CRIRES search for planets around the lowest-mass stars

High Precision Radial Velocity Measurements in the Infrared: A First Assessment of the RV Stability of CRIRES

A way to measure radial velocities in the infrared region of the spectrum is important for finding planets around M-type stars and brown dwarfs.

Detecting Planets Around Very Low Mass Stars with the Radial Velocity Method
http://arxiv.org/abs/0909.0002

Abstract wrote:

The detection of planets around very low-mass stars with the radial velocity method is hampered by the fact that these stars are very faint at optical wavelengths. We investigate the precision that can be achieved in radial velocity measurements of low mass stars in the near infrared (nIR) Y-, J-, and H-bands, and we compare it to the precision achievable in the optical. For early-M stars, radial velocity measurements in the nIR offer no or only marginal advantage in comparison to optical measurements. Although they emit more flux in the nIR, the richness of spectral features in the optical outweighs the flux difference. We find that nIR measurement can be more precise than optical measurements in stars of spectral type ~M3, and from there the nIR offers significant gains in precision towards cooler objects. We studied potential calibration strategies in the nIR finding that a stable spectrograph with a ThAr calibration probably offers the best choice with currently available technology. Furthermore, we simulate the wavelength-dependent influence of activity (cool spots) on radial velocity measurements. Our spot simulations reveal that the radial velocity jitter does not decrease as dramatically towards longer wavelengths as often thought. The jitter strongly depends on the details of the spots, i.e., on spot temperature and the spectral appearance of the spot. At low temperature contrast (~200K), the jitter shows a decrease towards the nIR up to a factor of ten, but it is substantially smaller with larger temperature contrast. Forthcoming nIR spectrographs will allow the search for planets with a particular advantage in mid- and late-M stars. Activity will remain an issue, but simultaneous observations at optical and nIR wavelengths can provide strong constraints on spot properties in active stars.

http://arxiv.org/pdf/0911.3148

Quote :

For example, our planet search project includes 31 objects with estimated masses below 0.2M(Sol), 22 of which have estimated masses below 0.15M(Sol). A previous high-precision radial velocity planet search utilizing a visible wavelength spectrograph on an identical telescope (UVES on the UT2 telescope of the VLT) was only able to target two objects with estimated masses below 0.2M(Sol), and only one of these two has an estimated mass below 0.15M(Sol) (Zechmeister et al. 2009).
In addition to enabling the search for planets around more low-mass stars, the gas cell and radial velocity measurement algorithm we have developed also opens up a new frontier on the search for potentially habitable planets. The orbital period range for a planet in the habitable zone around a star with a mass M = 0.10M(Sol) would be 3 – 21 days (Selsis et al. 2007). The 5ms−1 precision obtainable with our method corresponds to the velocity semi-amplitudes induced by a 2.5M(Earth) planet or a 4.6M(Earth) planet on the inner and outer edges of the habitable zone respectively. Therefore, it should be possible to detect Super-Earth type planets in the habitable zones of very low-mass stars with a reasonable expenditure of observing time on current facilities.

This is looking like a promising program. Of course, whether detectable planets exist in such systems is another matter

Incidentally we should probably merge this thread with this one and perhaps this one too... keep Borislav's title, we should keep the CRIRES prominent.

Lazarus wrote:

Incidentally we should probably merge this thread with this one and perhaps this one too... keep Borislav's title, we should keep the CRIRES prominent.

Sure, it'll help with organisation at least.

Radial Velocities with CRIRES: Pushing precision down to 5-10 m/s
http://arxiv.org/abs/0912.2643

Abstract wrote:

With the advent of high-resolution infrared spectrographs, Radial Velocity (RV) searches enter into a new domain. As of today, the most important technical question to address is which wavelength reference is the most suitable for high-precision RV measurements.
In this work we explore the usage of atmospheric absorption features. We make use of CRIRES data on two programs and three different targets. We re-analyze the data of the TW Hya campaign, reaching a dispersion of about 6 m/s on the RV standard in a time scale of roughly 1 week. We confirm the presence of a low-amplitude RV signal on TW Hya itself, roughly 3 times smaller than the one reported at visible wavelengths. We present RV measurements of Gl 86 as well, showing that our approach is capable of detecting the signal induced by a planet and correctly quantifying it.
Our data show that CRIRES is capable of reaching a RV precision of less than 10 m/s in a time-scale of one week. The limitations of this particular approach are discussed, and the limiting factors on RV precision in the IR in a general way. The implications of this work on the design of future dedicated IR spectrographs are addressed as well.

http://www.eso.org/sci/publications/messenger/archive/no.140-jun10/messenger-no140-41-45.pdf

Quote :

Our search for planets around very lowmass stars is continuing until at least the end of P85 and we have recently proposed to continue the programme for another two years. One of the goals of the new programme is to continue monitoring the 36 objects surveyed in the previous study in order to confirm planet candidates that we have identified around some of the stars, and to probe for lowermass and longer-period planets around all the stars. The new measurements should enable us to be sensitive down to even terrestrial-mass planets in the habitable zones around many of the stars we are targeting. We also aim to widen our
survey to include 30 new targets that we will search for gas giant companions in a
sort of shallow–wide survey. In addition to our work with CRIRES, we are also beginning a similar survey in the northern hemisphere using the NIR spectrograph IRCS on the Subaru telescope with a copy of our ammonia cell. The radial velocity precision obtained with this
facility will be significantly less than with the VLT + CRIRES due to the IRCS’s
lower spectral resolving power (20 000) and throughput, but it will still be useful to
search low-mass stars for gas giant planets. Together, the new CRIRES programme and the Subaru programme are volume complete for all known stars with masses < 0.2 MA and spectral type earlier than T0 out to 10 pc. Therefore, we are adding important information to the
census of planets around nearby stars. Technically speaking, our results have shown that obtaining NIR radial velocity precisions comparable to those which are routinely obtained in the visible is possible, and we see no reason why a level of precision of 1 ms–1 could not be
obtained in the future with a new approach or instrument. Our current measurements
are limited by the presence of telluric lines in the window in which we are observing, and it is likely we would be doing about a factor of two better if these lines were not present. Therefore, if a new type of gas cell for the NIR was designed that yielded useful calibration lines where interesting stars also have lines and where the Earth’s atmosphere does not, then that would probably give a boost in performance. However, our experience suggests such a breakthrough is unlikely. Instead, it is more likely that improving on our results will require applying the HARPS concept of building a highly stabilised instrument for the NIR.
Indeed, such new instruments are currently being considered for the immediate
future for telescopes in the 4-metre class range, and in the longer-term for 8–10-
metre class telescopes and even ELTs.

Definitely CRIRES is one of the most interesting surveys going, will definitely be interesting to see what the planet formation processes are doing around such low-mass stars. Wonder if they have found any planetary systems around brown dwarfs.

Lazarus wrote:

Definitely CRIRES is one of the most interesting surveys going, will definitely be interesting to see what the planet formation processes are doing around such low-mass stars. Wonder if they have found any planetary systems around brown dwarfs.

I have some doubts about the uniqueness of the new method.

UVES
http://arxiv.org/abs/0807.1452


CRIRES
http://www.eso.org/sci/publications/messenger/archive/no.140-jun10/messenger-no140-41-45.pdf



Then why the ideal goal (oceanides Gliese 1214b) observations conducted not CRIRES - then HAPRS?

Borislav - the wavelength ranges of UVES and CRIRES are different: according to its instrument page, UVES operates in two ranges: 300-500nm and 420-1100nm. The overview page for CRIRES states it operates at 0.95-5.2µm (i.e. 950-5200 nm). It is rather more difficult to do radial velocity measurements in the CRIRES range. For comparison, HARPS operates in the range 378-691 nm, according to the overview page.

The CRIRES instrument therefore seems to be best for faint, ultracool dwarfs like VB 10. Proxima is sufficiently bright (V=10) that other spectrographs which operate at shorter wavelengths can do better.