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Stellar collisions and mergers in the cores of globular clusters
Physics Reports 311 (1999) 363—369
Stellar collisions and mergers in the cores of globular clusters Michael M. Shara Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
Abstract Rare and exotic stars are being discovered with Hubble Space Telescope (HST) observations in large numbers in the cores of globular clusters. Stellar collisions and/or mergers are likely responsible for many of these bizarre objects. I review the theoretical arguments which predict substantial populations of blue stragglers and cataclysmic binaries — the most ubiquitous of globular cluster core exotica. HST observations confirm that blue stragglers are, as predicted, very common. However, dwarf and classical novae appear to be much rarer. A possible explanation for this apparent discrepancy is that most globular cataclysmic binaries are strongly magnetic, and can effectively hide from all but the deepest HST searches. 1999 Elsevier Science B.V. All rights reserved. PACS: 97.80.!d; 97.20.Rp Keywords: Binaries; Blue stragglers; Cataclysmic binaries
1. Introduction It was common wisdom in the early part of this century that stellar collisions must be so rare that they could be utterly neglected (Jeans, 1928). By the 1970s, this view began to change as globular cluster X-ray sources made their unexpected appearance. Katz (1975) and Clark (1975) noted that the &10 strong X-ray sources amongst the 10 stars of the Galaxy’s globular clusters implied an over abundance of a factor of &1000 relative to the galactic stellar population at large. The elegant tidal capture mechanism of Fabian et al. (1975) provided a simple and logical method for forming very close binaries where one of the two stars is compact. As a degenerate star sweeps by a main sequence field star, tides are raised on the latter which dissipate orbital energy as the stars separate after perihelion passage. For impact parameters x, where 1.5R 4x43R ,
tidal capture into a highly elliptical orbit occurs. Circularization on a time scale of &10 yr then leads to cataclysmic and X-ray binaries with, respectively, white dwarf and neutron star primaries. A detailed population and evolution synthesis model (Di Stefano and Rappaport, 1994) predicts the presence of &100 cataclysmic variables in each of 47 Tucanae and Omega Cen. 0370-1573/99/$ - see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 0 - 1 5 7 3 ( 9 8 ) 0 0 1 1 5 - X
364
M.M. Shara / Physics Reports 311 (1999) 363—369
Impact parameters x41.5R lead to direct, physical collisions between stars (e.g. Benz and
Hills, 1987, Rasio and Shapiro, 1995). The merger product of two colliding, coalescing main sequence stars is widely believed to be a blue straggler. White dwarfs or neutron stars colliding (with small impact parameter) with main sequence stars almost certainly destroy the latter (Shara and Shaviv, 1977; Shara and Regev, 1986). The shock-induced temperature rise and thermonuclear burning liberate several times the main sequence binding energy in a few hundred seconds, dooming the non-degenerate star to rapid dispersal. The fates of non-degenerate stars involved in non-zero impact parameter collisions with degenerate stars is much less certain. A particle-in-cell (PIC) simulation by Soker et al. (1987) suggest that a massive disk might form in orbit around the compact star. As few as 10—100 very close binaries in the core of a globular cluster can drive the dynamical evolution and stellar population history of the 10—10 cluster stars. Hills and Day (1976) were the first to show that thousands of collisions must occur in dense cluster cores over a Hubble time. About 5% of all the stars in 47 Tucanae’s central regions must have undergone a collision, while 40% of the stars in M80’s core have suffered a similar fate.
Fig. 1. The HST color-magnitude diagram for NGC 121. Fortytwo candidate blue stragglers are shown as diamonds.
M.M. Shara / Physics Reports 311 (1999) 363—369
365
Fig. 2. Normalized cumulative radial distributions of several stellar populations in the core of NGC 121.
Is there any observational basis to support these claims of high collision rates? Until the launch of HST globular cluster cores were essentially unresolved in even the best ground-based images. The wide field and planetary cameras and the faint object camera of HST have completely resolved the cores of even the densest galactic and magellanic clusters. Some of the remarkable stellar populations (which support the theoretical collision predictions) discovered in these images are described below.
2. Blue stragglers Blue stragglers have been considered to be anomalous curiosities ever since their detection by Sandage (1953). Very blue stars exhibiting luminosities 10 times larger than the turnoff luminosity of the host globular cluster should not exist according to canonical stellar evolution theory. The detection by Paresce et al. (1991) of a rich blue straggler sequence in the core of 47 Tuc was only the first in a long string of HST detections. Dozens of galactic globular clusters’ cores have now been imaged with HST, and blue stragglers are found in every case. These stars are always strongly centrally concentrated relative to e.g. the sub-giants and the turnoff stars. We show in Fig. 1 the color magnitude diagram of NGC 121, the oldest and most populous star cluster in the small magellanic cloud. The blue straggler sequence is strikingly obvious. Clearly,
366
M.M. Shara / Physics Reports 311 (1999) 363—369
Fig. 3. An F439W (B band) image of the central regions of the dense globular cluster NGC 6093 (M80).
blue stragglers are as common a phenomenon in extra-galactic star clusters as in those belonging to the Milky Way. Fig. 2 is a plot of the relative concentrations of several different star populations in NGC 121. The difference in radial distribution of the blue stragglers from the other populations is highly statistically significant. The simplest explanation for this difference is equipartition of energy: the massive blue stragglers’ low velocity dispersion congregates them deep in the cluster potential well, within a few core radii of the cluster center. The two currently viable theories of blue straggler formation are 1. stellar collisions leading to coalescence, and 2. close binary mergers (Leonard and Livio, 1995; Livio, 1993). Observationally distinguishing between these two possibilities is challenging. Model 1 predicts slowly rotating blue stragglers (Leonard and Livio, 1995) while model 2 suggests that at least some
M.M. Shara / Physics Reports 311 (1999) 363—369
367
Fig. 4. (a) and (b) Quiescence and outburst images of a dwarf nova in M80.
Table 1 Cataclysmic binaries in globular clusters NGC C
NGC NGC NGC NGC NGC NGC NGC NGC NGC NGC NGC NGC NGC NGC NGC NGC NGC
6397 6681H 7078H 6441H 6624H 6293H 1851H 7099H 6752H 104H 6093H 5904 5927H 6637H 6402 6171H 6352H
Other name
M 15
M 30 47 Tuc M 80 M5 M 69 M 14 M 107
C of epochs
C of erupting DN
C of CVs known
Central density (¸ /pc) >
1 5 3 2 3 2 2 3 1 12 6 1 2 2 1 5 2
0 0 0 1 1 0 0 0 0 2 2 1 0 0 0 0 0
3 0 0 1 1 0 0 0 2—4 3 3 1 0 0 1 0 0
5.69 5.42 5.37 5.31 5.24 5.19 5.17 5.05 4.92 4.87 4.82 3.94 3.90 3.83 3.31 3.14 3.05
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M.M. Shara / Physics Reports 311 (1999) 363—369
blue stragglers will be rapidly rotating. A determination of vsini &155 km/s from HST spectra has recently been made for one of the brightest blue stragglers in 47 Tuc (Shara et al., 1997). Further HST spectrographic observations of globular cluster blue stragglers are planned for the near future.
3. Cataclysmic binaries The prediction by Di Stefano and Rappaport (1994) that dozens of bright cataclysmics should exist in the core of 47 Tuc is testable with HST. Two erupting dwarf novae (DN) and zero-nova-like variables have been found to date in over a dozen epochs of observation. If there are, indeed, dozens of cataclysmics in this cluster, then they must be very faint (M (10) and they must erupt very infrequently or not at all. Fig. 3 is an HST image of NGC 6624, which we have also searched for erupting dwarf novae. The one detected DN is shown in Fig. 4 (in quiescence and eruption). Table 1 lists all the clusters that we have searched to date and the number of DN found in each; typically 0,1 or 2. We conclude that DN are as rare in globular clusters as in the field. Grindlay et al. (1995) found several Ha bright stars in the core of NGC 6397 with HST narrow band imaging. Subsequent spectroscopy demonstrates that these objects are cataclysmic, and all have strong HeII 4686 emission lines. A likely explanation is that these stars are magnetic cataclysmics, which do not exhibit DN eruptions. If this is correct, and if most globular cataclysmics resemble these faint, Ha bright objects, then we can reconcile the population synthesis models predictions of many cataclysmics with Table 1. Why should the core of a globular cluster be dominated by magnetic cataclysmics? Grindlay et al. (1995) have suggested that rapidly rotating blue stragglers might evolve to form highly magnetic white dwarfs, which in turn tidally capture main sequence stars to form magnetic binaries in the cores of clusters.
4. Conclusion High-resolution observations with the Hubble Space Telescope have revealed fascinating populations of blue stragglers and cataclysmic binaries in the cores of globular clusters. Tidal capture and close binary merger are probably both active in this environment, but much additional work must be done to clarify the formation and evolution of these stars.
5. For Further Reading: The following reference is also of interest to the reader: Shara et al. (1996)
References Clark, G.W., 1975. Astrophys. J. Lett. 199, L143. Di Stefano, R., Rappaport, S., 1994. Astrophys. J. 423, 274.
M.M. Shara / Physics Reports 311 (1999) 363—369
369
Fabian, A.C., Pringle, J.E., Rees, M.J., 1975. Mon. Not. R. Astron. Soc. 172, 15P. Grindlay, J.E., Cool, A.M., Callanan, P.J., Bailyn, C.D., Cohn, H.N., Lugger, P.M., 1995. Astrophys. J. Lett. 455, L47. Hills, J.G., Day, C.A., 1976. Astrophys. Lett. 17, 87. Jeans, J., 1928. In: Astronomy and Cosmology. Dover, New York, p. 319. Katz, J.I., 1975. Nature 253, 698. Leonard, P.J.T., Livio, M., 1995. Astrophys. J. Lett. 447, L121. Livio, M., 1993. In: R. Saffer, (Eds.), ASP Conf. Ser. 53, Blue Stragglers. San Francisco, ASP, p. 3. Paresce, F., Shara, M.M. et al., 1991. Nature 352, 277. Rasio, F.A., Shapiro, S.L., 1995. Astrophys. J. 438, 887. Sandage, A.R., 1953. Astrophys. J. 58, 63. Shara, M.M., Bergeron, E., Gilliland, R., Saha, A., Petro, L., 1996. Astrophys. J. 471, 804. Shara, M.M., Regev, O., 1986. Astrophys. J. 306, 543. Shara, M.M., Saffer, R.A., Livio, M., 1997. Astrophys. J. Lett. 489, L59. Shara, M.M., Shaviv, G., 1977. Mon. Not. R. Astron. Soc. 179, 705. Soker, N., Regev, O., Livio, M., Shara, M.M., 1987. Astrophys. J. 318, 760.
Stellar collisions and mergers in the cores of globular clusters Michael M. Shara Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
Abstract Rare and exotic stars are being discovered with Hubble Space Telescope (HST) observations in large numbers in the cores of globular clusters. Stellar collisions and/or mergers are likely responsible for many of these bizarre objects. I review the theoretical arguments which predict substantial populations of blue stragglers and cataclysmic binaries — the most ubiquitous of globular cluster core exotica. HST observations confirm that blue stragglers are, as predicted, very common. However, dwarf and classical novae appear to be much rarer. A possible explanation for this apparent discrepancy is that most globular cataclysmic binaries are strongly magnetic, and can effectively hide from all but the deepest HST searches. 1999 Elsevier Science B.V. All rights reserved. PACS: 97.80.!d; 97.20.Rp Keywords: Binaries; Blue stragglers; Cataclysmic binaries
1. Introduction It was common wisdom in the early part of this century that stellar collisions must be so rare that they could be utterly neglected (Jeans, 1928). By the 1970s, this view began to change as globular cluster X-ray sources made their unexpected appearance. Katz (1975) and Clark (1975) noted that the &10 strong X-ray sources amongst the 10 stars of the Galaxy’s globular clusters implied an over abundance of a factor of &1000 relative to the galactic stellar population at large. The elegant tidal capture mechanism of Fabian et al. (1975) provided a simple and logical method for forming very close binaries where one of the two stars is compact. As a degenerate star sweeps by a main sequence field star, tides are raised on the latter which dissipate orbital energy as the stars separate after perihelion passage. For impact parameters x, where 1.5R 4x43R ,
tidal capture into a highly elliptical orbit occurs. Circularization on a time scale of &10 yr then leads to cataclysmic and X-ray binaries with, respectively, white dwarf and neutron star primaries. A detailed population and evolution synthesis model (Di Stefano and Rappaport, 1994) predicts the presence of &100 cataclysmic variables in each of 47 Tucanae and Omega Cen. 0370-1573/99/$ - see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 0 - 1 5 7 3 ( 9 8 ) 0 0 1 1 5 - X
364
M.M. Shara / Physics Reports 311 (1999) 363—369
Impact parameters x41.5R lead to direct, physical collisions between stars (e.g. Benz and
Hills, 1987, Rasio and Shapiro, 1995). The merger product of two colliding, coalescing main sequence stars is widely believed to be a blue straggler. White dwarfs or neutron stars colliding (with small impact parameter) with main sequence stars almost certainly destroy the latter (Shara and Shaviv, 1977; Shara and Regev, 1986). The shock-induced temperature rise and thermonuclear burning liberate several times the main sequence binding energy in a few hundred seconds, dooming the non-degenerate star to rapid dispersal. The fates of non-degenerate stars involved in non-zero impact parameter collisions with degenerate stars is much less certain. A particle-in-cell (PIC) simulation by Soker et al. (1987) suggest that a massive disk might form in orbit around the compact star. As few as 10—100 very close binaries in the core of a globular cluster can drive the dynamical evolution and stellar population history of the 10—10 cluster stars. Hills and Day (1976) were the first to show that thousands of collisions must occur in dense cluster cores over a Hubble time. About 5% of all the stars in 47 Tucanae’s central regions must have undergone a collision, while 40% of the stars in M80’s core have suffered a similar fate.
Fig. 1. The HST color-magnitude diagram for NGC 121. Fortytwo candidate blue stragglers are shown as diamonds.
M.M. Shara / Physics Reports 311 (1999) 363—369
365
Fig. 2. Normalized cumulative radial distributions of several stellar populations in the core of NGC 121.
Is there any observational basis to support these claims of high collision rates? Until the launch of HST globular cluster cores were essentially unresolved in even the best ground-based images. The wide field and planetary cameras and the faint object camera of HST have completely resolved the cores of even the densest galactic and magellanic clusters. Some of the remarkable stellar populations (which support the theoretical collision predictions) discovered in these images are described below.
2. Blue stragglers Blue stragglers have been considered to be anomalous curiosities ever since their detection by Sandage (1953). Very blue stars exhibiting luminosities 10 times larger than the turnoff luminosity of the host globular cluster should not exist according to canonical stellar evolution theory. The detection by Paresce et al. (1991) of a rich blue straggler sequence in the core of 47 Tuc was only the first in a long string of HST detections. Dozens of galactic globular clusters’ cores have now been imaged with HST, and blue stragglers are found in every case. These stars are always strongly centrally concentrated relative to e.g. the sub-giants and the turnoff stars. We show in Fig. 1 the color magnitude diagram of NGC 121, the oldest and most populous star cluster in the small magellanic cloud. The blue straggler sequence is strikingly obvious. Clearly,
366
M.M. Shara / Physics Reports 311 (1999) 363—369
Fig. 3. An F439W (B band) image of the central regions of the dense globular cluster NGC 6093 (M80).
blue stragglers are as common a phenomenon in extra-galactic star clusters as in those belonging to the Milky Way. Fig. 2 is a plot of the relative concentrations of several different star populations in NGC 121. The difference in radial distribution of the blue stragglers from the other populations is highly statistically significant. The simplest explanation for this difference is equipartition of energy: the massive blue stragglers’ low velocity dispersion congregates them deep in the cluster potential well, within a few core radii of the cluster center. The two currently viable theories of blue straggler formation are 1. stellar collisions leading to coalescence, and 2. close binary mergers (Leonard and Livio, 1995; Livio, 1993). Observationally distinguishing between these two possibilities is challenging. Model 1 predicts slowly rotating blue stragglers (Leonard and Livio, 1995) while model 2 suggests that at least some
M.M. Shara / Physics Reports 311 (1999) 363—369
367
Fig. 4. (a) and (b) Quiescence and outburst images of a dwarf nova in M80.
Table 1 Cataclysmic binaries in globular clusters NGC C
NGC NGC NGC NGC NGC NGC NGC NGC NGC NGC NGC NGC NGC NGC NGC NGC NGC
6397 6681H 7078H 6441H 6624H 6293H 1851H 7099H 6752H 104H 6093H 5904 5927H 6637H 6402 6171H 6352H
Other name
M 15
M 30 47 Tuc M 80 M5 M 69 M 14 M 107
C of epochs
C of erupting DN
C of CVs known
Central density (¸ /pc) >
1 5 3 2 3 2 2 3 1 12 6 1 2 2 1 5 2
0 0 0 1 1 0 0 0 0 2 2 1 0 0 0 0 0
3 0 0 1 1 0 0 0 2—4 3 3 1 0 0 1 0 0
5.69 5.42 5.37 5.31 5.24 5.19 5.17 5.05 4.92 4.87 4.82 3.94 3.90 3.83 3.31 3.14 3.05
368
M.M. Shara / Physics Reports 311 (1999) 363—369
blue stragglers will be rapidly rotating. A determination of vsini &155 km/s from HST spectra has recently been made for one of the brightest blue stragglers in 47 Tuc (Shara et al., 1997). Further HST spectrographic observations of globular cluster blue stragglers are planned for the near future.
3. Cataclysmic binaries The prediction by Di Stefano and Rappaport (1994) that dozens of bright cataclysmics should exist in the core of 47 Tuc is testable with HST. Two erupting dwarf novae (DN) and zero-nova-like variables have been found to date in over a dozen epochs of observation. If there are, indeed, dozens of cataclysmics in this cluster, then they must be very faint (M (10) and they must erupt very infrequently or not at all. Fig. 3 is an HST image of NGC 6624, which we have also searched for erupting dwarf novae. The one detected DN is shown in Fig. 4 (in quiescence and eruption). Table 1 lists all the clusters that we have searched to date and the number of DN found in each; typically 0,1 or 2. We conclude that DN are as rare in globular clusters as in the field. Grindlay et al. (1995) found several Ha bright stars in the core of NGC 6397 with HST narrow band imaging. Subsequent spectroscopy demonstrates that these objects are cataclysmic, and all have strong HeII 4686 emission lines. A likely explanation is that these stars are magnetic cataclysmics, which do not exhibit DN eruptions. If this is correct, and if most globular cataclysmics resemble these faint, Ha bright objects, then we can reconcile the population synthesis models predictions of many cataclysmics with Table 1. Why should the core of a globular cluster be dominated by magnetic cataclysmics? Grindlay et al. (1995) have suggested that rapidly rotating blue stragglers might evolve to form highly magnetic white dwarfs, which in turn tidally capture main sequence stars to form magnetic binaries in the cores of clusters.
4. Conclusion High-resolution observations with the Hubble Space Telescope have revealed fascinating populations of blue stragglers and cataclysmic binaries in the cores of globular clusters. Tidal capture and close binary merger are probably both active in this environment, but much additional work must be done to clarify the formation and evolution of these stars.
5. For Further Reading: The following reference is also of interest to the reader: Shara et al. (1996)
References Clark, G.W., 1975. Astrophys. J. Lett. 199, L143. Di Stefano, R., Rappaport, S., 1994. Astrophys. J. 423, 274.
M.M. Shara / Physics Reports 311 (1999) 363—369
369
Fabian, A.C., Pringle, J.E., Rees, M.J., 1975. Mon. Not. R. Astron. Soc. 172, 15P. Grindlay, J.E., Cool, A.M., Callanan, P.J., Bailyn, C.D., Cohn, H.N., Lugger, P.M., 1995. Astrophys. J. Lett. 455, L47. Hills, J.G., Day, C.A., 1976. Astrophys. Lett. 17, 87. Jeans, J., 1928. In: Astronomy and Cosmology. Dover, New York, p. 319. Katz, J.I., 1975. Nature 253, 698. Leonard, P.J.T., Livio, M., 1995. Astrophys. J. Lett. 447, L121. Livio, M., 1993. In: R. Saffer, (Eds.), ASP Conf. Ser. 53, Blue Stragglers. San Francisco, ASP, p. 3. Paresce, F., Shara, M.M. et al., 1991. Nature 352, 277. Rasio, F.A., Shapiro, S.L., 1995. Astrophys. J. 438, 887. Sandage, A.R., 1953. Astrophys. J. 58, 63. Shara, M.M., Bergeron, E., Gilliland, R., Saha, A., Petro, L., 1996. Astrophys. J. 471, 804. Shara, M.M., Regev, O., 1986. Astrophys. J. 306, 543. Shara, M.M., Saffer, R.A., Livio, M., 1997. Astrophys. J. Lett. 489, L59. Shara, M.M., Shaviv, G., 1977. Mon. Not. R. Astron. Soc. 179, 705. Soker, N., Regev, O., Livio, M., Shara, M.M., 1987. Astrophys. J. 318, 760.