Recursos y actividades seleccionados para la enseñanza de la EA2

Since the original observations of G 29−38, more than forty white dwarfs with cir- cumstellar debris discs are now known from infra-red excesses (von Hippel et al., 2007; Jura et al., 2007b; Farihi et al., 2008; Brinkworth et al., 2009; Farihi et al., 2010b; Melis et al., 2010; Debes et al., 2011; Kilic et al., 2012; Farihi et al., 2012; Brinkworth et al., 2012; Bergfors et al., 2014; Rocchetto et al., 2015; Dennihy et al., 2016; Barber et al., 2016), and in all cases these white dwarfs are found to be metal polluted.7 _{Despite the ever increasing number, it actually took 18 years before the} second disc hosting white dwarf was discovered at GD 362 (Kilic et al., 2005; Becklin et al., 2005). The atmosphere of this star is extremely metal-rich, and while first identified as a DAZ (Gianninas et al., 2004), the star was later shown to have a he- lium dominated atmosphere (Zuckerman et al., 2007), with hydrogen only present as a trace element. The total metal abundance remains the highest detected for any white dwarf, demonstrated by the detection of trace elements Sc, V, Co, Cu, and Sr. In particular, the latter two of these have yet to be detected in the atmosphere of any other white dwarf. These large metal abundances and the bright infrared excess indicate that GD 362 is still accreting metals at a high rate.

It has been customary since the work of Jura (2003) to fit the white dwarf in- 7

PG 0010+280, possesses an infrared excess and so far no metals have been detected in its photosphere (Xu et al., 2015). However, in this case the infrared colours are indicative of an irradiated substellar companion.

0.5 1.0 2.0 5.0 10.0 20.0 Wavelength [µm] 1 10 Fl ux [m Jy ]

Figure 1.7: The Spitzer observations of Reach et al. (2005, 2009) demonstrate un- ambiguous 10µm silicate emission at G 29₋38. Photometry are from SDSS, APASS, 2MASS, WISE, and Spitzer. A 12 000 K DA model is plotted against the optical photometry to emphasise the flux excess beyond 1.5µm.

frared photometry with a model based on concentric rings each emitting a blackbody spectrum. Of course this is only an approximation, although certainly a useful one for determining disc parameters, however the underlying disc spectrum is more com- plicated. Spectroscopic observations of G 29₋38 with Spitzer8 _{(Reach et al., 2005,} 2009) revealed strong 10µm silicate emission (Fig. 1.7) which has been attributed to a mixture of enstatite and forsterite dust grains. In addition to G 29−38, only a few objects, including GD 362, have proved to be sufficiently bright enough for spec- troscopic follow-up with Spitzer (Jura et al., 2007a, 2009). With JWST9 _available in the near future, it will be possible to detect molecular emission at additional ob- jects, and includes the prospect of carrying out detailed mineralogy in the brightest systems like G29₋38.

While the metallic discs of these white dwarfs are usually detected via the infrared emission of dust grains, some are also visible through material in the gas phase. The first gaseous disc was identified at SDSS J122859.93+104032.9 by

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Spitzer is an infra-red space telescope with imaging and spectroscopic instrumentation covering 3.6–160µm. Its primary mirror has a diameter of 0.85 m.

9_{JWST is an upcoming space telescope expected to launch in 2019. It is chiefly designed for} infra-red observations with an array of imaging and spectroscopic instruments covering 0.7–27µm in wavelength. Among space-based observatories, its 6.5 m diameter primary mirror will provide an unprecedented collecting area and spatial resolution.

8500 8550 8600 8650 8700 Wavlength [ ˚A] 0 1 2 3 4 5 6 Fl ux [ × 10 − 16 er gs − 1 cm − 2˚ A − 1 ]

Figure 1.8: Gaseous emission observed at SDSS J122859.93+104032.9 from the infrared Caii triplet. The double peaked structure is indicative of a disc, with

material moving towards and away from the observer on each side of the disc. The laboratory wavelengths are marked by the red dotted lines.

G¨ansicke et al. (2006). The SDSS10 _{discovery spectrum is shown in Fig. 1.8, un-} ambiguously exhibiting double-peaked emission profiles from the Caiitriplet. Such

emission profiles are commonly encountered for astrophysical discs, including cata- clysmic variables and active galactic nuclei. Essentially the double-peaked structure results from the distribution of Doppler-shifts for gas moving towards and away from the observer on each side of the disc. Furthermore, SDSS J1228+1040 is found to exhibit an infrared excess (Brinkworth et al., 2009) as well as an atmosphere rich in metals (G¨ansicke et al., 2012).

In principle, all white dwarf discs ought to contain a gaseous component, with the gas-to-dust fraction reaching unity close to the white dwarf. In practice, gas discs are rarely detected except in the case of very high accretion rates. Since this time, the number of confirmed detections of gaseous discs totals seven (G¨ansicke et al., 2007, 2008; G¨ansicke, 2011; Farihi et al., 2012; Dufour et al., 2012; Melis et al., 2012; Wilson et al., 2014), with a candidate gas disc reported by Guo et al. (2015). An exciting aspect to the gaseous components to these discs is their recently discovered variability. Wilson et al. (2014) were the first to observe a dynami- cally active disc, showing that SDSS J161717.04+162022.4 displayed only weak gas

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emission (if at all) in its 2006 SDSS spectrum, but peaked later in 2008 SDSS obser- vations, and subsequently decayed in strength over the next six years. Wilson et al. (2014) speculated that this could indicate impact of an additional exoplanetesimal onto an already existent debris disc, producing new gas. In their monitoring of SDSS J1228+1040, Manser et al. (2016a) were able to exploit twelve years observa- tions to show slow precession of the disc. From Fig. 1.8, it is clear that in all three components, the red peaks are stronger suggesting an asymmetric disc. The data presented by Manser et al. (2016a) showed this asymmetry eventually equalising, before transitioning to a structure dominated by the blue peaks, which they were able to visualise (in velocity space) via Doppler tomography. Manser et al. (2016b) also identified similar gaseous variability at SDSS J104341.53+085558.2 (originally identified by G¨ansicke et al. 2007), which they argued could be explained through general relativistic precession of the disc. In summary, the gas components to these discs often vary on observable timescales, and thus offer a window into the dynamic nature of exoplanetesimal accretion onto white dwarfs.