telescope Interferometer (VLTI) using the beam-combiner instruments AMBER (near-IR interferometry with three 8.3 m Unit Telescopes or UTs) and MIDI (mid-IR interferometry with two UTs). Our observations of the CSE are well fitted by an elliptical Gaussian model with FWHM diameters varying linearly with wavelength. Typical diameters measured are ≅ 1.8 × 3.4 mas or ≅ 4.5×8.5 AU (adopting a distance of 2.5 kpc) at 2.2 μm, and ≅ 12×15 mas or ≅ 30 × 38 AU at 12 μm. We show that a spherical dust model reproduces the SED but it underestimates the MIDI visibilities, suggesting that a dense equatorial disk is required to account for the compact dust-emitting region observed. Moreover, the derived major-axis position angle in the mid-IR (≅ 144°) agrees well with previous polarimetric data, hinting that the hot-dust emission originates in a disk-like structure. Our results support the non-spherical CSE paradigm for B[e] supergiants

Chesneau, O

Domiciano de Souza, A

Driebe, T

Hofmann, K-H

Kraus, S

Miroshnichenko, AS

Ohnaka, K

Petrov, RG

Preibisch, T

Stee, P

Weigelt, G

Open Research Exeter

arXiv:astro-ph/0510736v1  26 Oct 2005Stars with the B[e] phenomenonASP Conference Series, Vol. **VOLUME**, 2005Michaela Kraus & Anatoly S. MiroshnichenkoThe vicinity of the galactic supergiant B[e] starCPD−57◦ 2874 from near- and mid-IR long baselinespectro-interferometry with the VLTI (AMBER andMIDI)A. Domiciano de Souza1, T. Driebe1, O. Chesneau2, K.-H. Hofmann1,S. Kraus1, A. S. Miroshnichenko1,3, K. Ohnaka1, R. G. Petrov4,Th. Preibisch1, P. Stee2, G. Weigelt11 Max-Planck-Institut fu¨r Radioastronomie, Auf dem Hu¨gel 69, 53121Bonn, Germany2 Observatoire de la Coˆte d’Azur, Gemini, CNRS UMR 6203, AvenueCopernic, 06130 Grasse, France3 Dept. of Physics and Astronomy, P.O. Box 26170, University ofNorth Carolina at Greensboro, Greensboro, NC 27402–6170, USA4 Laboratoire Universitaire d’Astrophysique de Nice (LUAN), CNRSUMR 6525, UNSA, Parc Valrose, 06108 Nice, FranceAbstract. We present the first spectro-interferometric observations of the cir-cumstellar envelope (CSE) of a B[e] supergiant (CPD−57◦ 2874), performed withthe Very Large Telescope Interferometer (VLTI) using the beam-combiner in-struments AMBER (near-IR interferometry with three 8.3 m Unit Telescopes orUTs) and MIDI (mid-IR interferometry with two UTs). Our observations of theCSE are well fitted by an elliptical Gaussian model with FWHM diameters vary-ing linearly with wavelength. Typical diameters measured are ≃ 1.8 × 3.4 masor ≃ 4.5× 8.5 AU (adopting a distance of 2.5 kpc) at 2.2µm, and ≃ 12× 15 masor ≃ 30 × 38 AU at 12µm. We show that a spherical dust model reproducesthe SED but it underestimates the MIDI visibilities, suggesting that a denseequatorial disk is required to account for the compact dust-emitting region ob-served. Moreover, the derived major-axis position angle in the mid-IR (≃ 144◦)agrees well with previous polarimetric data, hinting that the hot-dust emissionoriginates in a disk-like structure. Our results support the non-spherical CSEparadigm for B[e] supergiants.1. IntroductionCPD−57◦ 2874 is a poorly-studied object, for which McGregor et al. (1988)suggested a distance of d = 2.5 kpc, assuming that it belongs to the CarinaOB association. A high reddening and the presence of CO emission bands at2.3 − 2.4µm makes it compatible with the supergiant B[e] (sgB[e]) class. Zick-graf (2003) obtained high-resolution optical spectra exhibiting double-peakedemission lines that are suggestive of a flattened CSE geometry, typical for sgB[e]stars. However, the physical parameters of neither the star nor its CSE havebeen studied in detail yet.122. ResultsWe report here our results on CPD−57◦ 2874 based on recent (Dec/2004 -Feb/2005) spectro-interferometric observations performed with the VLTI instru-ments AMBER (e.g., Petrov et al. 2003) and MIDI (e.g., Leinert et al. 2004).Figures 1 and 2 show the spectra and visibilities obtained with AMBERand MIDI, respectively. CPD−57◦ 2874 is resolved in both spectral regions atall projected baselines Bp and position angles PA. As a zero-order size estimatethese figures also show the uniform disk angular diameters θUD obtained fromthe visibilities at each spectral channel. Figure 3 (left) shows the θUD as afunction of the baseline position angle observed with AMBER and MIDI. Themeasured sizes are clearly different on the 4 spectral channels chosen (2.2µm,Brγ line, 7.9µm, and 12.1µm); in particular, the mid-IR sizes are much largerthan those in the near-IR. Indications of a flattened CSE is more evident in thenear-IR than in the mid-IR but, as we will show in Sect. 3., both AMBER andMIDI observations suggest a non-spherical CSE.The AMBER observations show a zero closure phase (Fig. 3 right) at allwavelengths (within the noise level of a few degrees). This is a strong indicationthat the near-IR emitting regions (continuum and Brγ line) have an approxi-mately centrally-symmetric intensity distribution.Since sgB[e] stars are thought to have non-spherical winds, we expect anelongated shape for their CSE, unless the star is seen close to pole-on. Here-after, we show that both AMBER and MIDI observations can indeed be wellreproduced by an elliptical Gaussian model for the CSE intensity distribution,corresponding to visibilities of the form:V (u, v) = exp{−pi2(2a)24 ln 2[u2 + (Dv)2]}(1)where u and v are the spatial-frequency coordinates, 2a is the major-axis FWHMof the intensity distribution (image plane), and D is the ratio between the minorand major axes FWHM (D = 2b/2a). Since, in general, 2a forms an angle αwith the North direction (towards the East), u and v should be replaced in Eq. 1by (u sinα+ v cosα) and (u cosα− v sinα), respectively. A preliminary analysisof V at each individual λ showed that D and α can be considered independenton λ within a given spectral band (K or N). On the other hand, the size varieswith λ, as one can see from the θUD(λ) curves in Figs. 1 to 3.2.1. Size and geometry in the K bandWe interpret the AMBER observations in terms of an elliptical Gaussian model(Eq. 1) with a chromatic variation of the size. The θUD(λ) curves in Fig. 1suggest a linear increase of the size within this part of the K band. In addition,the AMBER visibilities decrease significantly inside Brγ, indicating that the line-forming region is more extended than the region responsible for the underlyingcontinuum. Based on these considerations, we adopted the following expressionfor the major-axis FWHM:2a(λ) = 2a0 + C1(λ− λ0) +C2 exp[−4 ln 2(λ− λBrγ∆λ)2](2)3Table 1. Model parameters and χ2red (reduced chi-squared) derived fromthe fit of an elliptical Gaussian (Eqs. 1 and 2) to the VLTI/AMBER andVLTI/MIDI visibilities. Angular sizes (in mas) correspond to FWHM diame-ters. The errors on the fit parameters are dominated by the calibration errorsof the instrumental transfer function, derived from the calibrator stars.Parameter AMBER MIDI (< 10µm) MIDI (> 10µm)λ0 (µm) 2.2 8.0 12.0major axis 2 a0 (mas) 3.4± 0.2 10.1 ± 0.7 15.3± 0.7C1 (mas/µm) 1.99 ± 0.24 2.58 ± 0.41 0.45± 0.22position angle α (◦) 173◦ ± 9◦ 145◦ ± 6◦ 143◦ ± 6◦D = 2b/2a 0.53 ± 0.03 0.76 ± 0.11 0.80± 0.10minor axis 2 b0 (mas) 1.8± 0.1 7.7 ± 1.0 12.2 ± 1.1Brγ: C2 (mas) 1.2± 0.1 – –Brγ: ∆λ (10−3 µm) 1.8± 0.2 – –χ2red 0.7 0.1 0.1where 2a0 is the major-axis FWHM at a chosen reference wavelength λ0(=2.2µm), and C1 is the slope of 2a(λ). The size increase within Brγ is modelled bya Gaussian with an amplitude C2 and FWHM ∆λ, centered at λBrγ = 2.165µm.Figure 1 shows a rather good fit of this model to the observed visibilities bothin the continuum and inside Brγ. The parameters derived from the fit are listedin Table 1.2.2. Size and geometry in the N bandSimilarly to the analysis of the AMBER visibilities, we interpret the MIDI ob-servations of CPD−57◦ 2874 in terms of an elliptical Gaussian model (Eq. 1)with a size varying linearly with λ as given in Eq. 2 (for the analysis of theMIDI data the parameter C2 is set to zero). Additionally, since θUD shows astronger λ-dependence between 7.9 and 9.8 µm compared to the region between10.2 and 13.5 µm (see Fig. 2), we performed an independent fit for each of thesetwo spectral regions. This elliptical Gaussian model provides a good fit to theMIDI visibilities as also shown in Fig. 2. The parameters corresponding to thefit in the two spectral regions within the N band are listed in Table 1.3. Discussion and conclusionsTo further investigate the geometry of the CSE, we attempted to simultaneouslyfit the MIDI visibilities and the spectral energy distribution (SED) using thespherical 1D code DUSTY (Ivezic´ & Elitzur 1997). To reproduce the featurelessspectrum around 10µm, we used large silicate grains and/or carbonaceous dust.The nature of the dust and the featureless mid-IR spectrum will be addressedin detail elsewhere. This spherical model in the optically-thin regime can nicelyfit the SED (as shown in Fig. 4), but it significantly underestimates the mid-IR visibilities, even when the model parameters are varied considerably. Thismeans that the measured dust-emitting region is too compact (2a ∼ 10−16 mas4or 25−40 AU; see Table 1) to be reproduced by a spherical model (even thoughthe SED fit is acceptable). Only a disk-like structure seems to allow the dust tosurvive within ∼ 10 AU from the star (see Kraus & Lamers 2003).Another argument against a spherical CSE comes from polarization mea-surements of Yudin & Evans (1998). After correction for the interstellar po-larization, Yudin (private communication) estimated an intrinsic polarizationposition angle ≃ 45◦ − 55◦. Interestingly, within the error bars this angle isperpendicular to the major-axis PA we derived from the MIDI data (α ≃ 144◦;see Table 1), as is expected from a disk-like dusty CSE.Thanks to the unprecedent combination of interferometric resolution, multi-spectral wavelength coverage and relatively high spectral resolution now avail-able from the VLTI we measured the size and geometry of the CSE of a sgB[e]star in the near- and mid-IR. We hope that the present work will open thedoor for new spectro-interferometric observations of these complex and intrigu-ing objects as well as motivate the development of interferometry-oriented andphysically-consistent models for such objects. A more complete description ofthe present work is given by Domiciano de Souza et al. (2005).Acknowledgments. A.D.S. acknowledges the Max-Planck-Institut fu¨r Radio-astronomie for a postdoctoral fellowship. We are indebted to Dr. R. V. Yudinfor his calculations on the intrinsic polarization vector.ReferencesDomiciano de Souza, A., Driebe, T., Chesneau, O., et al. 2005, A&A, acceptedDrilling, J. S. 1991, ApJS, 76, 1033Egan, M. P., Price, S. D., Kraemer, K. E., et al. 2003, The Midcourse Space ExperimentPoint Source Catalog (v.2.3)Ivezic´, Zˇ., & Elitzur, M. 1997, MNRAS, 287, 799Joint IRAS Science Working Group 1988, Point Source CatalogKraus, M., & Lamers, H. J. G. L. M. 2003, A&A, 405, 165Leinert, Ch., van Boekel, R., Waters, L.B.F.M., et al. 2004, A&A, 423, 537McGregor, P. J., Hyland, A. R., & Hillier, D. J. 1988, ApJ, 324, 1071Petrov, R. G., Malbet, F., Weigelt, G., et al. 2003, SPIE proc., 4838, 924Sloan, G. C., Kraemer, K. E., Price, S. D., & Shipman, R. F. 2003, ApJS, 147, 379Yudin, R. V., & Evans, A. 1998, A&AS, 131, 401Zickgraf, F.-J. 2003, A&A, 408, 25752.08 2.10 2.12 2.14 2.16 2.18 2.20 2.22 2.240.00.20.40.60.81.00.00.20.40.60.81.00.00.20.40.60.81.03456789345678934567891.01.52.0PA = 74 o ; Bp = 89 m Wavelength ( m) V (UT4-UT2)PA = 99 o ; Bp = 60 m V (UT3-UT4)PA = 38 o ; Bp = 44 m  V (UT2-UT3)UD (mas) UD (mas)  UD (mas)Mg II Na IBr  Norm. FluxFigure 1. VLTI/AMBER observations of CPD−57◦ 2874 obtained aroundBrγ with spectral resolution R = 1500. The normalized flux is shown inthe top panel and the visibilities V for each baseline are given in the otherpanels (the corresponding projected baselines Bp and position angles PA areindicated). The errors in V are ≃ ±5%. The dotted lines are the uniform diskangular diameters θUD in mas (to be read from the scales on the right axis),computed from V at each λ as a zero-order size estimate. The visibilitiesobtained from the elliptical Gaussian model fits the observations quite well(smooth solid lines; Eqs. 1 and 2, and Table 1). In contrast to the Brγ line,the Mg ii and Na i lines do not show any clear signature on the visibilities.60510150.00.20.40.60.81.00.00.20.40.60.81.00.00.20.40.60.81.08 9 10 11 12 13 140.00.20.40.60.81.010203040102030401020304010203040  F (10-12 W/m2 ) MIDI ISO-SWSPA = 105 o ; Bp= 61 m V (UT3-UT4)PA = 35 o ; Bp= 44 m V (UT2-UT3)PA = 80 o ; Bp= 55 m V (UT3-UT4)PA = 19 o ; Bp= 45 m Wavelength ( m ) V (UT2-UT3)UD (mas) UD (mas) UD (mas) UD (mas)Figure 2. VLTI/MIDI observations of CPD−57◦ 2874 obtained in the mid-IR with spectral resolution R = 30. V and θUD are shown as filled and opencircles, respectively. The MIDI and ISO-SWS (Sloan et al. 2003) spectra (toppanel) show no clear evidence of a silicate feature around 10µm. The MIDIvisibilities are well fitted with an elliptical Gaussian model (solid lines; Eqs. 1and 2, and Table 1).70246810121403060901201501802102402703003302468101214ENUniform disk angular size (mas) AMBER @ 2.2  m AMBER @ Br MIDI      @ 7.9   m MIDI      @ 12.1 m2.08 2.10 2.12 2.14 2.16 2.18 2.20 2.22 2.24-90-60-3003060901.01.52.0 Wavelength ( m) Closure Phase (deg)Mg II Na IBr   Norm. FluxFigure 3. Left: Uniform disk sizes dependence on the baseline position an-gle (in degrees) for 2 selected wavelengths both from AMBER (continuum at2.2µm and center of the Brγ line) and from MIDI (7.9µm and 12.1µm). Thecentral star is supposed to be at the center of the plot and the distance between2 symmetrical points gives the corresponding uniform disk angular diameter(θUD) of the CSE. The error bars on the AMBER θUD have similar sizes astheir corresponding symbols. Although the θUD is a zero-order estimate, thisfigure allows us to directly compare the CSE sizes in the near- and mid-IR.Right: VLTI/AMBER closure phase for CPD−57◦ 2874 obtained around Brγwith R = 1500. Within the noise level (≃ ±10◦) the closure phase is zero(continuum and Brγ line) suggesting a centrally-symmetric intensity distribu-tion in the near-IR. Spherical and non-spherical (with axial-symmetry) CSEsare thus compatible with the zero closure phase measured with AMBER.-15-14-13-12-11-10-9-8 0.1  1  10  100log (λFλ) [Wm-2 ]λ [µm]τ0.55µm = 0.030τ9.85µm = 0.036Teff = 20000 KR✸ = 53 RsunTin = 2000 Ka = 5.0 µmFbol = 3.2 * 10-10 W/m2d=2.5 kpcAv=5.9mISO (Sloan et al. 2003)MIDI (this paper)Drilling (1991)McGregor et al. (1988)MSX (Egan et al. 2003)IRAS PSCFigure 4. The SED of CPD−57◦ 2874 constructed from several sources (seefigure label). The solid lines are the observed and dereddened SEDs modelledwith the code DUSTY (Ivezic´ & Elitzur 1997), while the dashed line indicatesthe assumed blackbody spectrum of the central star. The parameters adoptedfor this model are listed in the figure: Tin is the temperature at the inner dustshell boundary, a is the diameter of the silicate grains, and the other symbolshave their usual meanings. A standard r−2 dust-density law was assumed.

The Vicinity of the Galactic Supergiant B[e] Star CPD-57\deg2874 from Near- and Mid-IR Long Baseline Spectro-Interferometry with the VLTI (AMBER and MIDI)

The Vicinity of the Galactic Supergiant B[e] Star CPD-57\deg2874 from Near- and Mid-IR Long Baseline Spectro-Interferometry with the VLTI (AMBER and MIDI)

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