Substellar companion around a hot subdwarf star

Discovery of a close substellar companion to the hot subdwarf star HD 149382 - The decisive influence of substellar objects on late stellar evolution

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Substellar objects, like planets and brown dwarfs orbiting stars, are by-products of the star formation process. The evolution of their host stars may have an enourmous impact on these small companions. Vice versa a planet might also influence stellar evolution as has recently been argued.Here we report the discovery of a 8-23 Jupiter-mass substellar object orbiting the hot subdwarf HD 149382 in 2.391 days at a distance of only about five solar radii. Obviously the companion must have survived engulfment in the red-giant envelope. Moreover, the substellar companion has triggered envelope ejection and enabled the sdB star to form. Hot subdwarf stars have been identified as the sources of the unexpected ultravoilet emission in elliptical galaxies, but the formation of these stars is not fully understood. Being the brightest star of its class, HD 149382 offers the best conditions to detect the substellar companion. Hence, undisclosed substellar companions offer a natural solution for the long-standing formation problem of apparently single hot subdwarf stars. Planets and brown dwarfs may therefore alter the evolution of old stellar populations and may also significantly affect the UV-emission of elliptical galaxies.

Reading the article, more likely a superplanet rather a brown dwarf.

How does a super-Jovian or brown dwarf (or anything) survive to a 2.391-day orbit around an sdB?

Any other way planets migrate other than by disks?
Could somehow the object orbiting through the extended envelope have caused the star to quickly end its enlarge phase? The drag would have lowered the orbit of the object.

Jean Schneider worried to come back from holidays to update EPE with this detection *LOL*

http://exoplanet.eu/star.php?st=HD+149382

Mass within Superjovian regime.

Sirius_Alpha wrote:

How does a super-Jovian or brown dwarf (or anything) survive to a 2.391-day orbit around an sdB?

Any other way planets migrate other than by disks?
Could somehow the object orbiting through the extended envelope have caused the star to quickly end its enlarge phase? The drag would have lowered the orbit of the object.

Seems to be a case of engulfment in the red giant envelope. Extreme mass ratio between star and planet means mass transfer is unstable, so doesn't form an accretion disc around the planet, but a common envelope. Planet would possibly have gained mass during the inspiral. (Indeed in the paper about the HW Virginis system, it is noted that the red dwarf companion could potentially be a former brown dwarf that became a star by accreting some of the primary's envelope during the red giant stage).

Very interesting evidence that planetary systems can have significant effects on the evolution of their parent stars, and perhaps even the spectrum of entire galaxies...

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HD 149382 b provides evidence that substellar companions can decisively change the evolution of stars, as they trigger extensive mass loss. They could be responsible for the formation of the single hot subdwarf population. These stars are not only numerous in our Galaxy, but also make elliptical galaxies shine in ultraviolet light.

One other thing to notice is the detection method: the other planet known around a single sdB star, V391 Peg b, was discovered by timing of pulsations, while this one was an RV detection - HD 149382 does not seem to be a pulsating sdB star.

I'm not 100% sure how they were able to determine the inclination of the star. I looked over the paper, but wasn't able to follow.

I was also surprised by the cleanness of the RV curve. I would've expected much more noise.

A novel search for low-mass companions to sdB stars does not detect a large number of small-sized (0.12 < r < 0.15 R_Sol) eclipsing secondaries.

Physical Properties of 29 sdB+dM Eclipsing Binaries in Zwicky Transient Facility

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The development of large-scale time-domain surveys provides an opportunity to study the physical properties as well as the evolutionary scenario of B-type subdwarfs (sdB) and M-type dwarfs (dM). Here, we obtained 33 sdB+dM eclipsing binaries based on the Zwicky Transient Facility (ZTF) light curves and Gaia early data release 3 (EDR3) parallaxes. By using the PHOEBE code for light curve analysis, we obtain probability distributions for parameters of 29 sdB+dM. R1, R2, and i are well determined, and the average uncertainty of mass ratio q is 0.08. Our parameters are in good agreement with previous works if a typical mass of sdB is assumed. Based on parameters of 29 sdB+dM, we find that both the mass ratio q and the companion's radius R2 decrease with the shortening of the orbital period. For the three sdB+dMs with orbital periods less than 0.075 days, their companions are all brown dwarfs. The masses and radii of the companions satisfy the mass--radius relation for low-mass stars and brown dwarfs. Companions with radii between 0.12R⊙ and 0.15R⊙ seem to be missing in the observations. As more short-period sdB+dM eclipsing binaries are discovered and classified in the future with ZTF and Gaia, we will have more information to constrain the evolutionary ending of sdB+dM.

Nevertheless at least three systems appear to host low-mass and substellar companions (0.055 < m₂ < 0.062 M_Sol) judging by the small mass ratio near or below 0.13 (assuming a 0.47 M_Sol primary), namely ZTF J162256.66+473051.1, ZTF J193555.33+123754.8 and ZTF J221339.18+445155.8. Measured radii for secondaries are 0.098, 0.18 and 0.2 R_Sol respectively, compatible with either compact and photoevaporating or inflated and irradiated objects.

In any case these are "transiting brown dwarfs" to me...