Flickering in AGB stars: probing the nature of accreting companions

Snaid, S.; Zijlstra, A. A.; McDonald, Iain; Barker, Helen; Marsh, T. R.; Dhillon, V. S.

doi:10.1093/mnras/sty660

Cited by 6 publications

(6 citation statements)

References 45 publications

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“…We note that much smaller-amplitude photometric variations have been recently reported at optical wavelengths (e.g., 0.06 mag peak-to-peak in the u ′ band with a typical timescale of 10 min) for Y Gem using ground-based differential photometry (Snaid et al 2018). Although a comparison with differential photometry of two field stars indicates that these variations are significant, the effects of aperture photometry in variable seeing conditions can produce spurious fluctuations that are not easily quantified.…”

Section: Results and Analysismentioning

confidence: 61%

“…Assuming that the combined luminosity of the hot and cool blackbodies results from accretion, i.e., L acc = 13 L ⊙ during exposures 40–50, then taking L acc ≲ GṀ acc M c / R c , where Ṁ acc is the accretion rate, and M c ~ 0.35 M ⊙ and R c = 0.44 R ⊙ (as above), we find that Ṁ acc > 5×10 −7 M ⊙ yr −1 . This relatively large accretion rate makes wind-accretion mechanisms such as Bondi-Hoyle or wind Roche-lobe overflow (e.g., Chen et al 2017, Huarte-Espinosa et al 2013) unfeasible because Y Gem does not have a detectable wind – the very weak, narrow CO J=2–1 emission line detected towards it likely arises in an extended disk (Setal11); and the infrared-excess is negligible or very low (McDonald et al 2012) implying a mass-loss rate of ≲10 −7 M ⊙ yr −1 (Snaid et al 2018). Therefore, either the primary overflows its Roche lobe and transfers material to the accreting star, or accretion occurs within a common-envelope (CE) configuration.…”

Section: Discussionmentioning

confidence: 99%

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Binarity and Accretion in AGB Stars: HST/STIS Observations of UV Flickering in Y Gem

Sahai

Contreras

Mangan

et al. 2018

ApJ

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Binarity is believed to dramatically affect the history and geometry of mass loss in AGB and post-AGB stars, but observational evidence of binarity is sorely lacking. As part of a project to search for hot binary companions to cool AGB stars using the GALEX archive, we discovered a late-M star, Y Gem, to be a source of strong and variable UV and X-ray emission. Here we report UV spectroscopic observations of Y Gem obtained with the that show strong flickering in the UV continuum on time-scales of ≲ 20 s, characteristic of an active accretion disk. Several UV lines with P-Cygni-type profiles from species such as Si IV and C IV are also observed, with emission and absorption features that are red- and blue-shifted by velocities of ~500 km s from the systemic velocity. Our model for these (and previous) observations is that material from the primary star is gravitationally captured by a companion, producing a hot accretion disk. The latter powers a fast outflow that produces blue-shifted features due to absorption of UV continuum emited by the disk, whereas the red-shifted emission features arise in heated infalling material from the primary. The outflow velocities support a previous inference by Sahai et al. (2015) that Y Gem's companion is a low-mass main-sequence star. Blackbody fitting of the UV continuum implies an accretion luminosity of about 13 , and thus a mass-accretion rate> 5 × 10 yr; we infer that Roche lobe overflow is the most likely binary accretion mode for Y Gem.

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Section: Results and Analysismentioning

confidence: 61%

Section: Discussionmentioning

confidence: 99%

Binarity and Accretion in AGB Stars: HST/STIS Observations of UV Flickering in Y Gem

Sahai

Contreras

Mangan

et al. 2018

ApJ

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“…In three out of four XMM-Newton observations (except in ObsID 0763050401), Y Gem shows L X /L UV < 0.1, and thus implies the existence of an accretion disk in Y Gem. The disk scenario is also supported by the photometric flickering in Y Gem (Snaid et al 2018).…”

Section: Estimating the System Properties Of Y Gemmentioning

confidence: 82%

“…For example, Sion et al (2002) found an outflow velocity of 170 km s −1 for RW Hydrae, which is a WD SySt. Moreover, the recent discovery of the photometric flickering in Y Gem also favors the existence of a WD companion (Snaid et al 2018). We then discuss if the accreting WD scenario can account for various observed properties of Y Gem as follows.…”

Section: Wd-syst Scenariomentioning

confidence: 99%

“…However, for an AGB star, the RLOF mass transfer to a 0.35M e MS companion is dynamically unstable (see Section 4.1.1 for details). Third, the photometric flickering found in Y Gem (Snaid et al 2018) indicates disk accretion onto a WD instead of an MS star. These inconsistencies deserve further investigations to improve our understanding of the nature of Y Gem, and more importantly, shed light on the origin and evolution of similar systems.…”

Section: Introductionmentioning

confidence: 97%

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Y Gem: A White Dwarf Symbiotic Star?

Shao

et al. 2022

ApJ

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In this work we conduct a thorough investigation of the X-ray and ultraviolet (UV) properties of Y Gem based on six archival XMM-Newton and Chandra observations to explore the nature of the system. The results show that Y Gem has strong (1032–34 erg s−1) X-ray emission, including a hard (with a maximum emission temperature of 8–16 keV) and a soft (with emission temperatures of 0.02–0.2 and 0.2–0.9 keV) component. The integrated UV luminosity of Y Gem reaches ∼1035 erg s−1. We show that the previous asymptotic giant branch-main-sequence (AGB-MS) Roche-lobe overflow (RLOF) scenario is dynamically unstable and can hardly explain the ∼10 keV X-ray emission temperature. We propose Y Gem as a symbiotic star, where a white dwarf (WD) accretes from its AGB companion based on its X-ray and UV properties. We make numerical simulations to examine the evolutionary history of this system. The simulations can produce the observed properties of Y Gem in the wind WRLOF scenario. An ∼0.8M ⊙ WD with a ∼1.0–1.8M ⊙ companion in a ∼2000–32,000 day initial orbit may evolve to a Y Gem-like system. Our finding implies a potential population of symbiotic stars that may have been misclassified as AGB-MS binaries. What is more, their high mass accretion rates may enable mass accumulation to the WD and makes them candidates of Type Ia supernovae progenitors.

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Planetary Nebulae, Morphology and Binarity, and the relevance to AGB Stars

Sahai

2018

Proc. IAU

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The dramatic transformation of the spherical outflows of AGB stars into the extreme aspherical geometries seen during the planetary nebula (PN) phase is widely believed to be linked to binarity and is likely driven by the associated production of fast jets and central disks/torii. We first briefly summarize results from the imaging surveys of large samples of young PNe and pre-PNe with HST which show that almost all objects have bipolar, multipolar and elliptical morphologies, with widespread presence of point-symmetric structure. We describe a relatively new technique of using UV photometic observations of large AGB star samples to search for binarity and associated accretion activity, and follow-up studies using UV spectroscopy and X-ray observations. We present results from studies of individual objects in transition to the PN phase, highlighting observational techniques being used to determine jet properties that can constrain the accretion modes that power these jets.

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Flickering in AGB stars: probing the nature of accreting companions

Cited by 6 publications

References 45 publications

Binarity and Accretion in AGB Stars: HST/STIS Observations of UV Flickering in Y Gem

Binarity and Accretion in AGB Stars: HST/STIS Observations of UV Flickering in Y Gem

Y Gem: A White Dwarf Symbiotic Star?

Planetary Nebulae, Morphology and Binarity, and the relevance to AGB Stars

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