Helium White Dwarfs and BY Draconis Binaries in the Globular Cluster NGC 6397

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Citation Taylor, J. M., J. E. Grindlay, P. D. Edmonds, and A. M. Cool. 2001. “Helium White Dwarfs and BY Draconis Binaries in the Globular Cluster NGC 6397.” The Astrophysical Journal 553 (2): L169–72. https://doi.org/10.1086/320676.

Citable link http://nrs.harvard.edu/urn-3:HUL.InstRepos:41399867

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HELIUM WHITE DWARFS AND BY DRACONIS BINARIES IN THE GLOBULAR CLUSTER NGC 63971 J. M. Taylor,2 J. E. Grindlay, and P. D. Edmonds Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138; [email protected], [email protected], [email protected] and A. M. Cool Department of Physics and Astronomy, San Francisco State University, 1600 Holloway Avenue, San Francisco, CA 94132; [email protected] Received 2000 December 24; accepted 2001 April 25; published 2001 May 18

ABSTRACT We examine 15 of Hubble Space Telescope observations of the nearby post–core-collapsed globular cluster NGC 6397 using narrowband photometry in Ha with broadband photometry in V, R, and I. We recover the candidate helium white dwarfs (He WDs) recently discovered by Cool et al., of which one was examined spectroscopically by Edmonds et al., along with a sequence of these objects extending down to the magnitude limit of the survey. We also find a sequence of with weak Ha emission and magnitudes characteristic of BY Draconis stars; their offset above the also suggests they are probable binaries. The radial distributions for both the He WD and BY Dra candidates are significantly more centrally concentrated than the main-sequence stars, suggesting that indeed both populations are in binary systems. Subject headings: binaries: close — binaries: eclipsing — globular clusters: general — globular clusters: individual (NGC 6397) — stars: individual (BY Draconis) — stars: neutron — stars: rotation — stellar dynamics — white dwarfs

1. INTRODUCTION other NFs might also be He WDs. While He WDs have been observed directly as the optical counterparts and secondaries Knowledge of stellar dynamics and stellar interactions in of binary MSPs in the field (e.g., van Kerkwijk, Bergeron, & globular clusters has advanced significantly over the past dec- Kulkarni 1996), this was the first to be found in a globular ade with the discovery of multiple cataclysmic variables (CVs) cluster. in each of three clusters with the Hubble Space Telescope (HST; In this Letter, we report much more sensitive HST obser- Cool et al. 1995; Grindlay et al. 1995; Bailyn et al. 1996; vations of NGC 6397 that provide strong evidence for a se- Knigge et al. 2001) and millisecond pulsars (MSPs) with in- quence of these objects by the identification of three more creasingly sensitive radio surveys (Camilo et al. 2000; probable He WDs, based on both blue colors and significant D’Amico et al. 2001). Together with the increasingly rich pop- Ha absorption, as expected for detached He WDs and as found ulations of low-luminosity X-ray sources in globulars (e.g., for the first three NFs. Verbunt & Hasinger 1998), these systems all are manifestations Our primary observational tool was deep Ha versus R im- of compact binaries containing white dwarfs (WDs) or neutron aging with the Wide Field Planetary Camera 2 (WFPC2) and stars (NSs). They are markers of the formation and evolution was motivated by our original objective to identify Ha emission of hard binaries that can, collectively, have profound effects objects to extend the Ha-selected CV candidates found by Cool on the dynamical evolution of the clusters themselves (Hut et et al. (1995) to much deeper limits. Our CV results are reported al. 1992). The new source populations, and particularly the separately (J. E. Grindlay, J. M. Taylor, P. D. Edmonds, A. M. MSPs, have sparked new theoretical understanding of the for- Cool, H. N. Cohn, & P. M. Lugger 2001, in preparation). In mation and evolution of the compact binaries needed (e.g., addition to the moderately bright Ha emitters (CVs), and weak Rasio, Pfahl, & Rappaport 2000). Ha absorbers (NFs), our Ha photometry also revealed a sep- Yet another marker of compact binary populations in glob- arate population of much weaker Ha emission objects. We ulars are helium white dwarfs (He WDs). As the stripped cores identify these as probable BY Draconis binaries, the first such of (sub)giants, He WDs are a natural outcome of compact population found in a globular cluster. binary evolution. Three objects in NGC 6397 were first iden- In the field, BY Dra stars are chromospherically active MS tified by Cool et al. (1998, hereafter CGC) as stars significantly stars (dK to dM) in binary systems (Alekseev 2000). Their bluer than the main sequence (MS) but lacking the flicker var- characteristics of weak Ha emission and variability typically iations generally seen in CVs. Follow-up HST/Faint Object on the order of days suggest chromospheric activity created by Spectrograph observations by Edmonds et al. (1999, hereafter fast rotation. However, since stellar wind mass loss and mag- EGC) of one of these “nonflickerers” (NF 2) yielded a spectrum Շ Ϫ1 netic braking slow their rotation down on timescales 1 Gyr ע consistent with an He WD and247 50 km s Doppler (Young et al. 1989), the necessarily much older BY Dra stars shift—suggesting it is in a binary with a dark companion, most in a globular must have their rotation maintained by tidal lock- probably a massive WD or an NS and by implication that the ing. Thus the identification of BY Dra systems provides a new tracer for the hard binary population in globular clusters. 1 Based on observations with the NASA/ESA Hubble Space Telescope ob- tained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy (AURA), Inc., under 2. OBSERVATIONS AND ANALYSIS NASA contract NAS5-26555. 2 Current address: Department of Astronomy, University of Tokyo, 7-3-1 The observations were obtained during 15 HST orbits on 1999 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan. April 3–4. Four separate visits were made to the target; the L169 L170 He WDs AND BY DRA BINARIES IN NGC 6397 Vol. 553

TABLE 1 He White Dwarf Candidates Ϫ Ϫ Identification x, y Da Dd V555 V I Ha R PC-1 ...... 628, 625 Ϫ13.2 7.6 18.99 0.09 0.32 PC-2 ...... 684, 457 Ϫ9.6 0.4 20.07 0.06 0.35 PC-3 ...... 468, 218 5.2 Ϫ0.1 20.25 0.10 0.34 PC-4 ...... 130, 688 0.6 25.7 22.76 0.58 0.96 PC-5 ...... 681, 259 Ϫ3.0 Ϫ5.7 22.76 0.62 0.79 PC-6 ...... 666, 171 0.4 Ϫ8.0 23.05 0.72 0.63 Note.—Pixel coordinates (x, y) refer to HST archive image u5dr0401r, and and are offsets in arcseconds from esti- mated cluster center, 17h40m41.96,s Ϫ53Њ40Ј27Љ.9 (J2000) or (546, 300) in PC coordinates. The center was determined by considering all MS and ! Љ brighter stars withV555 23 within 10 of the self-consistent core and weighting each by masses derived from the models of Baraffe et al. 1Љ and note that choiceע We estimate this center to be accurate to .(1997) of the blue straggler masses, magnitude cutoff, and self-consistent radius led to shifts in the center, but all fell within our suggested accuracy. Absolute magnitudes may be compared to those of CGC by using Ϫ p (m M)V 12.29.

∼ resulting total time span was 37 hr. We obtained 128 WFPC2 Ϫ Ϫ Fig. 1.—V555 vs. V555 I814 and Ha R675 CMDs for the PC (top) and the exposures with F555W, F675W, F814W, and F656N filters (here- WF (bottom) chips. He WD candidates are marked with diamonds, and BY afterVR555 , 675 , I 814 , and Ha). Exposure times were dominated Dra candidates are marked with circles. ∼ by Ha (30 at 700–900 s), which roughly matched the R675 24 sensitivity reached in 38 exposures at 40 s. Nearly as deep # sequence was calculated through iterative sigma-clipping in limits were reached inVI555 and 814 in24 40 s exposures each. The exposure time sampling yielded a sampling Nyquist fre- bins by magnitude. An object was retained as a candidate if quency of5 # 10Ϫ4 Hz, allowing consistent period searches be- the photometric error estimate combined in quadrature with the tween ∼1 and 20 hr. We dithered to 15 distinct pointings to width of the MS at that magnitude yielded a 3 j or stronger correct for pixel variations and maximize resolution. result; otherwise it was considered an MS . In addition to After performing the standard pipeline calibrations, we used CV candidates, we found two classes of objects by this tech- the STSDAS DITHER and DITHERII packages (Fruchter & nique: moderately strong Ha absorbers (NFs) and weak Ha emitters (BY Dra binaries), lying ∼0.1 mag to the left of the Hook 1997) to create a temporary median-combined, super- Ϫ sampled (“drizzled”) image for each filter. Transforming the MS in Ha R675. higher resolution temporary master back to the original image We also analyzed the time series available from the 15 orbits coordinates by “blotting,” we created images for cosmic-ray of data. PSF photometry using the master star list was deter- removal. After creating cosmic-ray masks, we then drizzled mined for every frame. A set of bright, nonvariable comparison additively to create the final master high-resolution image for stars were selected for each filter and chip. These were used each filter. These drizzled master images were used for our star to calculate differential photometry correction formulae with list construction and photometry, while the individual cosmic- empirical adjustments for variations in position (Gilliland & ray cleaned images were reserved for variability studies. The Brown 1988; Taylor et al. 1999). To look for periodic variation drizzled masters had the advantage of fully sampling the point- in our unevenly spaced data, we calculated the Lomb-Scarle spread function (PSF) on the PC and only marginally under- periodograms (Scargle 1982; Horne & Baliunas 1986; Press & sampling the PSF on the WF chips. Teukolsky 1988) and produced corresponding phase plots for We created star lists first by automated finding using all the Ha interesting objects. Periods were considered signif- DAOFIND and cross-comparison between filters to remove icant if the highest peak in the periodogram was not a multiple PSF artifacts, followed by iterative PSF subtraction of the cur- of the HST and had a false alarm probability (FAP) of rent list and addition by hand of any remaining stars, repeating less than 1%. Finally, all NF and BY Dra candidates were until no more stars could be found. To locate very dim stars, filtered for cluster membership using the multiepoch HST we combined all four drizzled images (one for each filter) and proper-motion studies of A. S. Bolton, A. M. Cool, & J. An- examined the combined image by eye. derson (2001, in preparation). Photometric calibrations, using the Holtzman et al. (1995) zero points, were done on isolated bright stars in the original 3. RESULTS AND DISCUSSION frames and then transformed to the drizzled master images. The Ϫ p 3.1. He White Dwarfs Ha zero point was chosen to obtain Ha R675 0 for stars with Ha equivalent widths (EWs) equal to 0 A˚ . This was The photometry yielded six blue stars with significant Ha accomplished by assuming that red giants have EW(Ha) p absorption, all in the PC chip. No comparably significant can- 1.0 A˚ and convolving the F656N filter bandpass with the ex- didates were found in the WF chips. These NFs, and thus He Ϫ pected line shape to determine the corresponding Ha R675 WD candidates, are listed in Table 1 and marked in Figure 1. magnitude. The three brightest are the NFs discovered by CGC. NFs 4–6 The calibrated PSF photometry was examined to find faint have very similar characteristics and together with NFs 1–3 Ϫ objects far from the MS in Ha R675. To determine which ob- mark out a sequence parallel to the CO WD sequence measured jects are statistically significant Ha absorbers or emitters, we about 5Ј from the cluster center by Cool, Piotto, & King (1996) first fit a cubic spline to the MS. An apparent width to the but ∼2 mag brighter in V. The standard deviation of the time No. 2, 2001 TAYLOR ET AL. L171

TABLE 2 BY Dra Candidates Ϫ Ϫ Identification x, y Da Dd V555 V I D(Ha R) PC-1a ...... 386, 266 6.3 4.01 17.08 0.93 0.08 PC-2 ...... 539, 96 7.0 Ϫ6.3 17.76 1.10 0.10 PC-3 ...... 424, 216 6.7 1.2 17.88 1.07 0.07 PC-4 ...... 691, 597 Ϫ14.4 4.6 18.28 1.16 0.06 PC-5 ...... 557, 640 Ϫ11.6 10.3 18.30 1.06 0.08 PC-6 ...... 390, 550 Ϫ3.2 12.9 19.01 1.23 0.09 PC-7 ...... 766, 296 Ϫ7.0 Ϫ7.3 21.04 1.66 0.14 WF2-1 ...... 85, 542 58.0 47.9 19.10 1.22 0.09 WF2-2 ...... 115, 246 35.1 28.7 19.93 1.49 0.02 WF3-1 ...... 591, 149 71.0 41.2 17.55 0.98 0.09 WF3-2 ...... 560, 93 64.8 42.8 18.99 1.20 0.06 WF3-3 ...... 374, 537 83.9 Ϫ1.1 20.20 1.39 0.09 WF3-4 ...... 363, 69 49.3 30.5 20.60 1.56 0.07 WF4-1b ...... 190, 223 22.3 Ϫ14.1 16.66 1.04 0.12 WF4-2c ...... 100, 222 16.1 Ϫ7.8 17.20 0.95 0.13 WF4-3 ...... 404, 391 25.8 Ϫ41.0 20.60 2.30 0.26 Note.—Pixel coordinates (x, y) refer to HST archive image u5dr0401r, and right ascension and declination are offsets in arcseconds from estimated cluster center, 17h40m41.96,s Ϫ53Њ40Ј27Љ.9 (J2000) or (546, 300) in PC coordinates. The center was determined by considering all MS and brighter stars with Fig. 2.—Cumulative radial distributions for MS stars (solid line), He WD V ! 23 within 10Љ of the self-consistent core and weighting each by masses candidates (dotted line), and BY Dra candidates (dashed line) as a function 555 derived from the models of Baraffe et al. (1997). We estimate this center to of distance from the cluster center and fraction of the total observed population, 1Љ and note that choice of the blue straggler masses, magnitudeע be accurate to weighted to correct for the irregular shape of the WFPC2, as discussed in the cutoff, and self-consistent radius led to shifts in the center, but all fell within text. our suggested accuracy. Absolute magnitudes may be compared to those of Ϫ p CGC by using (m M)V 12.29. series for each of the NFs is within with the estimated pho- a Variable (W UMa,period p 6.16 hr, FAP p 1.1 # 10Ϫ6 ). tometric error for an MS star of comparable magnitude, con- b Variable (period p 33.88 hr, FAP p 2.5 # 10Ϫ12 ). c Ϫ13 firming the absence of flicker in all six candidates. Variable (W UMa,period p 6.52 hr, FAP p 3.1 # 10 ); star V7 of A comparison of the radial distribution of NFs and cluster Kaluzny (1997). MS stars is shown in Figure 2. To correct for the irregular shape of the WFPC2 in calculating radial distributions, stars dMe/dKe star hypothesis untenable. However, we notice that in partially sampled annuli were weighted by the inverse of not only are these objects slightly Ha-bright, but in the Ϫ the fraction of the area of 1Љ.5 wide annuli centered on the core V555 I814 color-magnitude diagram (CMD; Fig. 1) they fall that fell on the WFPC2. We omitted the stars beyond 80Љ,or above the MS by Շ0.75 mag, consistent with their being MS the distance from the center of the cluster to the edge of WF4, binary stars. The rapid rotation that provides the chromospheric where the numbers of available MS stars begins to fall off. For activity can be sustained by tidal locking, suggesting these are ∼ the stars within 1 mag of theV555 24 limit, selection effects binary BY Dra–type systems. The comparison in Figure 2 of due to bright stars interfering with finding such dim stars were their cumulative radial distribution with the MS reveals a pop- significant. Artificial star tests revealed that at least 20% of ulation much more centrally concentrated (probability only 2% such stars were most likely missed. However, this is a loss of that they are distributed like the MS), implying a larger mass only ∼1% of the total population and thus is negligible for the and further supporting the binary hypothesis. distribution determination. A Kolmogorov-Smirnov test shows Periodogram analysis of the BY Dra candidates’ light curves that the NFs and MS radial distributions have only 0.002% shows that three are periodic variable stars: PC-BY1, WF4- probability of being the same. Thus, the strong concentration BY1, and WF4-BY2. Their amplitude variations are 0.10, 0.21, of NFs toward the cluster center implies that they are relatively and 0.65 mag, respectively; all are highly significant detections massive systems. The low masses of He WDs in turn suggest (see Table 2). Our variable WF4-BY2 is an independent re- that they must have reasonably massive binary companions. covery of this particularly isolated and bright variable discov- The latter are most likely WDs or NSs, given that all but the ered in ground-based photometry by Kaluzny (1997, his V7). lowest mass MS companions are ruled out on the basis of PC-BY1 and WF4-BY2 have periods of 6.16 and 6.52 hr, re- broadband colors (CGC). spectively—short for BY Dra systems, although the 11.17 hr field BY Dra binary identified by Chevalier & Ilovaisky (1997) 3.2. BY Dra Systems provides a precedent. The 33.88 hr period of the WF4-BY1 object is a more typical period for BY Dra-type systems. As The weak Ha emission objects below the turnoff are ∼0.10 BY Dra-type systems sometimes show flarelike variations, we Ϫ to the left of the MS in Ha R675. Following Young et al. computed the standard deviation of the time series for each Ϫ (1989), we measure the difference in Ha R675 between each candidate; as with the NFs, no significant nonperiodic varia- BY Dra candidate and MS stars of the same magnitude; these tions were found. Ϫ “excess” Ha R675 values are listed in last column of Table 2. The corresponding emission EWs are typically EW(H ) ∼ a 4. CONCLUSIONS 2–5 A˚ , similar to field dMe and BY Dra systems (Young et al. 1989). As noted before, the expected chromospheric activity Using the HST, we have imaged the core and central regions lifetime of single dMe and dKe stars is only on the order of of the collapsed core globular cluster NGC 6397 in a combined 1 Gyr as the mass loss and magnetic braking slows down the search for variability and Ha emission or absorption in stars ∼ rapid rotation that induces the activity, rendering the single as faint asV555 24 , 4 mag fainter than any previous Ha sur- L172 He WDs AND BY DRA BINARIES IN NGC 6397 Vol. 553

տ vey. Examination of the very dim stars (V555 20 ) confirms cools too far), as would the question of the relative frequency and extends our prior evidence for a sequence of He WDs as with which the NS would exchange in for the CO WD as Ha absorbers below the MS and establishes a new class of opposed to the lower mass He WD. Still, the fact that NGC weak Ha emission stars, probably BY Dra systems, Շ0.75 mag 6397 has undergone core collapse, and the corresponding rash above the MS. of recent binary interactions to halt collapse, should make such The populations of MSPs and He WDs are strongly linked, double-exchange interactions more likely. In this scenario, the and the He WD cooling (vs. MSP spin-down) timescales provide He WD is not the original companion to the MSP and thus powerful constraints on the system age and formation. Indeed, presumably not the star responsible for spinning it up. Alter- the He WD sequence reported here is shown by B. M. S. Hansen, natively, if the He WD progenitor were an evolved blue strag- V. Kalogera, E. Pfahl, & F. A. Rasio (2001, in preparation) to gler rather than a normal subgiant, unstable mass transfer could be too luminous and thus too young to still be in binaries with potentially occur even with an NS primary. The relatively large NS primaries; CO WD companions, with their lower masses, numbers of massive blue stragglers present in the central region are more likely. of NGC 6397 (Sepinsky et al. 2000) suggest this could be a At the same time, two independent analyses of an existing nonnegligible pathway. deep ROSAT/HRI image of NGC 6397 have led to the sug- Whether either of these mechanisms merits further investi- ∼ gestion that NF3 may have an X-ray counterpart with L X gation hinges on the possible association of the He WDs and 3 # 10 31 ergs sϪ1 (Metchev 1999; Verbunt & Johnston 2000; MSPs in the cluster. It will be of great interest to compare the S. Metchev & J. Grindlay 2001, in preparation). Significant timing positions, when derived, of the first MSP in NGC 6397 crowding caused by four brighter sources also near the cluster (D’Amico et al. 2001), and the others now likely to be found, center make the identification of this possible fifth source some- with the NFs. Deep X-ray studies of NGC 6397 with Chandra what uncertain, and the limited positional accuracy (∼3Љ) leaves will test whether NF3 and others of the current NF sequence open the possibility of a chance coincidence. Nevertheless, we are detectable. consider here alternatives that may allow at least some young Our identification of a sequence of stars with small Ha excess Ϫ He WDs to have NS companions. An NS companion would and lying on the binary sequence inV555 versus V555 I814 CMD naturally explain the suggested X-ray luminosity of NF3 (which strongly suggests they are BY Dra binaries, the first to be is typical of LX-values for MSPs; Becker & Tru¨mper 1999), identified as such in a globular. Likely orbital periods have whereas a CO WD companion would be unlikely to provide been measured for three of the 16 candidates. The two systems p the requisite X-ray flux in the absence of accretion. While withPorb 6.1 and 6.5 hr are necessarily contact binaries (in- Verbunt & Johnston (2000) note that the source corresponding deed, WF4-BY2/V7 is classified as such by Kaluzny 1997). to NF3 may be variable, higher resolution observations are Although BY Dra systems are detected in the field as low- ∼ 27 30 Ϫ1 needed. The lack of observable flicker and relatively cool tem- luminosity X-ray sources withL X 10 –10 ergs s (Demp- perature of the He WD secondary [∼(1–2) # 10 4 K; EGC] sey et al. 1997; Chevalier & Ilovaisky 1997), they are below both argue against present-day accretion in these systems, sug- the detection limits for the current ROSAT observations of NGC gesting that higher resolution X-ray observations will be nec- 6397 and none are seen (S. Metchev & J. Grindlay 2001, in essary to resolve this potential variability. preparation). One possible mechanism by which a young He WD might be found in a binary with an MSP is via an exchange interaction following the mass transfer event that created the He WD. The We thank H. Cohn and P. Lugger for discussions of the timescale for CO WD/He WD binaries to undergo such ex- analysis. This work was supported in part by NASA grant GO- changes would be critical (having to occur before the He WD 07335 to Harvard and San Francisco State University.

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