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A Refined Search for Pulsations in White Dwarf Companions To MNRAS 479, 1267–1272 (2018) doi:10.1093/mnras/sty1546 Advance Access publication 2018 June 12 A refined search for pulsations in white dwarf companions to millisecond pulsars Mukremin Kilic,1,2† J. J. Hermes,3‡ A. H. Corsico,´ 4 Alekzander Kosakowski,1 Warren R. Brown,5 John Antoniadis,6 Leila M. Calcaferro,4 A. Gianninas,1 Leandro G. Althaus4 and M. J. Green7 1Department of Physics and Astronomy, University of Oklahoma, 440 W. Brooks St., Norman, OK 73019, USA 2Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK 3Department of Physics and Astronomy, University of North Carolina, Chapel Hill NC 27599, USA 4Facultad de Ciencias Astronomicas´ y Geof´ısicas (UNLP), La Plata, Argentina 5Smithsonian Astrophysical Observatory, 60 Garden St, Cambridge MA 02138, USA 6Dunlap Institute for Astronomy & Astrophysics, University of Toronto, 50 St. George Street, Toronto M5S 3H4, Canada 7Astronomy and Astrophysics Group, Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK Accepted 2018 June 7. Received 2018 May 27; in original form 2018 April 11 ABSTRACT We present optical high-speed photometry of three millisecond pulsars with low-mass (<0.3 M) white dwarf companions, bringing the total number of such systems with follow-up time-series photometry to five. We confirm the detection of pulsations in one system, the white dwarf companion to PSR J1738+0333, and show that the pulsation frequencies and amplitudes are variable over many months. A full asteroseismic analysis for this star is underconstrained, but the mode periods we observe are consistent with expectations for an M = 0.16 − 0.19 M white dwarf, as suggested from spectroscopy. We also present the empirical boundaries of the instability strip for low-mass white dwarfs based on the full sample of white dwarfs, and discuss the distinction between pulsating low-mass white dwarfs and subdwarf A/F stars. Key words: stars: oscillations – stars: variables: general – white dwarfs – pulsars: individual: PSR J1738+0333, PSR J1911– 5958A, PSR J2234+0611. Pulsation frequencies and amplitudes depend on the internal 1 INTRODUCTION structure of the WD, and thus its evolutionary age (Winget & Kepler White dwarfs (WDs) are the most common type of companion 2008). We present an observational study of MSPs with low-mass detected around millisecond pulsars (MSPs, Lorimer 1998), and WD companions with effective temperatures in the regime where thus play a crucial role in establishing the mass and equation of pulsations may be excited by the onset of a surface convection zone. state of neutron stars. Pulsating WDs can provide a second clock Hydrogen-dominated WDs with canonical carbon–oxygen cores in these systems, constraining pulsar spin-down and magnetic-field (M ∼ 0.6 M) exhibit gravity-mode pulsations between effective decay (e.g. Kulkarni 1986). The characteristic MSP spin-down ages temperatures of ≈10,500 and 13,000K. Detailed asteroseismolog- do not necessarily represent their true ages (Tauris 2012). Hence, ical analyses of these stars provide unique constraints on the core cooling ages of their WD companions provide the only reliable age carbon–oxygen ratio (Giammichele et al. 2017), as well as the thick- measurements in these systems (Istrate et al. 2014). ness of the surface hydrogen and helium layers, which regulate the cooling of the star. Following the discovery of several extremely low-mass (ELM, Based on observations obtained at the Gemini Observatory, which is oper- M 0.3M) WDs in the field (Kilic et al. 2010) and as companions ated by the Association of Universities for Research in Astronomy, Inc., to MSPs, Steinfadt, Bildsten & Arras (2010) predicted that ELM under a cooperative agreement with the NSF on behalf of the Gemini WDs should also pulsate in a similar temperature range (see also partnership: the National Science Foundation (United States), the National Corsico´ et al. 2012; Van Grootel et al. 2013). Their initial search Research Council (Canada), CONICYT (Chile), the Australian Research did not find any pulsators (Steinfadt et al. 2012), but later searches Council (Australia), Ministerio´ da Ciencia,ˆ Tecnologia e Inovac¸ao˜ (Brazil) by Hermes et al. (2012, 2013a,b) found oscillations in several ELM and Ministerio de Ciencia, Tecnolog´ıa e Innovacion´ Productiva (Argentina). † E-mail: [email protected] WDs with pulsation periods ranging from about 20 min to more ‡ Hubble Fellow than an hour. C 2018 The Author(s) Downloaded fromPublished https://academic.oup.com/mnras/article-abstract/479/1/1267/5036529 by Oxford University Press on behalf of the Royal Astronomical Society by Mount Royal University user on 18 July 2018 1268 M. Kilic et al. However, the instability strip for low-mass WDs is complicated by an overlapping population of subdwarf A-type (sdA) stars, stars that have spectroscopic surface gravities comparable to WDs (Ke- pler et al. 2015, 2016) but which may be mostly metal-poor main- sequence stars (Brown, Kilic & Gianninas 2017; Pelisoli, Kepler &Koester2018). For example, Bell et al. (2017) found a 4.3 h dominant pulsation mode in J1355+1956, a star which has Teff = 8050 ± 120K and logg = 6.10 ± 0.06 based on pure hydrogen atmosphere models. However, this period significantly exceeds the theoretical limit for pulsations in ELM WDs (Corsico´ & Althaus 2016;Belletal.2017). J1355+1956 is better understood as a SX Phe or δ Scuti variable (Brown et al. 2017). Given the problems with distinguishing bona-fide pulsating low- mass WDs and pulsating sdA-type stars based on optical spec- troscopy, low-mass WD companions to MSPs provide a more re- liable opportunity to constrain the boundaries of the ZZ Ceti in- stability strip for these stars. Because neutron stars are spun up to millisecond periods through accretion in a compact binary system, MSP companions are expected to be low-mass white dwarfs, and not sdA stars. There are currently five companions to MSPs with spectro- scopic temperatures and surface gravities within 1500 K of the Figure 1. The Gemini South light curve (top panel) and its Fourier trans- extended, low-mass ZZ Ceti instability strip. Kilic et al. (2015) form (bottom panel) for the optical companion to PSR J1911−5958A. The discovered pulsations in the WD companion to PSR J1738+0333. dashed line marks the 3A significance level (17.4 mmag), as described in Here, we present the results from Gemini follow-up photome- the text. try of two additional MSP companions, and additional observa- tions of J1738+0333 from three ground-based facilities. We de- standard IRAF Gemini GMOS routines and the daily bias and sky scribe the results for PSR J1911−5958A, PSR J2234+0611, and flats to reduce and calibrate the data, and corrected our times to the PSR J1738+0333 in Sections 2–4, respectively. We discuss the con- solar-system barycentre using the tools of Eastman, Siverd & Gaudi straints on the instability strip for low-mass WDs in Section 5 and (2010). PSR J1911−5958A has a relatively bright source 3 away. conclude. To minimize contamination from this nearby source, we performed point spread function photometry, and used 15 non-variable refer- ence stars to calibrate the differential photometry. Given the colour 2 PSR J1911–5958A differences between the WD and the relatively red reference stars, PSR J1911−5958A is a 3.3 ms pulsar with a B = 22.2 mag WD we fit a second-degree polynomial to the light curve to remove the companion in a 0.87 d orbit (D’Amico et al. 2001). Bassa et al. long-term trend due to differential extinction. (2006) used optical spectroscopy of the companion to constrain Fig. 1 shows the Gemini light curve and its Fourier transform − its parameters, Teff = 10090 ± 150 K and logg = 6.44 ± 0.20. for the ELM WD companion to PSR J1911 5958A. The median Given the recent improvements in the Stark broadening calculations amplitude A in the Fourier transform is 5.8 mmag, but there are no of the hydrogen lines in dense plasmas by Tremblay & Bergeron frequency peaks above 12.5 mmag. Hence, there is no evidence of (2009), we refit the same spectrum (kindly provided to us by C. pulsations in PSR J1911−5958A. All but one of the known pulsat- Bassa) with 1D pure hydrogen model atmospheres that include ing ELM WDs in short-period binary systems show pulsations with these improvements (Gianninas et al. 2014). The best-fit model has amplitude larger than this limit. The exception is SDSS J1112+1117, Teff = 10270 ± 140 K and logg = 6.72 ± 0.05, and the Tremblay which displays pulsations with a maximum amplitude of 8.1 mmag et al. (2015) 3D model corrections change these parameters to Teff (Hermes et al. 2013a). Such low-level variations would not be de- = 9980 ± 140 K and logg = 6.65 ± 0.05. tectable for our data on PSR J1911−5958A. Bassa et al. (2006)used23× 600 s and 30 × 30 s B−band acquisition images to check for optical variability, but found no 3 PSR J2234+0611 significant variations. They found an rms scatter of 0.02-0.05 mag for the WD, but this was comparable to the scatter seen in reference PSR J2234+0611 is a 3.6-ms pulsar with a g = 22.2 mag companion stars of similar brightness. Steinfadt et al. (2012) obtained Hubble in an eccentric, e = 0.13, 32.0 d orbit (Deneva et al. 2013). Anto- Space Telescope observations of PSR J1911−5958A over four or- niadis et al. (2016) performed synthetic photometry of this target by bits, but with gaps in the data due to occultations by Earth. They convolving their 26 × 1420 s follow-up spectra with the g −band ruled out pulsations with amplitudes larger than 16 mmag.
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