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Implications for the formation of blue straggler stars from HST ultraviolet observations of NGC 188

Implications for the formation of blue straggler stars from HST ultraviolet observations of NGC 188
Implications for the formation of blue straggler stars from HST ultraviolet observations of NGC 188
We present results of a Hubble Space Telescope (HST) far-ultraviolet (FUV) survey searching for white dwarf (WD) companions to blue straggler stars (BSSs) in open cluster NGC 188. The majority of NGC 188 BSSs (15 of 21) are single-lined binaries with properties suggestive of mass-transfer formation via Roche lobe overflow, specifically through an asymptotic giant branch star transferring mass to a main sequence secondary, yielding a BSS binary with a WD companion. In NGC 188, a BSS formed by this mechanism within the past 400 Myr will have a WD companion that is hot and luminous enough to be directly detected as a FUV photometric excess with HST. Comparing expected BSS FUV emission to observed photometry reveals four BSSs with WD companions above 12,000 K (younger than 250 Myr) and three WD companions with temperatures between 11,000 and 12,000 K. These BSS+WD binaries all formed through recent mass transfer. The location of the young BSSs in an optical color–magnitude diagram (CMD) indicates that distance from the zero-age main sequence does not necessarily correlate with BSS age. There is no clear CMD separation between mass transfer-formed BSSs and those likely formed through other mechanisms, such as collisions. The seven detected WD companions place a lower limit on the mass-transfer formation frequency of 33%. We consider other possible formation mechanisms by comparing properties of the BSS population to theoretical predictions. We conclude that 14 BSS binaries likely formed from mass transfer, resulting in an inferred mass-transfer formation frequency of approximately 67%.
1538-4357
1-12
Gosnell, Natalie M.
ca36477c-cf40-44c0-a70e-b4921b383780
Mathieu, Robert D.
f8406264-e1b5-41be-9074-b612cd9e967f
Geller, Aaron M.
358fcd0f-def3-47e2-8595-ab16cdb12f3a
Sills, Alison
2f7067f6-89a0-4738-b60e-a25e8b7a434d
Leigh, Nathan
90069e68-a317-4cce-92e3-2401e3518751
Knigge, Christian
ac320eec-631a-426e-b2db-717c8bf7857e
Gosnell, Natalie M.
ca36477c-cf40-44c0-a70e-b4921b383780
Mathieu, Robert D.
f8406264-e1b5-41be-9074-b612cd9e967f
Geller, Aaron M.
358fcd0f-def3-47e2-8595-ab16cdb12f3a
Sills, Alison
2f7067f6-89a0-4738-b60e-a25e8b7a434d
Leigh, Nathan
90069e68-a317-4cce-92e3-2401e3518751
Knigge, Christian
ac320eec-631a-426e-b2db-717c8bf7857e

Gosnell, Natalie M., Mathieu, Robert D., Geller, Aaron M., Sills, Alison, Leigh, Nathan and Knigge, Christian (2015) Implications for the formation of blue straggler stars from HST ultraviolet observations of NGC 188. The Astrophysical Journal, 814 (2), 1-12. (doi:10.1088/0004-637X/814/2/163).

Record type: Article

Abstract

We present results of a Hubble Space Telescope (HST) far-ultraviolet (FUV) survey searching for white dwarf (WD) companions to blue straggler stars (BSSs) in open cluster NGC 188. The majority of NGC 188 BSSs (15 of 21) are single-lined binaries with properties suggestive of mass-transfer formation via Roche lobe overflow, specifically through an asymptotic giant branch star transferring mass to a main sequence secondary, yielding a BSS binary with a WD companion. In NGC 188, a BSS formed by this mechanism within the past 400 Myr will have a WD companion that is hot and luminous enough to be directly detected as a FUV photometric excess with HST. Comparing expected BSS FUV emission to observed photometry reveals four BSSs with WD companions above 12,000 K (younger than 250 Myr) and three WD companions with temperatures between 11,000 and 12,000 K. These BSS+WD binaries all formed through recent mass transfer. The location of the young BSSs in an optical color–magnitude diagram (CMD) indicates that distance from the zero-age main sequence does not necessarily correlate with BSS age. There is no clear CMD separation between mass transfer-formed BSSs and those likely formed through other mechanisms, such as collisions. The seven detected WD companions place a lower limit on the mass-transfer formation frequency of 33%. We consider other possible formation mechanisms by comparing properties of the BSS population to theoretical predictions. We conclude that 14 BSS binaries likely formed from mass transfer, resulting in an inferred mass-transfer formation frequency of approximately 67%.

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Accepted/In Press date: 6 October 2015
Published date: 1 December 2015
Organisations: Astronomy Group

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Local EPrints ID: 396480
URI: http://eprints.soton.ac.uk/id/eprint/396480
ISSN: 1538-4357
PURE UUID: 12828b99-fb2b-4e37-ba73-f874af56fa49

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Date deposited: 09 Jun 2016 14:27
Last modified: 16 Dec 2019 19:53

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