The Globular Cluster M 80
3.4 FUV Variable Sources in the Core of M 80
3.4.3 Previously Known Variable Sources
In addition to the sources described above, which were identified in the variability search of the FUV data described in Section 3.3.1, the FUV counterparts to two well-known variable sources, the classical nova T Sco and the dwarf nova DN 1 were also investigated.
3.4.3.1 T Scorpii
Source number 2129 in the FUV catalogue was identified (see Section 3.1.5.1) as the counterpart to the classical nova T Scorpii (T Sco). T Sco is one of only two novae known to have occurred along the line of sight to a GC. Source 2129 was not strongly variable in the FUV observations. In fact, after subtracting trends due to PSF changes over HST’s orbital period that were present in the light curves of the brightest sources (see Figure 3.13), T Sco exhibits only very little evidence for real variability in the FUV data, with amplitude< 0.1 magnitudes. The (detrended) light curve is shown in the top panel of Figure 3.20. Bruch (1992) suggests typical nova flickering amplitudes of a few tenths of a magnitude, and one would expect more variation in the bluer wavebands, but this was not detected. The Lomb-Scargle
98 Chapter 3. The Globular Cluster M 80
Figure 3.20: Light curves for the previously known variable sources, T Sco (top panel) and DN 1 (bottom), showing the variation from the mean magnitude (∆FUV= FUV − FUVMean). All light curves are detrended to remove trends due to the period of HST’s orbit which are visible in the light curves of the brightest sources.
3.4 FUV Variable Sources in the Core of M 80 99
0 50 100
0 1 2 3 4
Figure 3.21: Lomb-Scargle periodogram of T Sco, after the light curve has been detrended to remove small changes in magnitude due to PSF changes over the course of HST’s orbit. No significant peaks are found (simulations show that the 50% confidence level is at LS power= 4.36).
0 50 100
0.5 1 1.5 2 2.5
Figure 3.22: Lomb-Scargle periodogram of DN 1. No significant peaks are found (simulations show that the 50% confidence level is at LS power= 4.26, far higher than any peaks shown here), indicating that the source does not exhibit detectable periodic variability in our FUV data.
100 Chapter 3. The Globular Cluster M 80
power spectrum for the detrended light curve is shown in Figure 3.21 and demon-strates that no strong indication of periodic variability is apparent in the light curve.
Simulations show that the highest peak in T Sco’s power spectrum is significant at a level of only ≈ 50% (50% of simulations created using the method described in Section 3.4.2.1 exhibited peaks higher than LS power= 4.36). I note, however, that T Sco, as one of the brightest sources in the catalogue, was used in the determina-tion of the trend based on HST’s orbit, which was later subtracted from all light curves. Some intrinsic variability might, therefore, have been removed during this process (see Figure 3.13).2
As described in Section 3.1.5.1, T Sco was among the brightest objects in the FUV catalogue (FUV= 15.44 ± 0.01 mag). It was also the bluest object in the cat-alogue; in fact, it was found to be unphysically blue (FUV − NUV = −3.81 mag, while an infinite temperature blackbody would have FUV − NUV = −1.8 mag), so must have decreased in brightness in the month between the FUV and NUV obser-vations. This implies that T Sco was in a high state during the FUV obserobser-vations. I suggest, therefore, that the flickering normally observed in the light curves of clas-sical novae might be suppressed because the source was caught in a high state. Sup-pressed flickering in high states has been observed in DNe (Warner, 2003). Bruch (1992) also shows that lower amplitude flickering is seen in novae with lower in-clinations, so the lack of observed flickering seen here suggests that the system has a very low inclination. The faint FUV magnitudes when at its faintest makes searching for flickering away from the outburst difficult; if it were achievable, mea-surements of flickering elsewhere in the light curve would allow for more certain conclusions to be drawn. The implied FUV outburst is interesting, as one cannot rule out the possibility that T Sco is, in fact, a recurrent nova. However, this is prob-ably a far-fetched explanation for the inferred FUV brightening; a DN eruption, for example, is a much more likely scenario.
3.4.3.2 DN 1
Source number 1387 in the catalogue is the relatively faint FUV counterpart to X-ray source CX 07 identified by Heinke et al. (2003). This source is also DN 1, one of the DNe found by Shara & Drissen (1995). As shown in the bottom panel of Fig-ure 3.20 and in FigFig-ure 3.22, this source did not exhibit detectable variability in the
2Note that although T Sco is expected to show flickering, the magnitude variation shown in Fig-ure 3.13 demonstrates that any such variability is very small. Intrinsic T Sco variability included in the ‘detrending’ step will not, therefore, have a detrimental effect on the conclusions drawn regarding variability in other sources.
3.5 Summary 101
FUV data. The LS-power indicated in Figure 3.22 is far smaller than even the 50%
confidence level (which has LS power= 4.26). However, DN 1 is a relatively faint FUV source (FUV= 22.578), so instrumental errors are large (up to 0.9 mag) and limit the ability to draw any conclusions about the presence or absence of flickering or orbital variations.
3.5 Summary
I used 32 individual FUV images from the UV survey of the core region of M 80 (Dieball et al., 2010) to search for variable sources in the FUV catalogue. Three sources exhibit strong evidence for variability.
TDK 1 (source 2817) is an RR Lyrae in the core of the cluster. The FUV light curve shows that it was observed from around 40 minutes before to 4.5 hours after maximum brightness, and further investigation using archival WFPC2 optical data showed that it is clearly variable in all wavebands. Its SED is reasonably well described by a star of temperature Te f f ≈ 6700 K and radius R ≈ 4.2 R⊙, consistent with expected parameters for an RR Lyrae star. More specifically, I show that TDK 1 is a type ab RR Lyrae, based on the asymmetry in the FUV light curve. This is the eighth RR ab found in M 80, and brings the fraction of c type RR Lyrae in the cluster to 50%, exactly in line with the expected fraction for an Oosterhoff II (‘metal-poor’) cluster (see Section 2.2).
This is only the third cluster in which an RR Lyrae star has been identified based on UV observations (others were found in NGC 1851 by Downes et al. (2004) and M 15 by Dieball et al. (2007)). UV surveys can be useful tools in identify-ing RR Lyraes and similar objects, particularly in the cores of (dense) GCs where optical surveys are seriously hampered by crowding.
TDK 2 (source 2238) is likely an SX Phoenicis star with a period of 55.42 ± 0.66 minutes and amplitude of ≈ 1 mag. TDK 3 (source 2324) might be another RR Lyrae or a Cepheid.
Finally, I discussed two well known variable sources, T Sco and DN 1, the FUV counterparts of which were recovered in the FUV survey. T Sco exhibited surpris-ingly little flickering in the FUV data, possibly because it was caught in a high state compared with the NUV observations a month later. DN 1 is a very faint UV source, so photometric errors dominate over any possible intrinsic flickering or other varia-tions.
After this paper was completed, I found that TDK 1 and TDK 3 are included in
102 Chapter 3. The Globular Cluster M 80
Kopacki’s variability survey of M 80 (Kopacki, private communication). Kopacki agrees with my classification of these two sources as RR Lyrae stars. A preliminary summary of his results, including periods but not including coordinates or finder charts for the sources, is given in Kopacki (2009).
Now my own suspicion is that the Universe is not only queerer that we suppose, but queerer than we can suppose.
J. B. S. HALDANE (1892 – 1964)