Binaries in the Center of NGC 6397: Observational Constraints on Numerical Simulations of Globular Clusters

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1 Hubble Space Telescope Cycle 20 GO Proposal 1049 Binaries in the Center of NGC 6397: Observational Constraints on Numerical Simulations of Globular Clusters Principal Investigator: Dr. Adrienne Cool Institution: San Francisco State University Electronic Mail: Scientific Category: RESOLVED STELLAR POPULATIONS Scientific Keywords: Cluster Binary Stars And Blue Stragglers, Detached Binaries, Eruptive Binary Stars And Cataclysmic Variables, Globular Clusters, White Dwarfs Instruments: ACS, WFC3 Proprietary Period: 12 Orbit Request Prime Parallel Cycle Abstract We propose deep multi-wavelength imaging designed to make NGC 6397 a benchmark for current and future N-body and Monte Carlo simulations of globular clusters. NGC 6397 is the nearest cluster with a collapsed core; owing to its proximity, unusually complete samples of both binary and single stars can be obtained even in its high-density core. Using F625W, F435W and F336W filters, and making use of archival imaging, we will measure magnitudes, colors, and membership of stars down to ~0.1 solar masses out to nearly the half-mass radius. Our primary objectives are to (1) identify a population of detached main sequence/white dwarf binaries in a globular cluster for the first time; (2) determine the mass and age distribution of the population of known double white dwarfs; and (3) measure the mass ratios and radial distribution of double main sequence stars. This study will provide unprecedented information about multiple closely related binary populations, including those that have been transformed through evolutionary and dynamical processes.

2 Investigators: Dr. Adrienne Cool : Binaries in the Center of NGC 6397: Observational Constraints on Numerical Simulations of Globular Clusters Investigator Institution Country PI Dr. Adrienne Cool San Francisco State University USA/CA CoI Ms. Glenna Dunn San Francisco State University USA/CA CoI Dr. Haldan N Cohn Indiana University System USA/IN CoI Dr. Phyllis M. Lugger Indiana University System USA/IN CoI Dr. Craig Heinke University of Alberta CAN CoI Dr. Jay Anderson Space Telescope Science Institute USA/MD CoI Mr. Srikar Srinath University of California Santa Cruz USA/CA CoI Ms. Rachel Strickler University of California - Santa Cruz USA/CA CoI* Dr. Aldo Serenelli Max-Planck-Institut fur Aeronomie DEU Number of investigators: 9 * ESA investigators: 1 Target Summary: Target RA Dec Magnitude NGC V = 6.68 Observing Summary: Target Config Mode and Spectral Elements Flags Orbits NGC-6397 ACS/WFC Imaging F435W 6 ACS/WFC Imaging F625W NGC-6397 WFC3/UVIS Imaging F336W 6 Total prime orbits: 12

3 Scientific Justification Binary stars play a vital role in the dynamical evolution of globular clusters. Following core contraction driven by two-body relaxation, clusters enter a binary burning phase in which the core is supported by kinetic energy supplied by the binaries. Once they have been depleted, the core goes into deep collapse [H03a]. The dramatic increase in sophistication of numerical simulations in recent years, incorporating single and binary star evolution, now makes it possible to model relatively realistic clusters. The latest studies find that the overall binary fraction in a cluster is fairly constant, with the loss of single stars across the tidal boundary offsetting the gradual depletion of binary stars. In the central regions, however, the binary fraction rises markedly over time. In an N-body simulation of a star cluster with 5% primordial binaries, for example, [H07] predict that the central binary fraction rises to 20% in a Hubble time (see also [C10a]). The ability of the latest N-body and Monte Carlo methods to predict the fate of binaries opens up new possibilities for testing numerical simulations against observational data. Existing observations, while supporting (and driving) the overall picture of cluster and binary evolution, are still limited in the direct constraints that they provide. Populations of exotic binaries (e.g., [G01, H05, P06]) provide abundant evidence for the unusual dynamical processes taking place in dense cluster cores, but tend to be relatively few in number in any given cluster. While main-sequence/main-sequence (MS-MS) binary fractions have been measured in a growing number of clusters e.g., [M12]), simulations predict that a majority of binaries in the central regions of a cluster will no longer be MS-MS pairs. Instead they will have undergone exchange interactions (e.g., with white dwarfs (WDs)), or have been transformed into double-degenerate systems [I06, H07, F09]. PROPOSED STUDY We propose to use deep multi-wavelength imaging to make NGC 6397 a benchmark against which existing and future N-body and Monte Carlo simulations can be tested in detail. Two groups have already developed such models of NGC 6397 in particular [H07, H09]. The data from the proposed study will provide a crucial complement to existing deep ACS/WFC observations of a region just beyond the half-mass radius (GO-10424, PI=Richer). While that field provided a measure of the primordial binary fraction, the proposed observations will provide unprecedented constraints on binaries in the central regions, including those that have been transformed through evolutionary and dynamical processes. NGC 6397 is the nearest core-collapsed cluster; its proximity ( 2.5 kpc) reduces the crowding that plagues even HST observations of high-density clusters, to the extent that unusually complete samples of both binary and single stars can be obtained even in its 10 6 M /pc 3 core. Combining ACS/WFC imaging in F435W and F625W filters with WFC3/UVIS imaging in F336W, we will measure magnitudes and colors of an estimated 25,000 stars in the cluster s core and extending out to its half-mass radius. Three distinct classes of binaries are already known in the central regions of NGC 6397: MS-MS binaries [C02, D08a, S11] (see Fig. 1); MS-WD binaries in the form of cataclysmic variables (CVs) (see Fig. 2; [S09, C10b]); and WD-WD binaries in which one of the compo- 1

4 nents has a helium core (HeWD) (see Fig. 2; [S09]). One quiescent low-mass X-ray binary [G01] and one millisecond pulsar [D01] are also present, along with numerous blue stragglers [A90, L92]. The proposed observations would identify detached MS-WD binaries in a globular cluster for the first time, and make NGC 6397 the first cluster for which the numbers, characteristics, and radial distributions of four different, inter-related classes of binary stars are known. Our principal objectives are to: (1) Find and characterize detached MS-WD binaries. This class of binaries is of particular interest as it provides a direct evolutionary link between the other three classes of binaries known in significant numbers in NGC MS-WD binaries are the inevitable outcome of the evolution of MS-MS binaries; they are also predicted to be frequent products of exchange interactions in dense clusters (e.g., [I06]). In the simulation of Hurley et al. that most closely resembles NGC 6397 [H07, H08]), binaries formed via exchanges make up about half of all the binaries in the core (and spend considerable time outside the core as well [H07]). Detached MS-WD binaries are, in turn, the precursors of CVs; accretion begins when the detached pair is brought into contact via magnetic braking or dynamical interactions with passing stars. In cases in which the MS component of a MS-WD pair evolves up the giant branch while the system is detached (but close), the result will instead be a He WD with a WD companion like those now known in NGC 6397 (see Fig. 2). MS-WD binaries can in principle be identified by their distinctive locations in a CMD (Fig. 3; also see Fig. 17 of [C10a]). A significant obstacle, however, is the large number of background stars that occupy the same region of the CMD as the binaries (compare regions to left of MS in Figs. 1 and 2). We will combine two techniques to overcome this obstacle. First, we will measure very high accuracy proper motions (comparable to those obtained for the outer field in NGC 6397 [R08]) to remove stars that do not share the cluster s motion. Second, we will require that candidates be detected in three separate filters, in order to construct color-color diagrams (see Fig. 4); owing to the presence of two temperature components, MS-WD binaries occupy a distinctive location in these diagrams. Magnitudes in three filters will enable us to constrain the temperatures and masses of both binary components. Characterizing the binaries in this way is an essential part of our mission to provide meaningful observational benchmarks. Combining these two criteria will ensure that we are not misled by a small number of stars that are either poorly measured or are in the background but by chance share the cluster s proper motion. We have carried out extensive artificial star experiments on the existing ACS/WFC data to determine the rate of chance MS-WD alignments (which would mimic the binaries we seek). These show that only if a WD lands within 1 pixel of a MS star does a false positive occur. Taking account of the number of WDs in the field (corrected for incompleteness) and the numbers of MS stars with which WDs of various magnitudes could be paired (and result in a detectable signal), we find that only 0.4 such chance alignments should occur over the entire field. (2) Measure the mass and age distribution of HeWD-COWD binaries. We have identified two dozen He WDs in NC 6397 whose radial distribution in the cluster implies that they have massive companions in the form of CO WDs [S09]. Their presence provides 2

5 unusual insight into the evolved binaries in a globular cluster and interesting constraints on the binaries from which they must have formed. Because the progenitor s orbital separation determines when a giant s growth will be interrupted, the masses of the He WDs (former giant cores) are directly tied to the orbital periods of their progenitors [R95]. Their formation rate, determined by comparison to theoretical cooling tracks, suggests that 1-5% of MS stars have WD companions with 1-20 day periods at the time they leave the turnoff [S09]. The photometric accuracy of the proposed observations will significantly improve the measurement of the mass distribution of WDs in the cluster. At present it appears bimodal, with one peak at M for the CO WDs, and a second at M for the He WDs [S09]. But whether the dip between the two peaks is real, or an artifact of our inability to distinguish He WDs from CO WDs for masses greater than about 0.3M, is currently unclear. The proposed observations will enable us to distinguish He WDs with masses as high as 0.4M. The mass distribution of the He WDs has important implications for the period distribution of their progenitor binaries: while 0.3M He WDs imply progenitors with 20-day periods, 0.4M He WDs have progenitor periods of 140 days. A tantalizing finding of our study of the He WDs in NGC 6397 is that the sequence appears to come to an end near R , well above the magnitude limit (see Fig. 2). A possible explanation is that the WDs have thick H envelopes that make them cool very slowly [S02]; the end of the sequence would then be a consequence of the finite age of the cluster [S09; see also H03b]. We can test this hypothesis by determining with greater certainty whether the He WD sequence really ends abruptly. At present there are a few marginal candidates below R (see Fig. 2). While our artificial-star experiments suggest that these are poorly measured CO WDs [S09], higher accuracy photometry is needed to draw a firm conclusion. Improved knowledge of the He WD cooling rate would in turn improve our constraint on the binary population in NGC The factor of 5 uncertainty the frequency of progenitor binaries (1-5%) is the direct result of the uncertain He WD cooling rate. (3) Measure the number, mass ratios, and radial distribution of MS-MS binaries. Our ACS/WFC study places a new upper limit of only 1.8% on the fraction of MS-MS binaries with q = M 2 /M 1 > 0.5 in the central regions of NGC 6397 (see Fig. 1; [S11]). Extrapolating to all mass ratios suggests that the total fraction is still under 4%. This is lower than that found by either study that used WFPC2 (5±1% [D08a]; < 5 7% [C02]), and only slightly higher than the fraction measured beyond the half-mass radius (1-2%, depending on assumed q distribution [D08a]). The proposed observations will provide significantly more accurate determinations of colors along the main sequence. The use of proper motions to remove non-members with a high degree of confidence will enable us to make a definitive measurement not only of the MS-MS binary fraction from the center of the cluster out to its half-mass radius, but also of the mass ratios of the MS-MS binaries we observe. The feasibility of this type of measurement was first demonstrated by [C02] using WFPC2. At present, observers and theorists alike must assume a distribution of q values. Based on our artificial star experiments, we estimate that only 6 chance coincidences between unrelated MS stars will mimic real binaries. This is <3% of the MS-MS binary candidates we have identified (see Fig. 1), and will thus not significantly bias our results. 3

6 REFERENCES [A90] Aurière, Lauzeral, & Ortolani 1990, Nature, 344, 638 [B95] Bergeron, Wesemael, & Beauchamp 1995, PASP, 107, 1047 [C10a] Chatterjee, Fregeau, Umbreit, & Rasio 2010, ApJ, 719, 915 [C10b] Cohn et al. 2010, ApJ, 722, 20 [C02] Cool & Bolton, ASP Conference Series, v263, 2002 [D01] DAmico, Lyne, Manchester, Possenti, & Camilo 2001, ApJ, 548, L171 [D08a] Davis et al. 2008, AJ, 135, 2155 [D08b] Dotter, Chaboyer, Jevremovic, Kostov, Baron, & Ferguson 2008, ApJS, 178, 89 [F09] Fregeau, Ivanova, & Rasio 2009, ApJ, 707, 1533 [G01] Grindlay, Heinke, Edmonds, Murray, & Cool 2001, ApJ, 563, L53 [H03b] Hansen, Kalogera, & Rasio 2003, ApJ, 586, 1364 [H03a] Heggie & Hut 2003, Cambridge University Press [H05] Heinke et al. 2005, ApJ, 625, 796 [H07] Hurley, Aarseth, & Shara 2007, ApJ, 665, 707 [H08] Hurley et al. 2008, ApJ, 135, 2129 [H09] Heggie & Giersz 2009, MNRAS, 397, L46 [I06] Ivanova, Heinke, Rasio, Taam, Belczynski, & Fregeau 2006, MNRAS, 372, 1043 [K07] Kalirai et al. 2007, ApJ, 657, L93 [K98] King, Anderson, Cool, & Piotto 1998, ApJ 492, L37 [L92] Lauzeral, Ortolani, Aurière, & Melnick 1992, A& A, 262, 63 [M12] Milone et al. 2012, astro-ph/ [CHECK FORMAT] [P06] Pooley & Hut 2006, ApJ, 646, L143 [R95] Rappaport, Podsiadlowski, Joss, di Stefano, & Han 1995, MNRAS, 273, 731 [R08] Richer et al. 2008, AJ, 135, 2141 [S02] Serenelli, Althaus, Rohrmann, & Benvenuto 2002, MNRAS, 337, 1091 [S11] Srinath, Cool, & Anderson 2011, BAAS, 43, 2011 [S09] Strickler, Cool, Anderson, Cohn, Lugger, & Serenelli 2009, ApJ, 699, 40 Description of the Observations The goals we have outlined above can be achieved with 6 orbits of ACS/WFC imaging with F435W and F625W filters and 6 orbits of WFC3/UVIS imaging with the F336W filter, as described below. The ACS/WFC will be pointed to provide maximal overlap with our existing ACS/WFC imaging (GO-10257), and will encompass all the known He WDs and CVs. The WFC3/UVIS pointings will be planned to provide full coverage of the area spanned by the the WFC by shifting the camera appropriately. The result will be deep 3-band imaging for the entire WFC field. Our choice of ACS/WFC and the F435W and F625W filters, together with UVIS and the F336W filter is driven by several factors. First, use of ACS/WFC means that we will encompass as much area as possible; in the corners, WFC will extend out to the half-mass radius of 2.3. This will enable us to encompass all the He WDs we found in [S09], including 4

7 16 17 R B435 - R625 Figure 1: The main sequence of NGC 6397 measured from existing ACS/WFC data [S11]. Crosses mark 185 MS-MS binary candidates lying > 4σ from the main-sequence ridge line (red; corresponding to q 0.5) and within 1σ of the equal-mass binary line (purple). This is only 1.8% of the MS stars in the same magnitude range as the binaries. Proper motions have been used to select cluster members. However, many stars in this CMD are still likely to be interlopers (see, e.g., stars on blue side of lower MS); the lack of a 2nd epoch of sufficiently deep observations limits the accuracy of the proper motion measurements, particularly at faint magnitudes. Figure 2: Helium-core white dwarfs (He WD) and cataclysmic variables (CVs) in the central regions of NGC 6397 (from [S09b]). The two dozen He WD candidates are marked with large cyan circles; known CVs are shown as pink triangles. At a given color, He WDs are brighter than CO WDs owing to their smaller masses and thus larger sizes. Cooling tracks for a 0.53 M CO WD are shown (dashed = hydrogen atmosphere; solid = helium atmosphere [B95]). Several marginal He WD candidates are shown as small cyan circles. In the Hα R625 diagram (right), CVs appear Hα-bright, while He WDs, like CO WDs, are Hα-faint owing to broad absorption lines. No proper-motion selection has been applied to these diagrams. 5

8 Msun 0.5 Msun 0.4 Msun Msun 0.2 Msun R Msun B 435 -R 625 Figure 3: Location of simulated MS-WD binaries between the WD sequence and main sequence on a CMD. Tracks are shown for MS stars with masses from M [D08b] combined with WDs with ages of 0.1, 0.5, 1, 2, and 4 Gyr [B95]; these ages are also indicated with triangles on the WD sequence. Proposed observations would provide sufficiently accurate B 435 R 625 colors to distinguish < 1 Gyr-old WDs paired with < 0.3M MS stars and < 2 Gyr-old WDs paired with < 0.2M MS stars U B Msun Msun B R 625 Figure 4: Location of the main sequence, giant branch, WD sequence, and simulated MS-WD binaries in a color-color diagram. MS-WD binaries occupy a distinct region above the MS. The hairpin turn at the upper left of the MS is the turnoff; the giant branch lies just below the main sequence. Symbols as in Fig. 3. 6

9 the marginal ones whose nature we aim to clarify, all the known CVs, and several X-ray sources that have yet to be identifed. Second, and most critically, we need to achieve the best possible separation of MS-WD binaries from the main sequence. As WDs are faint (particularly ones that are old enough to be numerous), the separations will not be large. We have explored all the filter combinations available with WFC and UVIS, and find that this filter and camera combination offers the most efficient way in which to carry out this science. Similar considerations also apply to the He WDs, and F625W and F435W are already known to separate them well from the CO WD sequence. Use of the proposed ACS/WFC filters has the added advantage that we already have the equivalent of about 0.7 orbits of exposure time in each of these filters from the GO program, and can combine the datasets directly to boost our S/N somewhat. The requested exposure time in F435W and F625W filters is driven by the need to achieve high S/N at the faint end of the WD sequence to clearly separate CO WDs from He WDs. At the bottom of the WD sequence in Fig. 2 ( 2 Gyr-old CO WDs), the separation between the CO WD cooling track and a 0.4M He WD cooling track is just over 0.2 mag [S09]. For a 3σ separation we therefore require colors accurate to 0.07 magnitudes (the quality of the separation will be correspondingly better at brighter magnitudes). According to ETC, this can be achieved with 9100s of exposure in the F435W filter and 6700s in the F625W filter, for a total of 6 orbits in the two filters combined. Here we have used a G0V star spectrum, a 0.1 aperture, R=26.1 and B=27.4 appropriate for a 2 Gyr-old WD, required S/N=14 in F435W and S/N=25 in F625W, and divided the time into 16 and 8 exposures for F435W and F625W, respectively. These exposures will also provide high S/N on the lower main sequence which is critical for identifying MS-WD binaries. To determine the exposure time required in F336W with UVIS, we scale from the 6 620s parallel exposures from GO (PI=Rich) that partially cover the field of interest. As a benchmark, we use a 1 Gyr-old WD paired with a 0.3 M main-sequence star. In F336W F625W colors, the 1 Gyr-old WD will make the binary 0.46 magnitudes bluer than the MS in the color-color diagram (see Fig. 4). For a 10σ detection we therefore require an accuracy of magnitudes, or S/N 22. The system will have a combined magnitude of U= 24.8 (used for ETC normalization). Using a 0.1 aperture, E(B-V)=0.18, and a K7V star spectrum, ETC predicts 120 electrons in a 620s exposure. Near the center of the cluster, the background is 20 e /pix. Taking account of this elevated background together with read noise, we find that a single 620s exposure yields S/N = 4.3. To reach S/N = 22 we therefore need (22/4.3) 2 26 exposures (each 620s), which can be obtained in 6 orbits. Similar considerations show that these exposures will yield 3σ detections of binaries containing a 0.15M MS star and a 2 Gyr-old WD. We note that the archival exposures include only about 1/4 of the needed exposure time. Moreover, they are not centered on the cluster, and miss about 1/2 of the area that we aim to cover (and therefore many of the He WDs, for example). These data will, however, provide additional S/N in the area that they do cover. We have verified using the archival F336W imaging that, from the point of view of detectability of faint stars, the reduced exposure times at the edges of the WFC3 field (due to the pattern of pointings required to cover the full ACS/WFC field with WFC3) will be 7

10 offset by the significantly lower background in those regions. We estimate that there are Gyr-old WDs in the field that will be covered, such that even if the fraction that is paired with MS stars is quite low, we will detect binaries. Considering the numbers of WDs and the MS companions with which they can potentially be observed, we estimate that if 5% of the WDs have MS companions of any kind, then we will detect 15 MS-WD binaries. While this is not a large number, it is enough to make a meaningful measurement of the MS-WD binary fraction in the cluster (or set a meaningful upper limit). Finally, in order to achieve the goals of this proposal, it is essential to remove nonmembers from the CMD with a high degree of confidence using proper motions. The planned F625W exposures, which are required to achieve the high photometric accuracy we require, will simultaneously provide excellent astrometric positions for all stars in the CMD. For stars with R 625 = 26, for example, each 340s F625W exposure provides S/N = 10. In our experience, 5 exposures with this S/N yields an astrometric accuracy of 0.1 pixel. This is the accuracy that will be achieved with the 1st epoch data (GO-10257), obtained in 2004/2005. With s exposures in the proposed program, the 2nd epoch will be even more accurate. NGC 6397 has a proper motion of 17.3 mas/yr in Dec. and 3.6 mas/yr in R.A. [K07]. In the 7.5-year baseline between the existing ACS/WFC imaging and the proposed new imaging, it will move a total of 0.14 relative to the background nearly 3 WFC pixels. Considering the anticipated astrometric accuracy above, we will achieve excellent (> 20σ) separation down to R 625 = 26. At R 625 = 27, it will still be better than 10σ. The result will be an extremely clean CMD all the way from the WD sequence to the MS. Special Requirements Coordinated Observations Justify Duplications No existing or planned observation covers the region of interest in this proposal, namely the region in which we have identified two dozen He WDs and 15 CVs using ACS/WFC in the Cycle 13 GO program (PI Anderson), with sufficient sensitivity to achieve the scientific objectives of the present proposal. The GO ACS/WFC observations (PI Sarajedini) are much too shallow, as are all existing WFPC2 observations. The GO program itself has insufficient exposure in the two broad bands (F435W and F625W) to achieve the science goals of the present proposal. The F336W UVIS parallels in the GO (Rich) program include only about 1/4 as much exposure time as is needed in that filter. Also, they are not centered on the cluster, and miss about 1/2 of the area that we aim to cover (and therefore many of the He WDs, for example). 8

11 Past HST Usage Cool is PI on GO-12354, a Cycle 18 program to search for He WDs in NGC The final set of images were taken in November 2011, and a first-pass analysis is complete. Thus it will not interfere with the effort required on the proposed program. While Co-I Anderson is involved in many ongoing HST programs, the effort involved here should not require a significant commitment of time for innovation, as it will involve the use of in-house routines that will already have been written for other programs. Cohn and Lugger have commitments to HST programs GO and GO AR The work on GO has resulted in a paper that is is now published. The work on AR is in support of the PI work that is being done at Harvard. Major publications from past HST observations: Haurberg, Lubell, Cohn, Lugger, Anderson, Cool, & Serenelli 2010, ApJ, 722, 158 Cohn, Lugger, Couch, Anderson, Cool, van den Berg, Bogdanov, Heinke, & Grindlay 2010, ApJ, 722, 20 Anderson & van der Marel 2010, ApJ, 710, 1032 Strickler, Cool, Anderson, Cohn, Lugger, & Serenelli 2009, ApJ, 699, 40 Anderson, Piotto, King, Bedin, & Guhathakurta 2009, ApJ, 697, L58 Lugger, Cohn, Heinke, Grindlay, & Edmonds 2007, ApJ, 657, 286L Haggard, Cool, Anderson, Edmonds, Callanan, Heinke, Grindlay, & Bailyn 2004, ApJ, 613, 512 9