A spectral classification system for hydrogen-deficient carbon stars

Crawford, Courtney L.; Tisserand, Patrick; Clayton, Geoffrey C.; Soon, J.; Bessell, M. S.; Wood, P. R.; García–Hernández, D. A.; Ruiter, Ashley J.; Seitenzahl, Ivo R.

doi:10.1093/mnras/stad324

Cited by 11 publications

(17 citation statements)

References 98 publications

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“…Our synthetic spectra around 4840 Å show that the C 2 Swan Δv = 0 may dominate the observed spectrum with the CN Violet lines on average stronger than the CH lines for a normal H abundance; establishing a H-deficiency will be difficult. This suggestion is confirmed by Crawford et al (2023) observation that the CH G-band was undetectable in spectra of HdC stars "even those with measurable Balmer lines. "…”

Section: Hydrogen Contentmentioning

confidence: 62%

The Carbon Star DY Persei May Be a Cool R Coronae Borealis Variable

García-Hernández

Rao

Lambert

et al. 2023

ApJ

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Optical and near-IR photometry suggests that the carbon star DY Persei exhibits fadings similar to those of R Coronae Borealis (RCB) variables. Photometric surveys of the Galaxy and Magellanic Clouds uncovered new DY Per variables with infrared photometry identifying them with cool carbon stars, perhaps, with an unusual tendency to shed mass. In an attempt to resolve DY Per’s identity crisis—a cool carbon giant or a cool RCB variable?—we analyze a high-resolution IGRINS H&K-band spectrum of DY Per. The CO first-overtone bands in the K band of DY Per show a high abundance of 18O such that 16O/18O = 4 ± 1, a ratio sharply at odds with published results for regular cool carbon giants with 16O/18O ∼ 1000 but this exceptionally low ratio is characteristic of RCB variables and HdC stars. This similarity suggests that DY Per indeed may be a cool RCB variable. Current opinion considers RCB variables to result from the merger of a He onto a CO white dwarf; observed abundances of these H-deficient stars including the exceptionally low 16O/18O ratios are in fair accord with predicted compositions for white dwarf merger products. An H-deficiency for DY Per is not directly observable but is suggested from the strength of an HF line and an assumption that F may be overabundant, as observed and predicted for RCB stars.

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Section: Hydrogen Contentmentioning

confidence: 62%

The Carbon Star DY Persei May Be a Cool R Coronae Borealis Variable

García-Hernández

Rao

Lambert

et al. 2023

ApJ

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“…Based on Table 5 of Crawford et al (2023), a spectral type of HdC5 or later corresponds to a stellar effective temperature of 4500 K. We thus conclude that IRAS 00450+7401 is a cool R Cor Bor star.…”

Section: Spectroscopic Classificationmentioning

confidence: 66%

“…Figure 1 shows a comparison between optical spectra for IRAS 00450+7401 and the earliest-type matching standard from Crawford et al (2023). Earlier-type standards show a clear mismatch in the position of the steep molecular shelves (e.g., near 4700 Å for IRAS 00450+7401) or a lack of such features altogether, ruling out the HdC4 and earlier classifications (see Figure 4 of Crawford et al 2023).…”

Section: Spectroscopic Classificationmentioning

confidence: 97%

“…Confounding a more exact spectral type estimate for IRAS 00450+7401 is the fact that the Kast spectra were obtained when the source was still recovering from a prolonged and deep dimming event (see Section 3.1). The Kast spectra appear to have been obtained when the source was ∼2.4 mag below maximum light while all spectral standards from Crawford et al (2023) were obtained near maximum light or at most 0.5 mag below it.…”

Section: Spectroscopic Classificationmentioning

confidence: 98%

“…In a recent work Crawford et al (2023) define a spectral classification system for hydrogen-deficient carbon stars, including R Cor Bor stars. While mostly based on observed spectral appearance, the new classification system allows one to make a rough estimate of the temperature for a star of known spectral type.…”

Section: Spectroscopic Classificationmentioning

confidence: 99%

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IRAS 00450+7401 and the Mid-infrared Fade/Burst Cycle of R Coronae Borealis-type Stars

Burris

Melis

Shafter

et al. 2023

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We present optical and infrared imaging and spectroscopy of the R Coronae Borealis-type (R Cor Bor) star IRAS 00450+7401. Optical spectra further confirm its classification as a cool R Cor Bor system, having a hydrogen-deficient carbon star spectral subclass of HdC5 or later. Mid-infrared spectroscopy reveals the typical ∼8 μm “hump” seen in other R Cor Bor stars and no other features. A modern-epoch spectral energy distribution shows bright emission from hot dust having T dust > 600 K. Historical infrared data reveal generally cooler dust color temperatures combined with long-term fading trends, but provide no discernible correlation between flux level and temperature. Investigating the most mid-infrared variable R Cor Bor stars found in IRAS, AKARI, and WISE data reveals similar fading trends, bursts that can show a factor of up to 10 change in flux density between epochs, and blackbody-fit dust color temperatures that span 400–1300 K. While some R Cor Bor stars such as IRAS 00450+7401 appear to undergo fade/burst cycles in the mid-infrared, significant gaps in temporal coverage prevent conclusively identifying any preferred timescale for their mid-infrared variability and circumstellar dust temperature changes.

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The dawn of a new era for dustless HdC stars withGaiaeDR3

Tisserand

Crawford

Clayton

et al. 2022

A&A

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Context. Decades after their discovery, only four hydrogen-deficient carbon (HdC) stars were known to have no circumstellar dust shell. This is in complete contrast to the ∼130 known Galactic HdC stars that are notorious for being heavy dust producers, i.e. the R Coronae Borealis (RCB) stars. Together they form a rare class of supergiant stars that are thought to originate from the merger of CO/He white dwarf (WD) binary systems, otherwise known as the double degenerate scenario. Aims. We searched for new dustless HdC (dLHdC) stars to understand their Galactic distribution, to estimate their total number in the Milky Way and to study their evolutionary link with RCB stars and extreme Helium stars, the final phase of HdC stars. Methods. We used primarily the 2MASS and GAIA eDR3 all-sky catalogues to select candidates that were then followed up spectroscopically. We studied the distribution of known and newly discovered stars in the HR diagram. Results. We discovered 27 new dLHdC stars, one new RCB star, and two new EHe stars. Surprisingly, 20 of the new dLHdC stars share a characteristic of the known dLHdC star HD 148839, having lower atmospheric hydrogen deficiencies. The uncovered population of dLHdC stars exhibit a Bulge-like distribution, like the RCB stars, but show multiple differences from those that indicate they are a different population of HdC stars following its own evolutionary sequence with a fainter luminosity and also a narrow range of effective temperatures, between 5000 and 8000 K. Not all the new dLHdC stars belong to that new population, as we found indication of a current low dust production activity around four of them: the warm F75, F152, C526 and the cold A166. Those could in fact be typical RCB stars passing through a transition time, going in or out of the RCB phase. Conclusions. For the first time we have evidence of a large range of absolute magnitude in the overall population of HdC stars, spanning more than 3 mag. In the favoured formation framework, this is explained by a large range in the initial total WD binary mass which leads to a series of evolutionary sequences with distinct maximum brightness and initial temperature. The cold Galactic RCB stars are also noticeably fainter than the Magellanic ones, possibly due to a difference in metallicity between the original population of stars resulting in different WD mass ratio. The unveiled population of dLHdC stars indicates that the ability to create dust could be linked to the initial total mass. In our Galaxy, there could be as many dLHdC stars as RCB stars.

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A spectral classification system for hydrogen-deficient carbon stars

Cited by 11 publications

References 98 publications

The Carbon Star DY Persei May Be a Cool R Coronae Borealis Variable

The Carbon Star DY Persei May Be a Cool R Coronae Borealis Variable

IRAS 00450+7401 and the Mid-infrared Fade/Burst Cycle of R Coronae Borealis-type Stars

The dawn of a new era for dustless HdC stars withGaiaeDR3

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