Very Low Mass White Dwarfs from Kepler

Not really exoplanets but really interesting.

Kepler Observations of Transiting Hot Compact Objects
Jason Rowe, W. J. Borucki, D. Koch, Kepler Team
NASA Ames Research Center.

Quote :

We present Kepler lightcurves of two A spectral class stars which show hot, compact transiting companions. Our analysis of 45 days of high duty cycle, ultra precise photometry show the companions have radii of 40% and 90% that of Jupiter based and effective temperatures greater than 10 000K based on the transit and eclipse lightcurve profiles. These objects have properties similar to white dwarfs as they are compact and hot. The lightcurves also suggest the companions have masses less than 10% of the Sun. Kepler was selected as the 10th mission of the Discovery Program. Funding for this mission is provided by NASA, Science Mission Directorate.

exoplanet wrote:

Not really exoplanets but really interesting.

Kepler Observations of Transiting Hot Compact Objects
Jason Rowe, W. J. Borucki, D. Koch, Kepler Team
NASA Ames Research Center.

Quote :

We present Kepler lightcurves of two A spectral class stars which show hot, compact transiting companions. Our analysis of 45 days of high duty cycle, ultra precise photometry show the companions have radii of 40% and 90% that of Jupiter based and effective temperatures greater than 10 000K based on the transit and eclipse lightcurve profiles. These objects have properties similar to white dwarfs as they are compact and hot. The lightcurves also suggest the companions have masses less than 10% of the Sun. Kepler was selected as the 10th mission of the Discovery Program. Funding for this mission is provided by NASA, Science Mission Directorate.

So what do we call these companions anyway? They're not brown dwarfs -- too hot. I think we need a new category.

Suggestions anyone?

Wow. That's so strange. White dwarfs have radii of far less than this. 0.1 solar masses is the late red dwarf range, but red dwarfs are usually larger than this.

What are these o_O??

It looks like a new record of among lowest mass white dwarfs

http://arxiv.org/abs/astro-ph/0611498

Quote :

The Lowest Mass White Dwarf
Abstract: Extremely low mass white dwarfs are very rare objects likely formed in compact binary systems. We present MMT optical spectroscopy of 42 low mass white dwarf candidates serendipitously discovered in a survey for hypervelocity B-type stars. One of these objects, SDSS J0917+46, has Teff= 11,288 \pm 72 K and log g = 5.48 \pm 0.03; with an estimated mass of 0.17 M_sun, it is the lowest gravity/mass white dwarf currently known. However, 40 of the low mass candidates are normal DA white dwarfs with apparently inaccurate SDSS g magnitudes. We revisit the identification of low mass white dwarf candidates previously found in the SDSS, and conclude that four objects have M < 0.2 M_sun. None of these white dwarfs show excess emission from a binary companion, and radial velocity searches will be necessary to constrain the nature of the unseen companions.

Check this graph that represents the Mass-Radius relationship for white dwarfs.

http://en.wikipedia.org/wiki/File:WhiteDwarf_mass-radius.jpg

Look at how fast the radius grows as the mass decreases. At the very limit of the graph, around 0.05 solar masses, you get a radius of 0.036 solar. This is very close to the lower limit of 40% the radius of Jupiter (0.1 Sun * 0.4 = 0.04 Sun Radius).

Could these be very low mass white dwarfs that resulted from close-in binary evolution. Is there a lower limit for the mass of a white-dwarf ?

It's interesting to note that the Kepler abstract never referred to these objects as white dwarfs. So perhaps this is a new type of object yet to be named.

Borislav, are you sure that the white dwarfs referenced in the "The Lowest Mass White Dwarf" abstract is the same type of object mentioned in the Kepler paper?

Oh wow, that is cool.

Call it firedwarfs.
Could they be roasted Brown Dwarfs? Is that possible?

Well evolution of close binary stars can produce a whole bunch of weird objects, such as undermassive white dwarfs, subdwarf B stars, rapidly rotating stars, symbiotic stars, etc. For example there seem to be several examples of X-ray binaries containing white dwarf donors that have been reduced to planetary masses.

Not entirely surprising that Kepler has turned up a few such examples. One of the things to watch out for in terms of white dwarf transits is the effect of gravitational lensing, which should be fairly significant.

http://arxiv.org/abs/1001.3420

Quote :

Kepler photometry has revealed two unusual transiting companions orbiting an early A-star and a late B-star. In both cases the occultation of the companion is deeper than the transit. The occultation and transit with follow-up optical spectroscopy reveal a 9400 K early A-star, KOI-74 (KIC 6889235), with a companion in a 5.2 day orbit with a radius of 0.08 Rsun and a 10000 K late B-star KOI-81 (KIC 8823868) that has a companion in a 24 day orbit with a radius of 0.2 Rsun. We infer a temperature of 12250 K for KOI-74b and 13500 K for KOI-81b.
We present 43 days of high duty cycle, 30 minute cadence photometry, with models demonstrating the intriguing properties of these object, and speculate on their nature.

KOI-81 b looks like hypothetical blue dwarf star, but blue dwarves cannot existe in our univers...

Whats really Odd is that the chart for KOI-74 gives its mass as ~ 3 Mj.
Obviously incorrect.

Blue Hot Jupiter anyone...

TheoA wrote:

Whats really Odd is that the chart for KOI-74 gives its mass as ~ 3 Mj.

The version of the paper I downloaded gives that as a lower limit, with a higher limit being ~10 times as much.

A WD that migrated close to the parent star and got stripped of materials. Then it puffed up? That what I read on the net as a suggestion.

What does KOI stand for? Is it a Keppler's catalog?

You tend to get undermassive white dwarfs in binary systems because of mass transfer from the giant star to the companion. My guess is that we're looking at particularly extreme end products of this process.

At present the masses are estimated from ellipsoidal variations of the host stars and have large error bars, it is mentioned at the end of the paper that radial velocity measurements are planned.

KOI-81b could be a low mas white dwarf.

But even ~ 30 Mj is staggeringly low.

Also doesn't it say that the system is very young and thought to still be cooling down.

KOI-74 could instead be the first transiting hydrogen-helium dwarf. The extreme low-mass is tricky, because also other objects (I remember the low-mass binary LY Aquarii) have turned out to be anything but planetary/brown dwarf object, despite the low mass.

It's just stellar degenerates. Interesting finding, a bit of shame about no coordinates or distances.

I was going to put them in my Celestia scripts but I casually found the second object only, in a Kepler-related page...

I can see how a white dwarf can be reduced to planetary mass due to accretion of matter onto a neutron star companion, but... these objects orbit A and B type main sequence stars. Accretion of mass would work the other way around if it were significant here in the system.

Also HD 44179 hosts a similar companion, I think...

Surely with those temperatures, these cannot be planets or brown dwarfs, even if "super-irradiated"...

KOI-8B could be a small star formed by accretion in the protoplanetary disc the huge star.

We know that the more a star is massive, the more its disc protoplanétaire is also. we also know that this disc can give birth to monsters as CoRoT-3b, why not objects of stellar mass?

A core of heavy elements would form in first in a scenario by accretion, preventing nuclear fusion indoors. Reaction will occur then in layer, as in a hypothetical blue dwarf.

This scenario is mine and I do not know if it is well, I am only a student. But possibility that an object of stellar mass could form as a planet thrills me a lot.

Stalker wrote:

We know that the more a star is massive, the more its disc protoplanétaire is also. we also know that this disc can give birth to monsters as CoRoT-3b, why not objects of stellar mass?

What you describe as a deuterium fusing brown dwarf, which will have effective temperatures of 500+ on their own, plus a decent irradiation from the star.

The problem is that these objects Kepler found aren't just 1,000 or 2,000 K, they're 12,500 - 13,500 K. This is much more than a brown dwarf can provide. These companions are hotter than their host stars(!). As Edasich points out,

Edasich wrote:

Surely with those temperatures, these cannot be planets or brown dwarfs, even if "super-irradiated"...

Also blue dwarfs are suspected to be red dwarf stars that have exhausted the hydrogen in their core, skipping the red giant phase. Even red dwarfs have masses greater than these objects Kepler is finding. (as a side question, what evolutionary path do brown dwarfs follow in the deep future?). As you point out, blue dwarfs aren't expected to exist yet.

Lazarus wrote:

You tend to get undermassive white dwarfs in binary systems because of mass transfer from the giant star to the companion. My guess is that we're looking at particularly extreme end products of this process.

Also, I'm unsure how this would work to get undermassive white dwarfs around main sequence stars. Doesn't the matter transfer from the main sequence star to the companion? For KOI-74 b, I get a density of ~8.7·10³ - 8.7·10⁴ kg m^-3. This seems low for a white dwarf by several magnitudes. I don't expect a mass-density relation would work much here, since the density of a white dwarf would be determined exclusively by electron-degeneracy pressure, IIRC. Unless these are white dwarfs in the sense of the evolutionary track, but have sub-white dwarf densities due to very low masses (and even then, what keeps them from collapsing into electron degeneracy pressure densities?).

I suspect that some active fusion must be present in order to keep these objects at the quoted radius and temperature, assuming the mass values derived aren't horribly off.

Could these be objects that were something else but evaporated down to the core? (what kind of things have core masses of 3-30 Jupiters?)

especulation,maybe something artificial? for me i don't think so,however we must discuss all possibilities...

No evidence to suggest it's artificial.
Why go through the effort of building a 3 - 30 Jupiter-mass object that is significantly hotter than the host star? Occam's Razor at least tells us to rule out other ideas first.

Whatever this object is, it is now on the EPE's unconfirmed planet's list, pending characterization.

http://exoplanet.eu/planet.php?p1=KOI-74&p2=b

Probably radial velocity follow-up will provide true mass and likely around 0.2 or 0.1 solar masses, in the range of the least massive stellar degenerates.

Perhaps so. That may suggest our understanding of the interiors and/or tidal dissipation in early-A, late-B stars needs to be severely revised. Such stars may be not near as easy to tidally distort as assumed, allowing for anomalously low mass estimates.

Regarding the mass transfer: the white dwarfs, representing a more evolved stage in the stellar lifetime, were originally the more massive star. Mass transfer begins when the companion goes through its giant phase, this should initially result in decrease of semimajor axis. The end product is a more massive primary star with an undermassive sdB star companion, which then contracts to a white dwarf.

For an example of a discussion of this process as applied to a real star system: The Past and Future History of Regulus, which consists of a B-type main sequence star and an undermassive white dwarf. (Regulus is less extreme than these systems, but illustrates the general principles.)

Looking at figure 3 in the paper, neither of these objects appears to be too far from the white dwarf models. As for the paper not saying that the system is young and cooling down, it doesn't say this is not the case either. Certainly undermassive white dwarfs in post-mass-transfer systems are a known way of producing hot compact objects in close-orbit binary systems. It looks like these systems will be useful datapoints for studying mass transfer and the evolution of binary stars.

KOI = Kepler Object of Interest