The Circumstellar Environments of the Cool Hypergiants
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RevMexAA (Serie de Conferencias), 30, 6{11 (2007) THE CIRCUMSTELLAR ENVIRONMENTS OF THE COOL HYPERGIANTS: IMPLICATIONS FOR THE MASS LOSS MECHANISM Roberta M. Humphreys1 RESUMEN Las hipergigantes fr´ıas son las estrellas m´as luminosas conocidas en la parte superior del diagrama HR en el aparente rango de temperatura representado por los tipos espectrales de la A a la M. La mayor´ıa de estas estrellas en este r´egimen son inestables, como lo ponen de manifiesto sus altas tasas de p´erdida de masa, lavariabilidad, y en algunos casos los grandes excesos infrarrojos as´ı como el material circunestelar expulsado. En este art´ıculo corto, describo la complejidad del material eyectado de dos hipergigantes notables, IRC+10420 y VY CMa, y la evidencia de episodios de alta perdida de masa en sus historias recientes, aparentemente impulsadas por actividad convectiva a gran escala. ABSTRACT The cool hypergiants are the most luminous known stars in the upper HR Diagram in the apparent temperature range represented by the spectral types A to M. Most of these stars in this regime are unstable as evidenced by their high mass loss rates, variability, and in some cases large IR excesses and circumstellar ejecta. In this brief paper, I describe the complex circumstellar ejecta of two remarkable hypergiants IRC+10420 and VY CMa and the evidence for high mass loss episodes in their recent history apparently driven by large-scale convective activity. Key Words: CIRCUMSTELLAR MATTER | STARS: MASS LOSS | STARS: SUPERGIANTS 1. WHAT IS A COOL HYPERGIANT? that most if not all of the intermediate tempera- ture hypergiants are post-RSGs. In their post-RSG A few highly unstable, very massive stars lie on blueward evolution these very massive stars enter a or near the empirical upper luminosity boundary in temperature range (6000{9000 K) with increased dy- the HR diagram (Figure 1). These include the Lumi- namical instability, a semi-forbidden region in the nous Blue Variables, the cool hypergiants, and even HR diagram, that he called the \yellow void", where rarer objects, all related by high mass loss phenom- Ed. G. García-Segura & E. Ramirez-Ruiz high mass loss episodes occur. ena, sometimes violent, which may be responsible for To better understand the evolution of these cool, the existence of the upper boundary (Humphreys & evolved stars near the upper luminosity boundary Davidson 1994). In this paper, I use the term `cool and the mass loss mechanisms that dominate the up- hypergiant' for the stars that lie just below this up- per HR diagram, we have obtained high resolution per luminosity envelope with spectral types ranging multi-wavelength images with HST/WFPC2 of sev- from late A to M. The cool hypergiants very likely eral of the most luminous known evolved cool stars { represent a very short-lived evolutionary stage, and the M-type hypergiants, µ Cep (M2e Ia), S Per (M3- are distinguished by their high mass loss rates. Many 4e Ia), NML Cyg (M6 I), VX Sgr (M4e Ia{M9.5 I), of them also show photometric and spectroscopic and VY CMa (M4{M5 Ia) and the intermediate-type variability, and some have large infrared excesses, (F and G{type) hypergiants, IRC+10420 (A{F Ia) © 2007: Instituto de Astronomía, UNAM - Circumstellar Media and Late Stages of Massive Stellar Evolution and extensive circumstellar ejecta. ρ Cas (F8p Ia), HR 8752 (G0-5 Ia) and HR 5171a The evolutionary state of most of these stars is (G8 Ia). The presence or lack of fossil shells, bipo- not known. They are all post main sequence stars, lar or equatorial ejecta, and other structures in their but the intermediate{type or \yellow" hypergiants ejecta will be a record of their current and prior mass could be either evolving to cooler temperatures or loss episodes and provide clues to their evolutionary be post-red supergiant (RSG) stars in transition to history. These stars were selected on the basis of warmer temperatures. de Jager (1998) has suggested their infrared emission, strong molecular emission, or 1 peculiar spectroscopic variations to give us a snap- Astronomy Department, 116 Church St. SE., Uni- versity of Minnesota, Minneapolis, MN 55455, USA shot of different steps in their evolution across the ([email protected]). top of the HR Diagram. 6 COOL HYPERGIANTS 7 Fig. 1. A schematic HR Diagram for the most luminous stars. The empirical upper luminosity boundary is shown as a solid line, and the cool hypergiants are identified with crosses. Our results are quickly summarized: we found no observed by Habing et al. (1982). Morris & Jura detections of circumstellar material associated with (1983) showed that the asymmetric \inverse" H II ρ Cas, HR 8752, HR 5171a and µ Cep; VX Sgr and S region was the result of the interaction of a spheri- Per, both OH/IR sources, were marginally resolved. cally symmetric, expanding wind from NML Cyg and NML Cyg's (OH/IR source) ejecta has been shaped photo-ionization from plane parallel Lyman contin- by its environment and IRC+10420 and VY CMa uum photons from the luminous, hot stars in the (OH/IR sources) have extensive and complex cir- nearby association Cyg OB2 (see Figures 1 and 2 in cumstellar nebulae. Morris & Jura 1983). The presence of ionized hy- In this short paper, I describe our results for drogen surrounding an M supergiant like NML Cyg NML Cyg, IRC+10420 and VY CMa and the im- was somewhat of an enigma. To explain its pres- plications for the episodic mass loss mechanism in ence, they suggested that the molecular material in the latter two objects. the wind is photo-dissociated closer to the star so Ed. G. García-Segura & E. Ramirez-Ruiz that it does not shield the atomic hydrogen from the 2. NML CYG { SHAPED BY ITS ionizing photons (from Cyg OB2) farther out. ENVIRONMENT Our images show circumstellar material much The powerful OH/IR source NML Cyg (M6 I) is closer to NML Cyg than the surrounding H II region approximately 1.7 kpc from the sun and ∼ 100 pc and coincident with the water masers, as well as SiO from the large association Cyg OB2 in the X-ray masers, suggesting that we are likely imaging the emitting Cygnus X superbubble (Humphreys 1978; molecular photo-dissociation boundaries. Schuster Morris & Jura 1983; Kn¨odlseder 2003). This dis- et al. (2006) propose that the shape of the envelope tance places it near the empirical upper luminosity seen in the WFPC2 images is the result of the inter- boundary for red supergiants with a luminosity of action between the molecular outflow from NML Cyg © 2007: Instituto de Astronomía, UNAM - Circumstellar Media and Late Stages of Massive Stellar Evolution 5 5 × 10 L (Mbol ∼ −9:5) and a mass loss rate and the near-UV continuum flux from Cyg OB2, i.e., −5 −1 of 6:4 × 10 M yr (Hyland et al. 1972; Mor- analogous to an \inverse Photo-Dissociation Region" ris & Jura 1983). HST/WFPC2 images (Schuster, (PDR). To test our hypothesis, Schuster et al mod- Humphreys, & Marengo 2006) show that NML Cyg eled the shape of the photo-dissociation boundary. has a very obvious but small circumstellar nebula Figure 2 shows the dissociation surface superim- with a peculiar asymmetric shape (Figure 2). posed on the HST/WFPC2 image. It is especially There are remarkable similarities between the interesting that the asymmetric one-sided distribu- small(∼ 000:3), asymmetric envelope that we see and tion of the water masers is not only similar in extent the much more distant (∼ 3000 from the star) 21 cm to the reflection nebula, but also matches its convex ionized hydrogen (H II) contours around NML Cyg shape. The dusty cocoon engulfing NML Cyg must 8 HUMPHREYS Fig. 2. The small bean-shaped asymmetric nebula sur- Fig. 3. A short exposure HST/WFPC2 image showing 00 rounding the optically obscured star NML Cyg. The the complex inner ejecta within 2 of the embedded star. positions of the H2O masers are superimposed on the image together with the dissociation surface modeled by Schuster et al. (2006) fit to the envelope of the nebula. large infrared excess and strong maser emission, they concluded that IRC +10420 is a post-red su- pergiant evolving back toward the blue side of the be the consequence of high mass loss in the RSG HR diagram, in an evolutionary phase analogous stage, but its envelope has most likely been shaped to the proto-planetary/post-AGB stage for lower by its interaction with and proximity to Cyg OB2. mass stars. HST/WFPC2 images (Humphreys et al. If the outflow from NML Cyg is bipolar (Richards 1997) revealed a complex circumstellar environment, et al. 1998), then it appears that the molecular ma- with a variety of structures including condensations terial SE of the star is preferentially shielded from or knots, ray-like features, and several small, semi- photo-dissociation. Even without assuming bipolar- 00 Ed. G. García-Segura & E. Ramirez-Ruiz circular arcs or loops within 2 of the star (Figure 3), ity, there is more maser emission to the ESE, con- plus one or more distant reflection shells. These fea- sistent with our model for NML Cyg's circumstellar tures are all evidence for high mass loss episodes dur- envelope. ing the past few hundred years. A few other intermediate-temperature hyper- 3. IRC+10420 { A POST{RED SUPERGIANT giants such as ρ Cas and HR 8752 occupy the same IRC +10420 may be one of the most important region in the HR diagram, but IRC +10420 is the stars in the HR diagram for understanding the final only one with obvious circumstellar nebulosity, mak- stages of massive star evolution.