Hα photometry of low-mass stars in 47 Tucanae: chromospheric activity and exotica

Abstract

We have used archival Hubble Space Telescope observations obtained with the Advanced Camera for Surveys to study the Hα emission properties of main sequence stars in the globular cluster 47 Tucanae. Using a combination of multi-band observations in the F606W, F814W and F658N bands, we search for stars showing Hα excess emission. An accurate photometric measurement of their Hα equivalent width allows us to identify objects with large Hα emission, which we attribute to mass accretion rather than enhanced chromospheric activity. The spatial position of some of these stars is coincident with that of known X-ray sources and their location in the colour-magnitude diagram allows us to classify them as active binaries or cataclysmic variables (CVs). We show that this method, commonly adopted to study accreting discs in young stellar objects, can be successfully used to identify and characterise candidate CVs.

The Optical Counterpart to the Accreting Millisecond X-Ray Pulsar SAX J1748.9-2021 in the Globular Cluster NGC 6440*

“…Thus, these observations indicate an ongoing mass transfer activity from the companion toward the neutron star when the AMXP is in a quiescence state. This is in line with what is commonly observed in different classes of interactive binaries (e.g., low-mass X-ray binaries, cataclysmic variables) that are experiencing accretion phenomena also during quiescence (e.g., Ferraro et al 2000;Torres et al 2008;Beccari et al 2014;Torres et al 2014).…”

“…In the bottom panel of --( ) color-color diagram of the entire cluster of stars (small gray dots), with all the stars detected in the region around the X-ray source position highlighted as large black dots. This diagram has been found to be particularly powerful in pinpointing Ha emitters (e.g., Beccari et al 2014;Pallanca et al 2017). All the stars detected in the X-ray source region appear to be standard stars, with the only exception of one object (the red dot) that shows a quite anomalous Ha color  from the nominal position of the X-ray source, and it is the closest star to the X-ray position.…”

“…As can be seen from Figure 5, such an excess corresponds to an EW of the Ha emission of 19±1 Å. Such a value is too large to be attributed to chromospheric activity (Beccari et al 2014). It is instead a typical value for a system with a low mass accretion rate.…”

“…In doing this, we followed the method reported by De Marchi et al (2010) and already used in previous papers (Pallanca et al 2013;Beccari et al 2014, Pallanca et al 2017. Briefly, the excess in the dereddened Ha m m On the basis of this relation, different curves at increasing Ha EW can be computed, and they are plotted in Figure 5.…”

Improved mass and radius constraints for quiescent neutron stars in ω Cen and NGC 6397

“…Bassa et al (2009) (Haggard et al 2004), giving an estimated ∼110Å EW. Beccari et al (2014) use Hα, V and I photometry to calculate Hα EWs of 28Å and 50Å for the quiescent NS LMXBs W125 and W58/X5 in the globular cluster 47 Tuc. Finally, three systems (PSR J1023+0038, M28I=IGR J18245-2452, and XSS J12270-4859) swing between radio pulsations and active accretion (Archibald et al 2009;Papitto et al 2013;Bassa et al 2014).…”