Hubble Space Telescope Imaging of the Mass-losing Supergiant VY

THE ASTRONOMICAL JOURNAL, 115 : 1592È1598, 1998 April ( 1998. The American Astronomical Society. All rights reserved. Printed in U.S.A.

HUBBL E SPACE T EL ESCOPE IMAGING OF THE MASS-LOSING SUPERGIANT VY CANIS MAJORIS JOEL H. KASTNER MIT Center for Space Research, NE80-6007, Cambridge, MA 02139 ; jhk=juggler.mit.edu

AND DAVID A. WEINTRAUB Department of Physics and Astronomy, Vanderbilt University, P.O. Box 1807, Station B, Nashville, TN 37235 ; david=ttau.phy.vanderbilt.edu Received 1997 October 21 ; revised 1997 December 19

ABSTRACT The highly luminous M supergiant VY CMa is a massive star that appears to be in its Ðnal death throes, losing mass at high rate en route to exploding as a supernova. Subarcsecond-resolution optical images of VY CMa, obtained with the Faint Object Camera (FOC) aboard the Hubble Space Telescope, vividly demonstrate that mass loss from VY CMa is highly anisotropic. In the FOC images, the optical ““ star ÏÏ VY CMa constitutes the bright, well-resolved core of an elongated reÑection nebula. The imaged nebula is D3A (D4500 AU) in extent and is clumpy and highly asymmetric. The images indicate that the bright core, which lies near one edge of the nebula, is pure scattered starlight. We conclude that at optical wavelengths VY CMa is obscured from view along our line of sight by its own dusty envelope. The presence of the extended reÑection nebula then suggests that this envelope is highly Ñattened and/or that the star is surrounded by a massive circumstellar disk. Such axisymmetric circumstellar density structure should have profound e†ects on postÈred supergiant mass loss from VY CMa and, ultimately, on the shaping of the remnant of the supernova that will terminate its postÈmain-sequence evolution. Key words : dust, extinction È stars : individual (VY Canis Majoris) È stars : mass loss 1.

INTRODUCTION

show highly polarized optical reÑection nebulosity within D4A of the purported central star, with the largest polarization lying in a Ðnger of nebulosity extending to the west. Herbig also reported that ““ the visual polarization of VY plus the nebula ÏÏ is 12%È18%. HerbigÏs conclusion that VY CMa is not a multiple star system was subsequently conÐrmed via visual observations by Worley (1972) and Holden (1976, 1978). Each observed only nebulosity, perhaps containing condensations ; neither observed stellar companions. Worley reported that ““ even the brightest condensation, A, does not appear starlike, although no doubt a star is immersed in the nebulosity.ÏÏ There have been no follow-up imaging studies of VY CMa. However, it is a notorious source of radio molecular emission lines, displaying thermal emission from CO and HCN (Zuckerman & Dyck 1986 ; Nercessian et al. 1989) and maser emission from OH, H O, and SiO (see, e.g., 2 ; Reid & Dickinson Bowers, Claussen, & Johnston 1993 1976). It is also one of the brightest midinfrared sources in the sky (Hyland et al. 1969 ; Danchi et al. 1994). These observations indicate that VY CMa is losing mass at an exceedingly high rate (D3 ] 10~4 M yr~1 ; Danchi et al. 1994). Furthermore, VY CMa appears_to be associated with a molecular cloud (Lada & Reid 1978), suggesting that it is so massive and has evolved so rapidly that it is still located near its birthplace (indeed, Lada & Reid speculated that VY CMa may be a massive preÈmain-sequence star). The unusually high degree of large-beam optical polarization measured by Herbig (1972) is reminiscent of the T Tauri star HL Tau, which, at the spatial resolution of HST , proves to be pure reÑection nebulosity (Stapelfeldt et al. 1995). Hence it seemed possible to us that high-resolution imaging would demonstrate that even the starlike core in HerbigÏs images of VY CMa is dominated by reÑection nebulosity, as WorleyÏs (1972) visual observations suggest. We have obtained HST images with the Faint Object

Hubble Space Telescope (HST ) images of the remnant of SN 1987A in the Large Magellanic Cloud have forced a reevaluation of the mechanisms that shape supernova remnants. These images depict a bipolar nebula consisting of an elegant system of coaxial rings (Burrows et al. 1995). Since the discovery of the SN 1987A rings, various models have been put forward to explain their origin. One possibility involves the shaping of a bipolar remnant by the interaction of supernova ejecta with a massive, low-velocity bipolar wind formed while the central star was still a supergiant (e.g., Wang & Mazzali 1992). This hypothesis also has been invoked, with much success, to model the production of bipolar planetary nebulae (Frank & Mellema 1994 ; Mellema & Frank 1995). This model, however, does not address the question of why the original low-velocity outÑow was bipolar rather than spherically symmetric. Alternatively, it is possible that the formation of the most prominent feature of SN 1987A, its central ring, coincided with the formation of the star itself (McCray & Lin 1994). Under either of these scenarios, the origin of the axisymmetric structure of the SN 1987A remnant well predated the supernova explosion itself. Thus, to understand the mechanisms that shape supernova remnants, as well as the evolutionary histories of their progenitor stars, we need to study massive stars in their most rapidly mass-losing (evolved supergiant) phases. The luminous red supergiant VY CMa (HD 58061) is one of the best and longest studied examples of such an object. As early as 1917, it was identiÐed as a star embedded in nebulosity and, even earlier, as a potential multiple system (Herbig 1972 and references therein). However, HerbigÏs optical (6500 Ó) polarization maps indicated that of the (up to six) visual ““ components ÏÏ of the VY CMa system, only the red supergiant itself (component A) appeared to be a source of direct (as opposed to scattered) light. These maps 1592

HST IMAGING OF VY CMa Camera (FOC) that appear to conÐrm this hypothesis, and which yield insight into the mass ejection history and circumstellar structure of this massive, rapidly dying star. 2.

OBSERVATIONS

FOC images of VY CMa were obtained on 1996 May 5. We used the FOC in its f/96 512 ] 512 conÐguration, which produces a 512 ] 512 image with a pixel scale of 0A. 01435 (or D7A on a side). Four images were obtained in two successive orbits, two images each through the medium-pass Ðlters F346M (3460 Ó) and F550M (5500 Ó). Hereafter we refer to these images as u- and y-images, respectively. To keep the count rate for VY CMa (V D 8) within the linear response regime of the photon-counting FOC, we used neutral-density Ðlters for all images. As the potential to reach the nonlinear regime when imaging this red supergiant was more of a concern at the longer wavelength, the two images through F550M were obtained through di†erent amounts (5 and 6 mag) of neutral density. Both images through F346M employed the same amount (4 mag) of neutral density. Integration times were 1176 and 996 s, respectively, for the y-images obtained through 5 and 6 mag of neutral density. Integration times were 1196 and 1272 s for the u-images. The measured peak count rates of 0.04 and 0.86 s~1 pixel~1, respectively, for the raw u- and (deeper) y-images indicate that all of the images are within the FOC linearity threshold (nonlinearity obtains at 1.0 s~1 pixel~1 ; HST Data Handbook, Volt 1997). Comparison of the two raw y-images conÐrms that the deeper y-image is fully within the linear regime. The images were reduced using the standard FOC processing pipeline. This standard reduction consisted of application of a geometric distortion correction and division by a Ñat-Ðeld image to correct for large-scale detector nonuniformities (see HST Data Handbook). We co-added the two u-images, and translated and rotated the y- and co-added u-images in order to align them in equatorial coordinates. FOC images taken through the F550M Ðlter are o†set in position by ([4,]11) pixels relative to images obtained through the F346M Ðlter (HST Data Handbook) ; we applied this translation to the y-images to properly align the u- and y-images. Since the u- and y-images were obtained in successive orbits, their relative astrometric accuracy (after accounting for image translation due to the Ðlter change) is limited only by telescope jitter, which was determined to be less than 3 mas (rms). However, as no reference stars are visible in our FOC images, the absolute astrometric accuracy of these images can be assumed to be no better than D0A. 5 (HST Data Handbook). We applied a rotation to each image to correct for the reported roll angle of HST (81¡.384), such that north is up and east is to the left in our Ðnal (u, y)-images. To convert count rates to surface brightnesses, we applied the standard count rate calibrations for FOC images as described in the HST Data Handbook. Based on these calibrations, we estimate that the (3 p) sensitivities of the u- and y-images are 6 ] 10~19 and 1.2 ] 10~16 ergs cm~2 s~1 Ó~1 pixel~1, respectively. 3.

RESULTS

3.1. FOC u- and y-Images The FOC images of VY CMa are presented in Figure 1 (only the deeper y-image is shown). In these images, the ““ star ÏÏ VY CMa appears as a bright core embedded in an

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extended reÑection nebula. The core is very near the eastern edge of the nebula, which is highly asymmetric and elongated. The extended nebula also appears quite clumpy, with its main features reproduced in both images. Aside from the core, however, the nebular clumps are broad and di†use. Furthermore, aside from A and C (see below), there is no clear correspondence between features in these FOC images and the ““ components ÏÏ of VY CMa reported in the early literature. Hence these images conÐrm earlier conclusions that VY CMa is not a multiple star system (Herbig 1972 ; Worley 1972 ; Holden 1976, 1978). At u and y, respectively, the nebula is detected out to D2A. 5 and D3A. 5 away from the core to the west-northwest. This prominent extension is, presumably, the (highly polarized) ““ curved nebulous tail toward 290¡ ÏÏ in the photographic plates obtained at 6500 Ó by Herbig (1972), which he identiÐed with ““ component ÏÏ C. In sharp contrast, the nebular surface brightness declines steeply east of the core, such that the nebula extends less than 0A. 1 (0A. 3) in this direction at u (y). Careful examination of the nebular core reveals that it is well resolved in the HST FOC images. This is evident from a visual comparison of the VY CMa images with images of stars obtained through the F372M and F550M Ðlters (Fig. 2). The VY CMa nebular core clearly is extended in comparison with the core of the HST FOC point-spread function as measured through these Ðlters. Furthermore, the core is elongated in both u- and y-images, and the major axis of elongation rotates noticeably from u to y. Perhaps most signiÐcantly, the position of the core is not the same in the u- and y-images. SpeciÐcally, the core is farther to the northeastÈi.e., it is closer to the eastern edge of the nebulaÈin the y-image than in the u-image. All of these visual inferences are conÐrmed via centroid measurements of, and Ðts of two-dimensional Gaussian functions to, the core regions of the VY CMa images (Table 1). The extension of the nebular core, and its changing position with wavelength, serve as strong evidence that this feature is pure scattered light and indicate that our HST FOC images of VY CMa contain no direct emission from the stellar photosphere (° 4). Furthermore, given the luminosity of this M supergiant (5 ] 105 L at 1.5 kpc ; Lada & Reid 1978), its photospheric _ be of the order of D15 AU (3 ] 103 R ), or radius should _ the D20 mas. Hence we should not be able to resolve photosphere. Yet the deconvolved angular FWHM of the bright core at both u and y, D100 mas (Table 1), suggests a physical radius of D75 AU. The extension of the image core thus lends further support to the hypothesis that we have TABLE 1 POSITION AND EXTENT OF THE CORE OF THE VY CANIS MAJORIS NEBULA Band

R.A. O†seta (mas)

Decl. O†seta (mas)

FWHMb (mas)

P.A. (deg)

u .......... y .......... u[y......

0 20 ^ 3 30 ^ 15

0 40 ^ 3 85 ^ 15

99 104 ...

[68 157 ...

a For the y-image, the position of the centroid of the bright core relative to the centroid of the bright core in the u-image. For u [ y, the position of the reddest part of the color map relative to the centroid of bright core in the u-image. b For comparison, the FWHM of the HST FOC PSF, as determined from images of standard stars, is 40 mas at these wavelengths.

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Vol. 115

FIG. 1.ÈHubble Space Telescope images of VY CMa obtained with the Faint Object Camera (FOC) through Ðlters F346M (u band, top) and F550M (y band, bottom). Only the FOC subÐeld containing detected nebulosity is displayed. Position (0, 0) in (R.A., decl.) o†sets is centered on the peak pixel of the u-image.

not directly detected the stellar photosphere in these images. Note that the size of the core in the u- and y-images is somewhat larger than that estimated for the inner radius of the ““ dust shell ÏÏ around VY CMa, on the basis of interferometry at 11 km (Danchi et al. 1994). 3.2. T he u [ y Colors and Color Map Although the F346M and F550M Ðlters used to image VY CMa with the HST FOC are narrower than the standard U and V bands, and F346M is centered slightly shortward of U (which is nominally centered at 3650 Ó), the

HST FOC images of VY CMa provide a reasonable comparison with ground-based determinations of U[V . The u [ y color of VY CMa obtained by integrating the Ñux in the FOC images is 3.85. This is roughly the U[V color of an early to mid-M supergiant (e.g., a Ori, M2 Iab, has U[V D 4) and is similar to the ground-based U[V color reported for VY CMa in the SIMBAD database (U[V \ 4.06). Thus, the integrated FOC u [ y color of VY CMa is consistent with its spectral classiÐcation of M3/M4 in the 1988 Michigan Spectral Survey (Houk & SmithMoore 1988).

No. 4, 1998

HST IMAGING OF VY CMa

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FIG. 2.ÈClose-ups of the HST FOC images displayed in Fig. 1, showing the core regions of VY CMa nebulosity (top). Position (0, 0) in (R.A., decl.) o†sets is centered on the peak pixel of the u-image. These close-ups are displayed above images of bright Ðeld stars obtained with the HST FOC through Ðlters F372M and F550M, respectively, to characterize its point-spread function (PSF) near the u band (no PSF image is available for Ðlter F346M) and at the y band. The cores of the u- and y-images of VY CMa are extended in comparison with the corresponding PSF images. In addition, the position of the VY CMa core can be seen to shift from the u to the y band.

We constructed a u [ y color map of the VY CMa nebula from the (u, y) image pair (Fig. 3). This map demonstrates that the reddest region of the nebula is far redder, u [ y D 8, than the integrated u [ y or ground-based U[V color measurements of VY CMa. This red region lies at the eastern edge of the nebula, very near the bright core but not spatially coincident with either the u or y surface brightness peaks. Instead, the reddest pixel in the u [ y map (u [ y \ 8.3) lies adjacent to, but north and slightly east of, the bright core of the y-image (Table 1). At the position of the y core, we measure u [ y \ 7.0, while at the position of the u core, we Ðnd u [ y \ 5.2. The u [ y map also shows that the red (u [ y [ 7) region of the nebula is elongated east-southeast to west-northwest, i.e., roughly perpendicular to the northeastward ““ migration ÏÏ of the image cen-

troids with increasing wavelength. In contrast, the more di†use region of the nebula to the west of the core is considerably bluer, with u [ y generally between 3.5 and 4.5. Hence the integrated u [ y color of VY CMa is dominated by the di†use, scattered light in the extended nebula as opposed to the core region. If the di†use, extended portion of the VY CMa reÑection nebula displays the classical tendency to appear bluer than its source of illumination, we would conclude that the radiation that illuminates the nebula cannot be characteristic of the photosphere of an M supergiant, but must be much redder. SpeciÐcally, if grains in the nebula scatter optical photons with an efficiency Q in the Rayleigh scattering regime, Q P j~4, then we predict that the illuminating source has an intrinsic U[V D 6. This in turn would

FIG. 3.ÈThe u [ y color map of VY CMa, constructed from the HST FOC images displayed in Fig. 1 (black regions of the map indicate that surface brightness is less than 1 p in the u- and/or y-images). Red corresponds to u [ y [ 7.5, violet corresponds to u [ y \ 4.5, and yellow, green, and blue correspond to (increasingly blue) u [ y colors between these extremes. Note the elongated region of very red nebulosity, extending roughly east-southeast to west-northwest. Superposed on this map are contours indicating the positions of the intensity peaks of the cores of the y-image (solid contour) and u-image (dashed contour). Note the northeastward ““ migration ÏÏ from the u peak to the y peak to the position of reddest nebulosity.

HST IMAGING OF VY CMa suggest either that (1) the photosphere of the illuminating star is far cooler than that of an M3/M4 supergiant, or (2) radiation from the star is heavily reddened (e.g., by a more or less spherically symmetric circumstellar dust envelope) prior to scattering by grains in the nebula. Conversely, if the nebula ““ sees ÏÏ a bare M3/M4 photosphere, we would conclude that the scattering dust includes a population of large grains (radii a [ 0.1 km) that scatter light with an efficiency more or less independent of wavelength. As in the case of the bipolar reÑection nebula OH 231.8]4.2 (Kastner & Weintraub 1995), it is not straightforward to distinguish between these possibilities solely on the basis of two-color imaging. 4.

DISCUSSION

The analysis in ° 3 demonstrates that the core of the VY CMa nebula is extended and that the position of the core is a function of wavelength. Both these results strongly suggest that the compact core in the u- and y-images of VY CMa is pure scattered light, rather than direct emission from the stellar photosphere. That is, the optical ““ star ÏÏ VY CMa is in fact merely the core of a cometary reÑection nebula. Indeed, the ““ migration ÏÏ of the bright core region with wavelength is typical of multiwavelength images of cometary or bipolar reÑection nebulae surrounding obscured central stars (see, e.g., Kastner & Weintraub 1995 ; Whitney, Kenyon, & Gomez 1997), wherein as wavelength increases (and dust opacity decreases) one sees deeper into the nebula, closer to the stellar photosphere itself. Thus we would conclude that VY CMa actually lies northeast of the bright core of the y-image, in the vicinity of the red peak in the u [ y color map (Fig. 3). Deeper, wider Ðeld, subarcsecond imaging is required to ascertain the position of the nebular core relative to the position of the star as inferred from, e.g., water maser mapping (Bowers et al. 1993), since our images lack positional reference stars and, hence, lack the requisite absolute astrometric accuracy. An even more deÐnitive test of our hypothesis that the star lies northeast of the core in the y-image would be provided by high-resolution (subarcsecond) optical polarimetric imaging, either with the HST Wide Field Planetary Camera 2 (WFPC2) or via adaptive optics techniques. Images at a second epoch could also establish whether any of the features in the nebulosity are transient, display proper motion, or reveal light-travel time e†ects as a consequence of the variability of the illuminating star (such e†ects are observed in OH 231.8]4.2, which harbors a Mira variable ; Kastner et al. 1992). We conclude that VY CMa is embedded deep within a dusty, axisymmetric circumstellar envelope, which manifests itself at optical wavelengths as a cometary reÑection nebula. To produce such morphology, the envelope of VY CMa must be Ñattened, and/or the star must be surrounded by a massive circumstellar disk.1 The equatorial region of the envelope (and/or the disk) occults the star at optical wavelengths along our line of sight but allows optical photons to escape along other lines of sight. SpeciÐcally, photons readily escape along a westward direction ÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈ 1 It is interesting to note that, nearly three decades ago, Herbig (1970) also concluded, on the basis of the optical and radio line spectrum and spectral energy distribution of VY CMa, that it was surrounded and enshrouded by an expanding circumstellar disk.

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and are then scattered, and these scattered photons produce the blue, di†use portion of the nebula that lies west of the central star. The implied axis of symmetry of the VY CMa system is thus more or less northeast-southwest, i.e., along a line connecting the u- and y-image intensity peaks. The blue reÑection nebula extending west-southwest of the bright core is then best interpreted as a forward-facing polar ““ lobe.ÏÏ By symmetry, therefore, we infer the presence of an oppositely directed ““ lobe ÏÏ of reÑection nebulosity extending to the east-northeast of the central star. As we have not detected such a structure, it appears that the equatorial region of the envelope must be large and opaque enough to completely attenuate any optical light that scatters o† dust in the hypothetical ““ rear ÏÏ lobe (if the forward scattering efficiencies of the grains are signiÐcantly larger than their backward scattering efficiencies, this would also play a role in the lack of detection of a rear-facing lobe). Note that this model of the VY CMa system is essentially identical to that invoked to explain the cometary appearance of nebulosity associated with HL Tau, and the lack of detection of direct photospheric emission from HL Tau, in HST WFPC2 images of this T Tauri star (Stapelfeldt et al. 1995). Progenitor mass appears to govern the appearance of bipolar structure during the late stages of evolution of intermediate-mass stars. Strongly axisymmetric planetary nebulaeÈi.e., those that retain massive, molecule-rich circumstellar disksÈare produced by stars with initial mainsequence (MS) masses Z1.5 M , i.e., stars of spectral type _ A and earlier (Corradi & Schwarz 1994 ; Kastner et al. 1996). We have argued (Kastner & Weintraub 1995 ; Kastner et al. 1996) that the relatively brief MS lifetimes of such relatively massive stars may explain the connection between initial MS mass and the manifestation of bipolar structure. That is, A stars evolve rapidly enough that they may retain the fossil vestiges of pre-MS disks throughout their MS lifetimes (the so-called Vega phenomenon) ; these disks then serve as the ““ seeds ÏÏ for subsequent post-MS bipolar structure. Our HST FOC imaging results for VY CMa are consistent with this hypothesis. Indeed, in the case of this massive supergiant, we are observing a star whose main-sequence lifetime is so short (D106 yr) that it very likely was still surrounded by a pre-MS disk and/or remnant envelope when it began to evolve away from the upper MS.2 Thus, the pre-MS circumstellar structure of VY CMa may have spawned the complex, but fundamentally axisymmetric, post-MS circumstellar structure we have detected with the HST FOC. We may then extrapolate that, when VY CMa explodes as a supernova, the remnant thus formed will have profound bipolar structure. This structure would have its origins in the formation of the star itself, as McCray & Lin (1994) contend in the case of SN 1987A.

Support for this research was provided by Space Telescope Science Institute grant GO-06416.01-95A to MIT. This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France. ÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈ 2 Ironically, Herbig (1970) hypothesized that VY CMa might possess a circumstellar disk in the context of the inference that it could be a massive pre-MS star.

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REFERENCES Bowers, P. F., Claussen, M. J., & Johnston, K. J. 1993, AJ, 105, 284 Kastner, J. H., Weintraub, D. A., Zuckerman, B., Becklin, E. E., McLean, I., Burrows, C. J., et al. 1995, ApJ, 452, 680 & Gatley, I. 1992, ApJ, 398, 552 Corradi, R., & Schwarz, H. 1994, A&A, 293, 871 Lada, C. J., & Reid, M. J. 1978, ApJ, 219, 95 Danchi, W. C., Bester, M., Degiacomi, C. G., Greenhill, L. J., & Townes, McCray, R., & Lin, D. N. C. 1994, Nature, 369, 378 C. H. 1994, AJ, 107, 1469 Mellema, G., & Frank, A. 1995, MNRAS, 273, 401 Frank, A., & Mellema, G. 1994, ApJ, 430, 800 Nercessian, E., Guilloteau, S., Omont, A., & Benayoun, J. J. 1989, A&A, Herbig, G. H. 1970, ApJ, 162, 557 210, 225 ÈÈÈ. 1972, ApJ, 172, 375 Reid, M. J., & Dickinson, D. F. 1976, ApJ, 209, 505 Holden, F. 1976, PASP, 88, 52 Stapelfeldt, K. R., et al. 1995, ApJ, 449, 888 ÈÈÈ. 1978, PASP, 90, 463 Volt, M., ed. 1997, HST Data Handbook, Vol. 1, Current Instruments Houk, N., & Smith-Moore, M. 1988, Michigan Catalogue of Two(version 3 ; Baltimore : STScI) dimensional Spectral Types for the HD Stars, Vol. 4 (Ann Arbor : Dept. Wang, L., & Mazzali, P. A. 1992, Nature, 355, 58 Astron., Univ. Michigan) Whitney, B. A., Kenyon, S. J., & Gomez, M. 1997, ApJ, 485, 703 Hyland, A. R., Becklin, E. E., Neugebauer, G., & Wallerstein, G. 1969, ApJ, Worley, C. E. 1972, ApJ, 175, L93 158, 619 Zuckerman, B., & Dyck, H. M. 1986, ApJ, 304, 394 Kastner, J. H., & Weintraub, D. A. 1995, AJ, 109, 1211 Kastner, J. H., Weintraub, D. A., Gatley, I., Merrill, K. M., & Probst, R. G. 1996, ApJ, 462, 777