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The mass-transfer formation frequency of blue straggler stars in the old open cluster NGC 188

The mass-transfer formation frequency of blue straggler stars in the old open cluster NGC 188
The mass-transfer formation frequency of blue straggler stars in the old open cluster NGC 188
The formation of blue straggler stars (BSSs), commonly categorized as stars bluer and brighter than the main sequence turnoff, has puzzled astronomers since their first detection over sixty years ago. The well-studied BSS population of the old (7 Gyr) open cluster NGC 188 has the potential to settle outstanding issues surrounding the frequency of different BSS formation mechanisms. NGC 188 contains 21 BSSs: 15 long-period single-lined binaries, two short-period double-lined binaries, and four non-velocity variables. We present results of the Hubble Space Telescope far-ultraviolet (FUV) ACS/SBC survey of the NGC 188 BSS population. This survey aims to detect white dwarf (WD) companions of BSSs that are indicative of a mass-transfer formation history. We directly detect FUV excesses consistent with four hot WD companions (Teff ≥ 12,000 K). We infer the presence of three additional WD companions with temperatures between 11,000-12,000 K. Since WDs cool as they age, these results indicate that seven BSSs formed through mass transfer within the past 400 Myr. These WD detections set a lower limit mass-transfer formation frequency of 33%. After taking into account other potential formation mechanisms we conclude that 14 long-period binary BSSs likely formed through mass transfer, setting a total NGC 188 BSS mass-transfer formation frequency of 67%. Comparing these results to a sophisticated N-body model of NGC 188 implies that binary population synthesis models underproduce mass transfer products, and the parameterization of stable mass transfer may need to be revisited. Finally, when comparing the optical CMD position of young BSSs to the zero-age main sequence (ZAMS), we find that distance from the ZAMS is not necessarily equivalent to BSS age. One must use caution before using standard single-star isochrones to age luminous BSSs.Support for Program number 12492 was provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555. This work was funded by the National Science Foundation grant AST-0908082 to the University of Wisconsin-Madison.
Gosnell, Natalie M.
ca36477c-cf40-44c0-a70e-b4921b383780
Mathieu, Robert D.
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Sills, Alison
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Geller, Aaron M.
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Leigh, Nathan
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Knigge, Christian
ac320eec-631a-426e-b2db-717c8bf7857e
Gosnell, Natalie M.
ca36477c-cf40-44c0-a70e-b4921b383780
Mathieu, Robert D.
f8406264-e1b5-41be-9074-b612cd9e967f
Sills, Alison
2f7067f6-89a0-4738-b60e-a25e8b7a434d
Geller, Aaron M.
358fcd0f-def3-47e2-8595-ab16cdb12f3a
Leigh, Nathan
90069e68-a317-4cce-92e3-2401e3518751
Knigge, Christian
ac320eec-631a-426e-b2db-717c8bf7857e

Gosnell, Natalie M., Mathieu, Robert D., Sills, Alison, Geller, Aaron M., Leigh, Nathan and Knigge, Christian (2015) The mass-transfer formation frequency of blue straggler stars in the old open cluster NGC 188. American Astronomical Society, AAS Meeting #225, United States. 04 - 08 Jan 2015.

Record type: Conference or Workshop Item (Paper)

Abstract

The formation of blue straggler stars (BSSs), commonly categorized as stars bluer and brighter than the main sequence turnoff, has puzzled astronomers since their first detection over sixty years ago. The well-studied BSS population of the old (7 Gyr) open cluster NGC 188 has the potential to settle outstanding issues surrounding the frequency of different BSS formation mechanisms. NGC 188 contains 21 BSSs: 15 long-period single-lined binaries, two short-period double-lined binaries, and four non-velocity variables. We present results of the Hubble Space Telescope far-ultraviolet (FUV) ACS/SBC survey of the NGC 188 BSS population. This survey aims to detect white dwarf (WD) companions of BSSs that are indicative of a mass-transfer formation history. We directly detect FUV excesses consistent with four hot WD companions (Teff ≥ 12,000 K). We infer the presence of three additional WD companions with temperatures between 11,000-12,000 K. Since WDs cool as they age, these results indicate that seven BSSs formed through mass transfer within the past 400 Myr. These WD detections set a lower limit mass-transfer formation frequency of 33%. After taking into account other potential formation mechanisms we conclude that 14 long-period binary BSSs likely formed through mass transfer, setting a total NGC 188 BSS mass-transfer formation frequency of 67%. Comparing these results to a sophisticated N-body model of NGC 188 implies that binary population synthesis models underproduce mass transfer products, and the parameterization of stable mass transfer may need to be revisited. Finally, when comparing the optical CMD position of young BSSs to the zero-age main sequence (ZAMS), we find that distance from the ZAMS is not necessarily equivalent to BSS age. One must use caution before using standard single-star isochrones to age luminous BSSs.Support for Program number 12492 was provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555. This work was funded by the National Science Foundation grant AST-0908082 to the University of Wisconsin-Madison.

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Published date: 1 January 2015
Venue - Dates: American Astronomical Society, AAS Meeting #225, United States, 2015-01-04 - 2015-01-08

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Local EPrints ID: 429000
URI: http://eprints.soton.ac.uk/id/eprint/429000
PURE UUID: 2b2d5495-778e-4949-8394-e39f84457eff

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Date deposited: 18 Mar 2019 17:30
Last modified: 15 Jan 2020 17:40

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Contributors

Author: Natalie M. Gosnell
Author: Robert D. Mathieu
Author: Alison Sills
Author: Aaron M. Geller
Author: Nathan Leigh

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