Astron. Astrophys. 348, 557-569 (1999)

1. Introduction

HR 4796 A is one of the A type stars showing a high infrared excess ( / ) due to the thermal emission from circumstellar dust. The excess is twice that of Pictoris, the best studied case among Vega-like stars so far. The star age, 82 Myr (Stauffer et al., 1995), is about 5 to 15 times less than Pictoris implying that HR 4796 A is tracing an evolutionary phasis prior to that of Pictoris.

Koerner et al. (1998) and Jayawardhana et al. (1998) resolved first the circumstellar dust at thermal infrared wavelengths (m) recently followed by Schneider et al. (1999) in scattered light ( and m). Interestingly, the disk lies nearly in the direction to HR 4796 B, a physically bound companion of HR 4796 A (Jura et al., 1993), located 7.7" away, i.e. 515 AU in projected distance according to the Hipparcos star distance pc.

According to the resolved images, the disk is extending only a few tens of AU outside the peak of the dust distribution assessed to about 70 AU (Schneider et al., 1999). Outer truncation of circumstellar disks within a binary system has been predicted at typical distances of 1/3-1/2 of the binary separation (Papaloizou & Pringle, 1977; Artymowicz & Lubow, 1994). However, the impact of the companion HR 4796 B on the disk extension is still unclear.

In the inner part of the disk, Jura et al. (1995) first suggested a depletion of dust close to the star so as to reproduce the 110 K color temperature deduced from IRAS data. The comparison between resolved mid-IR images and first order modeling (see below) confirmed the necessity of an inner hole in the disk at 5515 AU from the star for Koerner et al. (1998) and 6020 AU for Jayawardhana et al. (1998).

According to Koerner et al. (1998), a second population of hotter grains may however lie closer to the star (inside the inner hole). Located at distances similar to those of the zodiacal dust in our Solar System, this dust population would be responsible for both the 12.5 µm detected excess and for a faint emission at 20.8 µm (less than 10% of the total flux at the wavelength) in excess of the resolved disk and centered on the star.

The grains properties are so far poorly constrained. According to Jura et al. (1995), grains smaller than about 3 µm are blown outward by radiation pressure. Considering the Poynting-Robertson effect, they found that the grains are probably larger than 40 µm under the assumption of a 40 AU inner hole. Koerner et al. (1998) used thermal absorption/emission laws for the grains following the model proposed by Backman et al. (1992) for Pictoris: is constant for and proportional to otherwise, the parameter is expected to be related to an effective grain size in the disk. Although Koerner et al. (1998) constrain to 2515 µm, they do not specify the ratio in the case of HR 4796 A. This ratio could vary by a factor 40 between the lower and the higher value (Backman et al., 1992).

Before being imaged in the mid-IR spectral range, no optical or near-IR observations successfully resolved the disk around HR 4796 A, if we except for a slight asymmetry in the coronographic adaptive optics system images performed with the ESO adaptive optics system in K' band Mouillet et al. (1997a). Because of the low signal to noise ratio in these data, these results were not presented as a positive detection. Knowing the position angle of the disk, we reduced again these observations and we now marginally detect the disk. After a brief summary of thermal available data, we present in Sect. 2 the newly reduced scattered light images and compare them with more recent and better signal to noise ratio scattered ligth images (Schneider et al., 1999).

As data become actually more numerous, we wish to better constrain the grain properties in the HR 4796 A disk. Sophisticated grains models developed by Greenberg et al. (1972) succeeded both in reproducing interstellar observations (extinction, polarization (Li & Greenberg, 1997)) and in fitting the shape of the SED of the disk surounding Pictoris (Li & Greenberg, 1998; Pantin et al., 1997). In addition, Li & Greenberg (1998) proposed a link between ISM particles and circumstellar evolved environments assuming that dust grains in the Pictoris disk are of cometary origin (see also Backman & Paresce (1993); Lecavelier des Etangs et al. (1996)) and could be fluffy aggregates of primitive interstellar dust.

In this paper, we adopt a similar approach for the grains in the HR 4796 A disk. We describe in Sect. 3 the disk model assumptions and try to fit (Sects. 4.1 and 4.2) the Spectral Energy Distribution (hereafter SED) to derive some physical and chemical grain properties such as typical size, porosity, presence of ice and finally to estimate if the grains are more similar to interstellar dust grains or to comet-like grains.

In addition, we test whether the presence of a second population responsible for the 10 µm excesses is necessary or not. Given the constraints on the grain distribution derived from the SED fitting, we try to reproduce the thermal and scattered light resolved images (Sect. 4.3). We finally discuss Sect. 5 the implications on the disk dynamics.

© European Southern Observatory (ESO) 1999

Online publication: July 26, 1999
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