Introduction

The defining characteristic of type II SNe is the presence of very broad Hα emission with a strong P-Cygni profile. A small number of peculiar type II SNe have been found in recent years which are either very bright in the optical continuum with very strong and broad Hα emission without a P-Cygni profile, or are strong radio sources then called radio supernovae.

Introduction

The circumstellar gas (CSG) around supernovae (SNe) provides information on the mass loss history of the dying star. When the SN explodes, the CSG is ionized by the radiation from both the SN and the gas shocked by the expanding ejecta. By looking at spectral signatures from the ionized CSG at increasingly large radii, we may peer deeper and deeper back through time into the mass loss history of the pre-SN. The recent bright and well-studied Type II SNe 1987A and 1993J have given us an unprecedented chance of doing so. Here we briefly discuss the CSG of these two SNe.

SN 1987A

Light curves for the narrow UV emission lines from SN 1987A (Sonneborn et al. 1994) show that the emission starts ∼ 70 days after the outburst with a roughly linear increase in strength until day ∼ 400, followed by a gradual decline up to day ∼ 1000 when the lines start to fall below detectability. In the optical there is very good information on the spatial flux distribution from observations with the NTT (Wampler et al. 1990; Wang & Wampler 1992) and the HST (Jakobsen et al. 1991; Plait et al. 1994); the emission mainly comes from a patchy, elliptically shaped ring with semimajor and semiminor axes of ∼ 0.830 (corresponding to ∼ 6.2 × 1017 cm at 50 kpc) and ∼ 0.605 arcsec, respectively.

Introduction

Supernovae are important for the study of several astrophysical problems – nuclear processing in stellar interiors, distance scale determinations, and the chemical enrichment of the interstellar medium have all been explored. Additionally, they may be used as background probes of interstellar gas by studying, for example, NaI and CaII lines, sampling the gas in the host galaxy, the Milky Way halo gas, and any intervening intergalactic gas. SN 1987A allowed the detailed study of gas towards and within the LMC to a distance D ≃ 0.05 Mpc (Vidal-Madjar et al. 1987, de Boer et al. 1987). With the unusually bright SN 1993J in M81 it is now possible to extend the search for interstellar/intergalactic absorptions beyond the local group of galaxies, out to the distance of the M81 group at a distance of D~3.25 Mpc. In this paper we present a preliminary study of high resolution optical interstellar spectra towards SN 1993J. The observations are described in section 2. The origin of the absorption lines, which fall into three distinct groups are discussed in section 3.

Observations

The observations of SN 1993J were obtained during the nights 1993 April 4–8, using the Utrecht Echelle Spectrograph on the William Herschel Telescope at the Observatorio del Roque de los Muchachos, La Palma.

Introduction

An early result from ultraviolet astronomy was that OB stars with bolometric magnitudes brighter than Mbol ≃ −6 suffer significant mass loss (Snow & Morton 1976). At about the same time the framework was laid down by Castor, Abbott & Klein (1975; hereafter CAK) for what, has proved since to be a very successful theory of mass loss from OB and other similarly high-luminosity stars. In bare outline the physical model is a simple one in which the outward force able to overcome gravity is the pressure exerted by the hot star's radiation field on its own atmosphere. In practical application, complexity arises from the fact that it is overwhelmingly the scattering of radiation in spectral lines that, mediates the force (a point first appreciated by Lucy & Solomon 1970).

Introduction

The standard picture of a Type II SN progenitor star is a red supergiant (RSG) that has evolved from a massive star with an initial main-sequence mass above ∼ 10M⊙. These RSGs are observed to have very massive, slow winds with terminal speeds in the range of 10 − 50 km s−1, and mass loss rates in the range of 10−7 − 10−5M⊙yr−1. These slow winds will gradually blow a stellar wind bubble of RSG wind into the relic main-sequence stellar wind bubble, building up a shell of shocked RSG wind at the edge of the expanding bubble.

Introduction

Axisymmetric outflows are associated with many nebulae (e.g. He 2-36, BI Cru, My Cn 18, IC 4406, K 3-72, Corradi & Schwarz 1993a, b, c; OH 17.7–2.0, La Bertre 1986; R Aquarii, Burgarella & Paresce 1992) and with Be stars. I will concentrate in the present review mainly on planetary nebulae (PNe).

An examination of the catalogue of narrow band images of Schwarz, Corradi and Melnick (1992) and other images in the literature reveals a few interesting morphologies. In some cases, almost perfect rings are observed (e.g. ScWe 3, ScWe 2, Schwarz, Corradi & Melnick 1992; Hen 1357, Bobrowsky 1993).

Introduction

If a supernova progenitor has undergone significant mass-loss then the expanding supernova ejecta will eventually collide with this circumstellar material (CSM). Shock waves arising from the collision will compress and heat both the ejecta and the CSM. The emission from the shocked material depends strongly on the density distributions of the ejecta and the CSM, thereby providing important information about the nature of the CSM.

SN 1987A

Images from the European Southern Observatory (ESO) (Wampler et al. 1990) and the Hubble Space Telescope (HST) (Jakobsen et al. 1991) revealed the presence of a ring-like structure at ∼ 6 × 1017 cm from SN 1987A. The outermost part of the supernova, ejecta is expanding at ∼ 104 km s−1 (Shigeyama & Nomoto 1990) and so is expected to collide with the ring ∼ 10 years after the explosion.

Hydrodynamical model

The progenitor of SN 1987A went through a red supergiant (RSG) phase, and then contracted to a blue supergiant (BSG) before the explosion (for reviews, see Arnett et al. 1989, Hillebrandt & Höflich 1989, Podsiadlowski 1992, and Nomoto et al. 1993a). This evolutionary scenario implies that the SN 1987A environment was formed as follows: the progenitor blew a stellar wind with a velocity ∼ 10 km s−1 and a mass loss rate ∼ 10−5M⊙ yr−1 during the RSG stage, and with corresponding values of ∼ 550 km s−1 and ∼ 10−6M⊙ yr−1 during the BSG stage (Lundqvist & Fransson 1991).

Introduction

The dusty envelopes of late type stars are fascinating objects on their own; they are also interesting for what they teach us about IS chemistry. From their velocity field and density profile, we can study the mass loss during a crucial phase of stellar evolution: since the gas expands at nearly constant velocity in all, but the innermost envelope, the velocity maps yield a 3-D view of the molecule spatial distribution; the distributions of the different molecular species show how photochemical, molecule-molecule and grainsurface reactions proceed with time in a well behaved environment.

The closest massive envelopes lie a few hundred parsecs away and have small angular sizes. The construction of large millimeter-wave interferometers, in particular the IRAM Plateau de Bure interferometer (Guilloteau et al. 1992), has provided a major breakthrough in their investigation.

Molecular emission in IRC+10216

The most remarkable and probably closest massive envelope surrounds the bright IR object IRC+10216 (CW Leo). Its outer radius, observed in the mm lines of 12CO, the most abundant molecule and the best shielded from photodissociation after H2, is R = 3′ (Guélin & Cernicharo, in preparation).

Introduction

The LMC star S119 is a member of the group of Ofpe/WN9 stars listed by Bohannan and Walborn (1989). The Ofpe/WN9 category, first defined by Walborn (1982), identifies peculiar supergiants whose spectra combine the typical Of characteristic emission lines of He II and N III with equally strong lower ionization emission features, such as those of He I and N II, and are believed to represent a transition phase in the evolution between massive O stars and WR stars.

High Resolution Echelle Observations

We have observed S119 with the high resolution echelle spectrograph EMMI, coupled to the NTT, ESO La Silla, on September 18, 1991. The spectra cover the wavelength range 4100Å- 7800Å, with a spectral resolution of 0.089 Å/pixel at 6563 Å. The selected slit width was 1.5″ × 5″, with a plate scale on the detector of 0.345″/pixel. In the spectrum, previously undetected nebular lines of Hα, Hβ, [NII], [SII] appear strong and spatially extended, an indication that S119 is surrounded by a bright gaseous nebula. We detect clear splitting of all the observed nebular lines. In Figure 1 we show the radial velocity map obtained from the [NII] 6583 Å line profile. During the observation the slit was oriented EW, and the star was not centered in the aperture, so that only the eastern portion of the nebula lies completely within the slit, while the western region is marginally covered (≃ 2″).

Introduction

This is a short summary of several calculations of the evolution of supernova remnants in a constant high density medium n0 ≥ 106−8 cm−3 and an abundance in the range 0.01Z⊙≤ Z ≤ 10Z⊙. The main difference found when comparing them with the standard calculation of a supernova evolving into a constant density medium n0 = 1 cm−3 is that radiative cooling becomes important very early in the life of the remnants. The radiative phase starts well before the ejecta is fully thermalized and while the expansion velocities are still in the range of several thousands of km s−1. Consequently, the remnants miss their Sedov evolutionary phase and, unlike the standard case, in these calculations full thermalization of the ejecta is only completed long after the moment of thin shell formation (see Terlevich et al. 1992, 1994a; hereafter referred to as papers I and II). The cooling event leads to large luminosities (≥ 109 L⊙) in spans of time of only a few years, causing a major rapid depletion of the supernova's stored thermal energy, in only a few weeks or months. Strong radiative cooling leads to an ionizing spectrum (see paper I) and thus to an HII region with multiple components, as it photoionizes the recombining, rapidly-moving swept-up gas and the outer unperturbed matter. The ionizing radiation is also absorbed by the still unshocked and dense expanding ejecta.

Such remnants, hereafter termed “compact SNRs”, are capable of producing the strong ionizing flux that makes them appear as Seyfert I impostors (Filippenko 1989) when occurring in dense regions far away from the nucleus of galaxies.

Introduction

Rapid progress has been made over the last few years in the study of the neutral gas in planetary nebulae (PNe), and it is now well established that at least some PNe are surrounded by massive envelopes of neutral gas (see, e.g., the review by Huggins 1993). The neutral envelopes provide a new perspective on the formation and evolution of the ionized nebulae, and allow the study of a range of circumstellar processes with different characteristics than those found in other circumstellar environments. In this paper we summarize the results of a recent survey of millimeter CO emission in PNe to study the molecular component of the neutral gas, and we comment on some of the issues raised by the observations.

The Molecular Gas in PNe

Millimeter CO emission has proved to be an especially useful probe of the neutral gas in PNe, since it can be used to determine the structure and kinematics of the envelopes, and to estimate the mass of the molecular component. In order to systematically study the differences between PNe, particularly evolutionary effects, we have undertaken extensive survey work in the 230 GHz CO (2–1) line using the IRAM 30 m and SEST 15 m telescopes. These provide access to both northern and southern PNe, with angular resolutions of 12″–24″. Our observations considerably extend the earlier survey work by Huggins ≈ Healy (1989), and are up to a factor of ≈ 6 times more sensitive, depending on the angular size of the PNe.

Introduction

It is well established that the bulk of mass loss from low and intermediate mass stars occurs during the asymptotic giant, branch (AGB) stage of evolution, leading to the well-defined sequence of mass-losing stars in the IRAS two-colour diagram (van der Veen and Habing 1988) and the formation of planetary nebulae (Abell and Goldreich 1966; Renzini 1981). However, a reliable theoretical understanding of the causes of mass loss is still not available, although progress is being made. An additional complication is that the time history of mass loss during AGB evolution is quite complex since AGB evolution is modulated by helium shell flashes which control the surface luminosity and thereby the mass loss rate. In this paper, mass loss rates from AGB stars are discussed and the effects of helium shell flashes on the mass loss are described and compared with observations. Time dependent winds produced by AGB stars are reviewed. Finally, the evolution of AGB stars that lose mass in binary mass transfer events is briefly described.

AGB mass loss rates

IRAS observations of stars in the solar neighbourhood indicate that those stars with substantial circumstellar shells - those with high mass loss rates - are nearly all AGB stars undergoing large-amplitude pulsation (Habing 1990).

Introduction

Practically all diffuse media of astrophysical significance are clumpy media which are responding to energy sources. The most important distinction between flows initiated in clumpy as opposed to homogeneous media, is that in the former, there is mass, momentum and energy interchange at clumptenuous plasma boundaries, i.e. in boundary layers. The consequences are major (Hartquist & Dyson 1993). This interchange reacts back on the dynamical, physical and even chemical state of the global tenuous plasma flow; conversely, the state of the global flow influences the interchange process.