Even excluding the possibility of contamination from background sources, the de- tection of an IR excess does not unambiguously correspond to the presence of a debris disc. Low-mass, close, unresolved companions (brown dwarfs, BD) to the white dwarfs may have an infrared signature similar to that of a debris disc and only far-infrared photometry and accurate SED modelling can discriminate between the two (e.g. Steele et al. 2011; Girven et al. 2011; Farihi et al. 2012). White dwarfs with close BD companions are per se very interesting systems: they are key to probe the parameter space spanned by BDs, and only ∼ 6 have confirmed to date (Burleigh et al., 2006; Casewell et al., 2012; Steele et al., 2013; Littlefair et al., 2014). However, the targets of our search are debris discs and we aimed, where pos- sible, to avoid WD+BD binaries. According to current theories all metal polluted white dwarfs have or must have had a circumstellar debris reservoir of pollutants. Therefore a debris discs is the most likely origin of any genuine (not caused by spu- rious photometric contamination) IR excess observed from a metal polluted white dwarf. In conclusion, a sample of spectroscopically confirmed metal polluted white dwarfs is the best starting point in a search for debris discs. Previous studies have extensively searched for possible IR excesses from the spectroscopically confirmed white dwarfs in SDSS DR7 using UKIDSS (Steele et al., 2011; Girven et al., 2011)
Table 8.9: Results of the cross matching of the newly identified SDSS DR9/10 metal polluted white dwarfs with UKIDSS and WISE.
new metal polluted white dwarfs in SDSS DR9/10 62 with at least one detection in UKIDSSY JHK 44 or in WISEw1, w2
infrared excess candidates 4
of which with both UKIDSS and WISE photometry 2
and WISE (Debes et al., 2011b). Therefore a similar search using our sample of newly identified metal polluted white dwarfs from SDSS DR9/10 represent a natu- ral progression in this type of work. We began by retrieving all available UKIDSS DR10 and WISE IR photomety for our 62 metal polluted white dwarfs. In their search for IR excess, Girven et al. (2011) compared the result of SDSS to UKIDSS coordinate cross matching as a function of matching radius. They determined that a radius of 2.5 arcseconds excluded nearly all spurious matches while missing only a handful of the highest proper motion objects. However the large proper motions of the white dwarfs and the now longer (few years since DR7) time interval between the two surveys can lead to some significant shifts in the coordinates of the objects. In our search we are striving for completeness and so decided to conduct our cross matching between SDSS ad UKIDSS using a matching radius of five arcseconds. Naturally this increased radius leads to higher chance of spurious matching so a great effort was later applied in manually eliminating those. Following a similar argument and taking into account the lower resolution of WISE (6”), we adopted a cross-matching radius of 10 arcseconds between SDSS and WISE.
Our cross-match results in a sample of 44 metal polluted white dwarfs with at least one detection in UKIDSSY JHK or WISE w1, w2 (Table 8.9).
For all these objects we constructed SED using the available SDSS, UKIDSS and WISE photometry. In order to evaluate whether or not the IR detection cor- responds to an excess we fit a blackbody model to the SDSS r and i magnitudes. In practice the SED of a white dwarfs is not a perfect blackbody, but the Rayleigh- Jeans tail of a blackbody curve can describe a white dwarf spectrum over regions with no significant absorption features; i.e. the equivalent wavelengths of r and i bands and the near and mid-infrared. We then proceeded to inspect the SEDs of our 44 objects with IR matches. We also closely inspected the available SDSS, UKIDSS and WISE images and discarded all objects for which the IR emission is likely caused by a contaminating source (e.g. Fig. 8.10).
Figure 8.10: SED of the metal polluted white dwarf SDSS J1204+0759 showing a spurious infrared excess. SDSS ugriz , UKIDSS Y JHK and WISE w1, w2 photometric measure- ments are shown as blue, green ad yellow points respectively. The grey solid line in the top panel indicates a blackbody fit to the SDSS ri photometry. The dashed red lines in the middle panel indicate the 3σthreshold. SDSS, UKIDSS and WISE images are included in the bottom panels from left to right respectively. The red cross on the WISE image indicates the coordinates of the SDSS objects, while the green cross indicates the location of the WISE detection. The IR excess visible in thew1, w2 bands is most likely caused by a nearby source also faintly visible in the SDSS and UKIDSS image.
one IR measurement in theH, K, w1, w2 bands in excess by a minimum of 3σ with respect to the blackbody model (Fig. 8.11).
We conclude that four of our metal polluted white dwarfs are strong debris disc candidates (Table 8.10). As mentioned above such IR excess could be caused by a debris disc or a low mass companion, but the metal polluted nature of these white dwarf corroborates the debris disc origin. Further support for the disc scenario comes from the two candidates with both UKIDSS and WISE IR photometry. For these two objects the UKIDSS photometry is consistent with the flux from the isolated white dwarf and the IR excess becomes evident only in thew1, w2 bands (e.g. Fig. 8.11). Aside from debris discs, the only objects which could cause IR excess at such log wavelengths are very late-type (L or later) brown dwarfs. White
Figure 8.11: SEDs of SDSS J2309+0608 and SDSS J1516−0040 constructed from GALEX f uv, nuv, SDSS ugriz, UKIDSS Y JHK and WISEw1, w2 photometry. The dotted line represents the white dwarf model fitted to the GALEX and SDSS photometry, the dashed lines represent emission from optically thick, flat disc models, and the dotdashed lines represent the sum of the stellar and circumstellar model fluxes. The parameters of the disc models are given in the panel.
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Figure 8.12: SEDs of SDSS J1548+2135 and SDSS J1018+3726 constructed from GALEX f uv, nuv, SDSS ugriz and, WISE w1, w2 photometry. The dotted line represents the white dwarf model fitted to the GALEX and SDSS photometry, the dashed lines represent emission from an optically thick, flat disc model, and the dotdashed lines represent the sum of the stellar and circumstellar model fluxes. The parameters of the disc models are given in the panel.
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Table 8.10: Four newly identified metal polluted white dwarfs from SDSS DR9/10 with infrared excess.
name ra dec class g(mag) UKIDSS
SDSS J1018+3726 10 18 38.74 +37 26 57.5 DABZ 17.70±0.02 − − −− SDSS J1516−0040 15 16 42.96 −00 40 42.7 DBAZ 17.21±0.02 Y JHK SDSS J1548+2135 15 48 15.24 +21 35 28.5 DBAZ 18.05±0.02 − − −− SDSS J2309+0608 23 09 53.03 +06 08 20.0 DABZ 17.87±0.02 Y JHK
dwarfs in close binaries with such low mass companions are extremely rare (Steele et al., 2011; Girven et al., 2011; Farihi et al., 2012) and the presence of a debris disc is a more likely explanation for the IR excess. For our four debris disc candidates we included Galex f uv and nuv photometry in the SEDs and attempted to fit the IR excess using the geometrically thin, optically thick disc model from Jura (2003). The free parameters in the disc model are the inner and outer disc temperature (Tin, Tout) and the disc inclination (i). Fig. 8.11 illustrates the result of our disc
modelling and shows that the IR excess are consistent with the presence of flat, optically thick debris discs. However the best fits achieved are not fully satisfactory, implying that the data at hand could be not completely reliable or that background contamination may still be a factor. To ultimately discern the origin of the IR excess and correctly characterize the possible disc, it will be necessary to acquire more accurate IR photometry (e.g. Spitzer IRAC).