Non-detection of Eps Eri b and HD 102365 b with HARPS

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Non-detection of Eps Eri b and HD 102365 b with HARPS

Post by Sirius_Alpha on 2nd December 2012, 11:24 pm

The planet search programme at the ESO CES and HARPS. IV. The search for Jupiter analogues around solar-like stars
http://arxiv.org/abs/1211.7263

In 1992 we began a precision radial velocity (RV) survey for planets around solar-like stars with the Coude Echelle Spectrograph and the Long Camera (CES LC) at the 1.4 m telescope in La Silla (Chile). We have continued the survey with the upgraded CES Very Long Camera (VLC) and HARPS, both at the 3.6 m telescope, until 2007. The observations for 31 stars cover a time span of up to 15 years and the RV precision permit a search for Jupiter analogues. We perform a joint analysis for variability, trends, periodicities, and Keplerian orbits and compute detection limits. Moreover, the HARPS RVs are analysed for correlations with activity indicators (CaII H&K and CCF shape). We achieve a long-term RV precision of 15 m/s (CES+LC, 1992-1998), 9 m/s (CES+VLC, 1999-2006), and 2.8 m/s (HARPS, 2003-2009, including archive data), resp. This enables us to confirm the known planets around Iota Hor, HR 506, and HR 3259. A steady RV trend for Eps Ind A can be explained by a planetary companion. On the other hand, we find previously reported trends to be smaller for Beta Hyi and not present for Alp Men. The candidate planet Eps Eri b was not detected despite our better precision. Also the planet announced for HR 4523 cannot be confirmed. Long-term trends in several of our stars are compatible with known stellar companions. We provide a spectroscopic orbital solution for the binary HR 2400 and refined solutions for the planets around HR 506 and Iota Hor. For some other stars the variations could be attributed to stellar activity. The occurrence of two Jupiter-mass planets in our sample is in line with the estimate of 10% for the frequency of giant planets with periods smaller than 10 yr around solar-like stars. We have not detected a Jupiter analogue, while the detections limits for circular orbits indicate at 5 AU a sensitivity for minimum mass of at least 1 M_Jup (2 M_Jup) for 13% (61%) of the stars.

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Re: Non-detection of Eps Eri b and HD 102365 b with HARPS

Post by Lazarus on 3rd December 2012, 3:15 am

Interesting to note about the astrometric results for Eps Eri:
For ε Eri b, the astrometric results (Benedict et al. 2006; Reffert & Quirrenbach 2011) seem to confirm the planet, but they do not constitute independent detections, since they rely on the combination with less precise RV measurements. Also the astrometric phase coverage is not complete. However, the same is true for our HARPS measurements. Hence, this planet is not yet fully disproved with our RV non-detection or by imaging non-detections (e.g. Janson et al. 2007, 2008).
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Re: Non-detection of Eps Eri b and HD 102365 b with HARPS

Post by Edasich on 3rd December 2012, 8:47 am

Rather I see some more details about the possible planetary companion of Eps Ind A (m sin i > 0.97 MJ, P > 30 yrs) Smile
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Re: Non-detection of Eps Eri b and HD 102365 b with HARPS

Post by jyril on 3rd December 2012, 11:17 am

Afraid you have to wait for a while... Wink

Though distant orbit + nearby star => good imaging candidate.

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Re: Non-detection of Eps Eri b and HD 102365 b with HARPS

Post by Lazarus on 3rd December 2012, 7:14 pm

Quite unfortunate that genuine Jupiter-analogues have the same periods as stellar magnetic cycles for solar-type stars. Makes the confirmation more difficult!

Interesting also the slight decrease in eccentricity of the orbital solutions for Iota Horologii and HD 10647 (which is now indistinguishable from circular), I guess this is a result of the various biases towards false eccentricity if there is insufficient RV coverage?
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Re: Non-detection of Eps Eri b and HD 102365 b with HARPS

Post by Edasich on 5th December 2012, 4:52 pm

About Eps Eri b I can agree there were certain evidences from disk (asteroid belt) morphologies which suggested a much lower orbital eccentricity to say (with e = ca. 0.25 rather 0.7).

However there is an oblique reference in the paper HARPS-TERRA Project I where an orbital solution with e=0.4 and lower minimum mass (M sin i = 0.69 MJup) for Eps Eri b was provided, including all RV datasets up to date.

But where do we place Reffert & Quirrenbach (2011) astrometric confirmation of the planet? The aforementioned authors of HARPS-TERRA I paper argue that astrometric detection could be not valid and that Eps Eri b could actually be... stellar activity. Neutral

If so, what kind of exoplanet detection method could we trust? Transit only?
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Re: Non-detection of Eps Eri b and HD 102365 b with HARPS

Post by Sirius_Alpha on 5th December 2012, 8:14 pm

There isn't one you can "trust" to always be right. It's the nature of science - there is some ambiguity either from multiple models to fit the observations (as microlensing suffers from), sunspot activity (affects RV), diluted eclipsing binaries (affects transits), and so on. It's just a matter of being careful and trying to rule out other possibilities. Science in its very nature carries uncertainty, this is a manifestation of that.

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Re: Non-detection of Eps Eri b and HD 102365 b with HARPS

Post by jyril on 6th December 2012, 8:03 am

Indeed. Fortunately, it always helps if you can get data from multiple sources which agree with your model (for example, transits + RV).

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Re: Non-detection of Eps Eri b and HD 102365 b with HARPS

Post by Edasich on 6th December 2012, 10:08 am

jyril wrote:Indeed. Fortunately, it always helps if you can get data from multiple sources which agree with your model (for example, transits + RV).

Transit is certainly the most direct evidence, but when you are dealing with edge-on orbits. When you have nearly face-on orbits you need astrometry for true masses.
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Re: Non-detection of Eps Eri b and HD 102365 b with HARPS

Post by Lazarus on 7th December 2012, 6:34 pm

Edasich wrote:But where do we place Reffert & Quirrenbach (2011) astrometric confirmation of the planet?
Good question...

arXiv link for that paper is http://arxiv.org/abs/1101.2227

The measurements presented there do not cover a full orbit, note that in figure 1 the coverage is outside the region of greatest curvature.

Section 6.1 of that paper covers this point in more detail:
As mentioned already the phase coverage of the astrometric data is not complete. This applies to both the HST/FGS as well as the Hipparcos data; the HST/FGS data cover 2.9 years, while the Hipparcos data cover 2.5 years. This is to be compared with the orbital period of 6.9 years. Unfortunately, neither of the two astrometric data sets covers the phase of the orbit where the motion in right ascension is reversed, which would be very helpful to distinguish between orbital curvature and linear proper motion.
It also seems to be impossible to combine the two astrometric datasets because of differences in the reference points used.

Furthermore the astrometric fit in the Reffert & Quirrenbach paper is NOT independent of the radial velocities: from the description of the method (section 3.1):
The only two or-bital parameters fitted for were the inclination and the ascending node; all other five orbital parameters (period, periastron time, eccentricity, longitude of periastron and mass function) were kept fixed at the literature values found via fits to the radial velocity data of each star.
In other words they fit the astrometric data under the assumption that the RV measurements give accurate planetary parameters.
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Re: Non-detection of Eps Eri b and HD 102365 b with HARPS

Post by Sirius_Alpha on 7th December 2012, 8:42 pm

Interesting! Thanks for looking back through that. Looks like this weakens the case for Eps Eri b, but I'd still like to see more work done on this system.

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Re: Non-detection of Eps Eri b and HD 102365 b with HARPS

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