Very Low Mass White Dwarfs from Kepler

Good point, about Regulus.

Lazarus wrote:

As for the paper not saying that the system is young and cooling down, it doesn't say this is not the case either.

It mentions this at the end of the first paragraph in the discussion.

Quote :

This could indicate prior evolution of the orbits or unique structure of the companions and we are watching the objects slowly cool.

Thanks Lazarus, I understand now.

EPE page for KOI-74 b lists a radius of 0.4 Jupiter radii.
I think we're not in planetary regime, since hydrogen/helium dwarf use to yield such shrunken radii.

I always recall the LY Aqr's case where degenerate secondary had mass of 27 Jupiter masses and 3 times the radius of Jupiter.

Here how I rendered KOI-74 b:

Nice rendering, Edasich (just a minor tid-bit, you wrote KIC twice =p)

*doh*. Fixed.

Borislav wrote:

http://arxiv.org/abs/1001.3420

Now up to v3, with the correct KOI-74 curve shown...

If confirmed, this analysis is nothing short of amazing !


A paradox resolved using observations of Doppler boosting in Kepler lightcurves

Authors: Marten H. van Kerkwijk (1 and 2), Saul A. Rappaport (3), Rene P. Breton (1 and 2), Stephen Justham (1), Philipp Podsiadlowski (4), Zhanwen Han (5) ((1) KIAA/PKU, (2) UofT, (3) MIT, (4) Oxford, (5) Yunnan Obs./NAOC)
(Submitted on 26 Jan 2010)

Quote :

Abstract: Among the initial results from Kepler are two striking lightcurves, for KOI-74 and KOI-81, in which the relative depths of the primary and secondary eclipses show that the more compact, less luminous object is hotter than its stellar host. That result becomes particularly intriguing because a substellar mass is derived for the secondary in KOI-74, which would make the high temperature challenging to explain; in KOI-81, the mass range for the companion is also consistent with a substellar object. We re-analyze the Kepler data and demonstrate that both companions are likely to be white dwarfs. We also find that the photometric data for KOI-74 show a modulation in brightness as the more luminous star orbits, due to Doppler boosting. The magnitude of the effect is sufficiently large that we can use it to infer a radial velocity amplitude accurate to 1 km/s. As far as we are aware, this is the first time a radial-velocity curve has been measured photometrically. Combining our velocity amplitude with the inclination derived from the eclipses and a primary mass estimate based on its spectral type, we infer a secondary mass of about 0.2 Msun. We use our estimates to consider the likely evolutionary histories and previous mass-transfer episodes of these binary systems.

Hang on, what? Kepler detected, photometrically, the Doppler blue-shifting of the star?

O... M... G...

I take back what I said about photometry being unable to allow for the true mass to be determined. This is absolutely amazing!

This is for KOI-74b, does thire made the same measures for KOI-81b?

Yes, and got a mass of ~0.3 solar masses.

Sirius_Alpha wrote:

Yes, and got a mass of ~0.3 solar masses.

it is consistent with a white dwarf density?

have you got links for this papers?

Paper is here.
http://arxiv.org/abs/1001.4539

I don't know about the density of the objects. I haven't read the paper too in-depth or calculated it myself.

We know that the radii of a white dwarf is inversly proportional to the cubic root of its mass. I made comparison between several white dwarfs and our two strange objects. I find for KOI-74b a theoretical radii included between 0.011R_Sol and 0.029R_Sol, and for KOI-81 b between 0.009R_Sol and 0.026R_Sol. It is lower 4,6 times for KOI-74b and 13 times for KOI-81b than the measured value.

How can they say that "A paradox resolved"?

Doppler boosting, that's pretty neat. Relativity to the rescue then?

Low-mass white dwarfs tend to be helium white dwarfs, if you are extrapolating from carbon-oxygen white dwarfs you are going to get theoretical radii which are too small. Furthermore the R~M^-1/3 relation is an approximation. Judging by the evolutionary tracks from this paper, the properties of the companions do not seem to be wildly inconsistent with the theoretical values.

But KOI-81b are bigger AND more massive than KOI-74b! For other white dwarves, if the star is more massive, it is smaller...

A lot of the white dwarf mass radius relationships are derived for zero-temperature objects, for massive white dwarfs this isn't so much of an issue. For lower mass white dwarfs, the actual temperature of the object has to be taken into account: you need more parameters than just the mass to determine the radius.

KOI-81b has a higher temperature than KOI-74b, and certainly it appears to fit quite well to the evolutionary tracks.

I thought that for degenerate matter, an increase in temperature did not result in an increase in radius? Which if I recall, is why when white dwarfs reach a specific mass, they detonate. Fusion starts and since the white dwarf cannot expand to accommodate the heating, the rest of the white dwarf undergoes fusion too.

A lot of red dwarves are a little less compact than KOI-81b...

IIRC degenerate matter has a weak temperature dependence.

The question is of course the star's structure - the core is presumably degenerate, but how important are the outer layers where degeneracy is less significant. For lower mass white dwarfs, the outer layers become more significant, as stated in the paper I quoted upthread.

KOI-74 ans KOI-81, blue Stragglers?
Kepler is able to detect gravitationnal lensing

Quote :

Kepler's first major discoveries are two hot objects orbiting stars in its
field. These may be the cores of stars that have each been eroded or disrupted
by a companion star. The companion, which is the star monitored today, is
likely to have gained mass from its now-defunct partner, and can be considered
to be a blue straggler. KOI-81 is almost certainly the product of stable mass
transfer; KOI-74 may be as well, or it may be the first clear example of a blue
straggler created throughthree-body interactions.

We show that mass transfer binaries are common enough that Kepler should
discover ~1000 white dwarfs orbiting main sequence stars. Most, like KOI-74 and
KOI-81, will be discovered through transits, but many will be discovered
through a combination of gravitational lensing and transits, while lensing will
dominate for a subset. In fact, some events caused by white dwarfs will have
the appearance of "anti-transits" --i.e., short-lived enhancements in the
amount of light received from the monitored star. Lensing and other mass
measurements methods provide a way to distinguish white dwarf binaries from
planetary systems. This is important for the success of Kepler's primary
mission, in light of the fact that white dwarf radii are similar to the radii
of terrestrial planets, and that some white dwarfs will have orbital periods
that place them in the habitable zones of their stellar companions. By
identifying transiting and/or lensing white dwarfs, Kepler will conduct
pioneering studies of white dwarfs and of the end states of mass transfer. It
may also identify orbiting neutron stars or black holes. The calculations
inspired by the discovery of KOI-74 and KOI-81 have implications for
ground-based wide-field surveys as well as for future space-based surveys.

Ok, not exactly exoplanets but quite spectacular anyway.

SOPHIE velocimetry of Kepler transit candidates. I. Detection of the low-mass white dwarf KOI-74b

Quote :

The Kepler mission has detected transits and occultations of a hot compact object around an early-type star, the Kepler Object of Interest KOI-74. The mass of this transiting object was photometrically assessed in a previous study using the presence of the relativistic beaming effect (so-called `Doppler boosting') in the light curve. Our aim was to provide a spectroscopic validation of this pioneering approach. We measured the radial velocity variations of the A1V star KOI-74 with the SOPHIE spectrograph at the 1.93-m telescope of the Observatoire de Haute-Provence (France). Radial velocity measurements of this star are challenging to obtain because of the high level of stellar pulsations and the few available spectral lines. Using a technique dedicated to early-type main-sequence stars, we measure radial velocity variations compatible with a companion of mass 0.252 +/- 0.025 Msun, in good agreement with the value derived from the Kepler light curve. This work strengthens the scenario suggesting that KOI-74 is a blue straggler orbited by a stellar core despoiled of its envelope, the low-mass white dwarf KOI-74b.

http://arxiv.org/abs/1009.1873

Nice to get more confirmation of the nature of the system. As they state, not many systems like this are known.

There it goes again.
A Third Hot White Dwarf Companion Detected by Kepler