Bye Bye HD 136118 b. Welcome to HD 136118 c?

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Bye Bye HD 136118 b. Welcome to HD 136118 c?

Post by Edasich on 25th November 2009, 5:28 am

Another brown dwarf in the desert *sigh*

The Mass of the Candidate Exoplanet Companion to HD136118 from Hubble Space Telescope Astrometry and High-Precision Radial Velocities

We use Hubble Space Telescope Fine Guidance Sensor astrometry and high-cadence radial velocities for HD136118 from the HET with archival data from Lick to determine the complete set of orbital parameters for HD136118b. We find an orbital inclination for the candidate exoplanet of i_{b} = 163.1 +- 3.0 deg. This establishes the actual mass of the object, M_{b} = 42^{+11}_{-18} MJup, in contrast to the minimum mass determined from the radial velocity data only, M_{b}sin{i} ~ 12 MJup. Therefore, the low-mass companion to HD 136118 is now identified as a likely brown dwarf residing in the "brown dwarf desert".

But residuals of a second companion are reported. Possibily a 0,35 Mj objects at 0.85 AUs (e=0.5).
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Re: Bye Bye HD 136118 b. Welcome to HD 136118 c?

Post by Sirius_Alpha on 25th November 2009, 9:59 am

I'm glad this one's gone. I hate the RV planets at and around 13 M_j (whether or not its an arbitrary barrier).



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Re: Bye Bye HD 136118 b. Welcome to HD 136118 c?

Post by Lazarus on 25th November 2009, 2:22 pm

Edasich wrote:Another brown dwarf in the desert *sigh*
Then again, these are quite rare (hence the term "brown dwarf desert" in the first place), so a fairly exotic system... as they note it falls right in the "driest" part of the brown dwarf desert. Seems to be a high degree of misalignment too: the star's rotation is apparently seen near edge-on, whereas the orbit seems to be near face-on.

Second companion not too plausible because of stability concerns... as they note, systematic errors are a possibility.
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Re: Bye Bye HD 136118 b. Welcome to HD 136118 c?

Post by Borislav on 25th November 2009, 3:47 pm

RV-planets from Encyclopedia


On theories of probability this else 20 false planets?

Sorry if offtop

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Re: Bye Bye HD 136118 b. Welcome to HD 136118 c?

Post by Lazarus on 25th November 2009, 6:01 pm

Good question. It is possible to do a naive estimate by assuming random inclinations and figuring out the probabilities that a planet of given minimum mass exceeds a certain threshold (e.g. the traditional cutoff of 13 Jupiter masses).

However it turns out that the mass function is not constant, and there are various populations of objects that exist in the distribution. Furthermore in some cases there are dynamical stability constraints, etc. Factoring this in is not trivial!

If you define the term "brown dwarf" as a population of objects that overlaps with the population of giant planets, things get even more interesting...
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Re: Bye Bye HD 136118 b. Welcome to HD 136118 c?

Post by Sirius_Alpha on 26th November 2009, 3:19 am

This all actually isn't news, but it's still interesting to see the paper on ArXiv.

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Re: Bye Bye HD 136118 b. Welcome to HD 136118 c?

Post by Sirius_Alpha on 28th November 2009, 5:31 am

Reffert & Quirrenbach (2006) measured astrometric masses for two exoplanet candidates HD 38529c (M = 37 +36/-19 M_Jup) and HD 168443c (M = 34 +/- 12 M_Jup). Both are likely brown dwarf companions around solar type stars like HD 136118 b.
I never heard of this before. Last I heard HD 38529 Ac's mass was only known to be 13.1 M_jup (minimum) and HD 168443 b having a minimum mass of 18.1 M_Jup. So can we cross these poseurs off the list now? Doing the math, I get inclinations of i = ~20 degrees for HD 38529 Ac (placing HD 38529 Ab at 2.4 M_J) and i = ~32 degrees for HD 168443 c (which places HD 168443 b at 8 M_J).

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Re: Bye Bye HD 136118 b. Welcome to HD 136118 c?

Post by Lazarus on 28th November 2009, 5:40 am

Sirius_Alpha wrote:I never heard of this before. Last I heard HD 38529 Ac's mass was only known to be 13.1 M_jup (minimum) and HD 168443 b having a minimum mass of 18.1 M_Jup. So can we cross these poseurs off the list now? Doing the math, I get inclinations of i = ~20 degrees for HD 38529 Ac (placing HD 38529 Ab at 2.4 M_J) and i = ~32 degrees for HD 168443 c (which places HD 168443 b at 8 M_J).
Unfortunately the paper in question is not freely available, however the relevant parameters are in the abstract:
The best fit solution for HD 38529, (i, Ω) = (160, 52), yields a mass of 37+36-19 M_Jup for the outer companion. For HD 168443, we derive best fit parameters of (i, Ω) = (150, 19), which imply a companion mass of 3412 M_Jup.Conclusions.The outer companions in both systems are thus brown dwarfs.
This nicely confirms your deductions of the inclinations: it isn't possible to distinguish between the inclinations θ and 180-θ.

If HD 168443 b is coplanar with c, then I get as the true mass 8.02/sin(150) = 16 Jupiter masses. Figuring out how this system formed is going to present an interesting challenge.
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Re: Bye Bye HD 136118 b. Welcome to HD 136118 c?

Post by Sirius_Alpha on 29th November 2009, 10:13 am

Lazarus wrote:If HD 168443 b is coplanar with c, then I get as the true mass 8.02/sin(150) = 16 Jupiter masses.
Ah, yeah, I got that now. Apparently someone can't do math.

This system looks similar to that one system (Iota Aurigae?) with two brown dwarfs. I wonder how common that is. If these two substellar objects were formed by gravitational collapse of a section of a disk, might they have significant relative inclinations?

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Re: Bye Bye HD 136118 b. Welcome to HD 136118 c?

Post by Lazarus on 29th November 2009, 6:38 pm

Regarding the other similar systems, there's the Iota Aurigae case which was reported to have a 2:1 resonance between the brown dwarf-mass objects, then there's BD+20 2457 which has a period ratio of about 1.6 (so probably also resonant).

HD 168443 is more hierarchical: the period ratio is about 30. So you could perhaps regard it as a triple star system, except that binary star formation processes seem to favour mass ratios near unity, especially at such small separations.

Or you could perhaps regard it as a binary star system with a superplanet orbiting one of the components, except then you've got the mass ratio issue in the binary and the problem of how to form such a massive planet with an outer companion truncating the disc.

Maybe you could regard it as a double superplanet system, except that you'd need an extremely massive disc to build a 34 Jupiter-mass planet. (Maybe it's at the low end of the range nearer 22 Jupiter masses, and things don't look quite so bad, but still...)
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Re: Bye Bye HD 136118 b. Welcome to HD 136118 c?

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