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Publ. Astron. Soc. Aust., 2000, 17, 241­243

The Variable Ca II Absorption in Pictoris during 1998
S. I. Barnes, William Tobin and K. R. Pollard
Mount John University Observatory & Department of Physics and Astronomy, University of Canterbury, Private Bag 4800, Christchurch 8020, New Zealand s.barnes, w.tobin, k.pollard@phys.canterbury.ac.nz Received 1999 August 3, accepted 2000 February 23 Abstract: Variable absorption features were observed in the visible and ultraviolet spectrum of Pictoris soon after this star gained attention in the early 1980s due to its large IRAS infrared excess and the discovery, from optical imaging, of an edge-on dust disk. The absorption has been attributed to the evaporation of infalling planetesimals or comet-like bodies (the falling evaporating bodies, or FEB, hypothesis). With a view to confronting this hypothesis with fuller observations, we monitored the Ca II H and K lines in Pictoris simultaneously during 1998, obtaining sequences of spectra on 50 nights. Variable absorption was usually present. The different oscillator strengths of the H and K lines permit the determination of covering factors, but detailed modelling is required to test whether all features can be explained by the FEB hypothesis. The blend of Ca II H with Balmer H means that the H and K photospheric profiles are different, and that the variable absorption features do not evolve in parallel. The behaviour of the variable absorption on November 27 is evocative of a body passing in front of the stellar disk in a prograde equatorial orbit. Keywords: circumstellar matter--line: profiles--stars: individual ( Pictoris)

1 Introduction Attention was drawn to the bright, southern A5 V star Pictoris by its large IRAS infrared excess. Imaging soon revealed that Pictoris is surrounded by an edge-on dust disk extending over 1000 AU from the star. Spectroscopy revealed the presence of circumstellar absorption features, some of which are variable, with velocities up to a few hundred km s ­1 in both lowionisation species such as Ca II and high-ionisation species like Al III and C IV. It is believed the system may be a transitional object between a young star surrounded by a protoplanetary disk and more evolved systems in which giant planets (at least) have formed. Recent reviews include those by Artymowicz (1997) and VidalMadjar, Lecavelier des Etangs & Ferlet (1998). 1.1 The FEB Hypothesis The variable absorption features have been ascribed to material evaporating from planetesimals or comet-like bodies on star-grazing orbits. This `falling evaporating bodies' (FEB) hypothesis has principally been developed by workers in Grenoble (e.g. Beust et al. 1998 and references therein). The FEB model explains several observed phenomena. For example, shocks can produce the Al III and C IV ions not expected from photoionisation by an A5 V star, and lower-velocity absorption features are longer-lived than higher-velocity ones because the FEB crosses the line of sight further from the star. However, puzzles remain, such as the extended lifetimes of lower-velocity features (possibly attributable to an infalling stream of FEBs) and the comparative rarity of blue-shifted features, which may be indicative of the presence of planetary resonances perturbing FEBs from circular into star-grazing orbits at
© Astronomical Society of Australia 2000

selected azimuths. The behaviour of the variable absorption requires better observational characterisation. 1.2 The Ca II H and K Absorption The Ca II H and K absorption lines have different oscillator strengths (that of K is twice that of H) and this makes it possible to determine the optical depth of the obscuring ionic cloud, K (= 2H), and the fraction of the stellar photosphere that it covers. Under the crude model of a uniform cloud and photosphere, these parameters are obtained simply by solving the equations for the line depths, pK = (1 - e-K) and pH = (1 - e-K/2), whose observed values should fall within the limits pH = p K (optically thin) and p H = p K (optically thick). Simple analyses of this sort have shown that the covering factors, , are smaller for higher-velocity features, as expected if radiation pressure close to the star inhibits expansion of the cloud of Ca+ ions. In reality, the physical situation is more complicated. The photosphere is limb-darkened, so an ionic cloud of a given size blocks proportionately more light when its projected location is closer to the centre of the stellar disk. More importantly, the photosphere is rotating fast ( V sin i 140 km s ­1 ). Since the FEB absorption is detected against the spectral background of the photospheric Ca II lines, whose profiles are a strong function of wavelength, the intensity of the photospheric light at any wavelength and at any point on the projected stellar disk is altered because of rotational Doppler shifts. A concrete example will make this clearer. If planetary perturbations cause planetesimals to become stargrazing, the FEB orbits are likely to be equatorial. The photospheric K profile increases in intensity away from its central wavelength. An FEB located in front of either the blue- or the red-shifted equatorial limb will therefore
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experience enhanced flux at the central wavelength of the absorption compared to what it would experience if the star was not rotating. Because of blending, however, the profile of the photospheric H line in Pictoris is not symmetric about the wavelength corresponding to the atomic transition. The intensity initially decreases on the long-wavelength side, where it is blended with Balmer H. An equatorial FEB therefore experiences enhanced intensity if it is located in front of the approaching limb, and reduced intensity if it is located in front of the receding limb. As Beust & Lissauer (1994) and Hubeny & Heap (1996) realised, the joint evolution of the differently-behaving H and K absorption features should permit tracking of the passage of FEBs across the stellar disk, and thus provide further evidence for the FEB hypothesis. Figure 1 shows predictions from Beust & Lissauer for the prograde, equatorial passage an FEB of a certain size and orbit; it is even possible for the H line to be stronger than the K line, despite its weaker oscillator strength. 1.3 Previous Observations Observations of the Ca II lines in Pictoris are thus of great interest, and intermittent Иchelle spectroscopy of this star has been under way from the Mt John University Observatory (MJUO) since 1992, sometimes as part of multi-observatory campaigns. Results from 1992 December have been published by Lagrange et al. (1996); data from 1994­96 (intensive during 1995) have been presented by Petterson & Tobin (1999). For these observations, the detector was too small to capture both the H and K lines, which were therefore observed sequentially, but difficulties arise in their joint interpretation because the absorption can evolve over the same timescale as that required to obtain a spectrum. In

order to permit simultaneous observations of the H and K lines, a spectrograph focal reducer was built (Tobin et al. 1998) and a larger CCD detector was acquired. A two-year program of intensive monitoring began in 1997 (Persson 1998). 2 The Ca II Absorption in 1998 2.1 New Observations The McLellan One-metre Telescope at MJUO was allocated to this research over 45 full and 39 shared nights during 1998. We were able to acquire 407 sets of spectra during 178 hours of observation on 50 nights between 1998 January 20 and December 2, the southern declination of this star allowing year-round observation. Reduced spectra were normalised by absorption-free photospheric profiles to isolate the circumstellar features, which, following earlier workers, we were able to well-fit with multiple Gaussian profiles. (We fitted a common velocity and width but separate depths, and hence equivalent widths, to corresponding H and K features.) For fuller details, see Barnes (1999). 2.2 Results A sample spectrum pair from 1998 November 27 is shown in Figure 2. The deep, sharp central feature is

Figure 1--The predictions of Beust & Lissauer (1994) for the prograde, equatorial passage of an FEB across the disk of Pictoris (their Figure 7). Heavy curve: evolution of Ca II K. Light curve: evolution of Ca II H. The closed and open circles are our K and H observations of the ~120 km s­1 feature on 1998 November 27, arbitrarily offset in abscissa and arbitrarily scaled in ordinate to emphasise the similarity in form, as explained in Section 2.2.

Figure 2--Sample reduced and normalised spectra of Pictoris obtained on 1998 November 27. The reference profiles used for the normalisation are also shown. The normalised spectra have been fitted by four Gaussians, whose central velocities are indicated by vertical bars. One corresponds to the deep, `stable' central feature at the stellar radial velocity of ~20 km s­1. One weak blueshifted and two stronger redshifted features have also been fitted. Had the multiple Gaussian fits been plotted, discrepancies between them and the normalised profiles would have been too small to see at the scale of this diagram.


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acquired a wealth of similar observational material. Some aspects of the entire dataset are the presence of absorption activity on almost every night (in our 407 pairs of spectra we detect some 940 variable features, i.e. excluding the central absorption), the clear detection of blueshifted absorption features, and the emergence of correlations between absorption velocities and absorption widths and depths. In addition, comparison of H and K line depths confirm that higher-velocity features obscure smaller fractions of the stellar disk. We feel we do not understand the scattered-light properties of our spectrograph sufficiently well to make any unequivocal statement concerning the stability or otherwise of the deep, central absorption feature. Most interestingly, the H and K equivalent widths of the ~120 km s­1 feature on 1998 November 27 evolve jointly in a way evocative of the predictions of Beust & Lissauer (1994) for an FEB in a prograde equatorial orbit. Our observations are compared with one of their models in Figure 1. It must be emphasised that the strength and timescale of the predicted variation depend on assumed properties of the FEB; all that we are claiming is that there is a similarity of form between model and observations. Although daylight occurred at the moment when the H line strength might have been expected to exceed that of K, which would have provided a clear confirmation, this may have been the first detection of an FEB transit. Our dataset now merits detailed modelling within the context of the FEB hypothesis. Acknowledgments
Figure 3--Parameters of the multiple Gaussian fits to the variable absorption features observed on 1998 November 27. The abscissa indicates the time at which different spectra were acquired. The horizontal dashed line represents the radial velocity of the star at which the deep, stable absorption feature (not plotted) occurs. Variable absorption features were found to occur at three distinct radial velocities during this night, each of which has been plotted with a different symbol (circle, square, triangle). A given feature is then plotted with the same symbol in all panels of the plot. The velocity and width of each fitted feature were constrained to be the same for the H and K lines, but depths and equivalent widths were fitted independently. These are plotted by open and filled symbols for the H and K lines respectively, and are slightly laterally offset from each other to reduce visual confusion.

This paper summarises a presentation made at the Second Joint Meeting of the Royal Astronomical Society of New Zealand and the Astronomical Society of Australia. We thank New Zealand's Marsden Fund for curiosity-driven research for its financial support. References
Artymowicz, P. 1997, Ann. Rev. Earth Planet. Sci., 25, 175 Barnes, S. I. 1999, MSc thesis, University of Canterbury Beust, H., & Lissauer, J. J. 1994, A&A, 282, 804 Beust, H., Lagrange, A.-M., Crawford, I. A., Goudard, C., Spyromilio, J., & Vidal-Madjar, A. 1998, A&A, 338, 1015 Hubeny, L., & Heap, S. R. 1996, ApJ, 470, 1144 Lagrange, A.-M., et al. 1996, A&A, 310, 547 Lagrange, A.-M., et al. 1998, A&A, 330, 1091 Persson, S. I. 1998, MSc thesis, University of Canterbury Petterson, O. K. L., & Tobin, W. 1999, MNRAS, 304, 733 Tobin, W., et al. 1998, Southern Stars, 37, 197 Vidal-Madjar, A., Lecavelier des Etangs, A., & Ferlet, R. 1998 Planet. Space Sci., 46, 629

always present and may be due to deceleration of Ca+ ions by a ring of H I surrounding Pictoris (Lagrange et al. 1998), in which case it may exhibit variability on a timescale of years. Figure 3 shows the parameters of the individual variable features for the whole of that night. We have