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The CRIRES search for planets around the lowest-mass stars. I. High-precision near-infrared radial velocities with an ammonia gas cell

Bean, Jacob L.; Seifahrt, Andreas; Hartman, Henrik LU ; Nilsson, Hampus LU ; Wiedemann, Guenter; Reiners, Ansgar; Dreizler, Stefan and Henry, Todd J. (2010) In Astrophysical Journal 713(1). p.410-422
Abstract
Radial velocities measured from near-infrared (NIR) spectra are a potentially powerful tool to search for planets around cool stars and sub-stellar objects. However, no technique currently exists that yields NIR radial velocity precision comparable to that routinely obtained in the visible. We are carrying out an NIR radial velocity planet search program targeting a sample of the lowest-mass M dwarfs using the CRIRES instrument on the Very Large Telescope. In this first paper in a planned series about the project, we describe a method for measuring high-precision relative radial velocities of these stars from K-band spectra. The method makes use of a glass cell filled with ammonia gas to calibrate the spectrograph response similar to the... (More)
Radial velocities measured from near-infrared (NIR) spectra are a potentially powerful tool to search for planets around cool stars and sub-stellar objects. However, no technique currently exists that yields NIR radial velocity precision comparable to that routinely obtained in the visible. We are carrying out an NIR radial velocity planet search program targeting a sample of the lowest-mass M dwarfs using the CRIRES instrument on the Very Large Telescope. In this first paper in a planned series about the project, we describe a method for measuring high-precision relative radial velocities of these stars from K-band spectra. The method makes use of a glass cell filled with ammonia gas to calibrate the spectrograph response similar to the "iodine cell" technique that has been used very successfully in the visible. Stellar spectra are obtained through the ammonia cell and modeled as the product of a Doppler-shifted template spectrum of the object and a spectrum of the cell, convolved with a variable instrumental profile (IP) model. A complicating factor is that a significant number of telluric absorption lines are present in the spectral regions containing useful stellar and ammonia lines. The telluric lines are modeled simultaneously as well using spectrum synthesis with a time-resolved model of the atmosphere over the observatory. The free parameters in the complete model are the wavelength scale of the spectrum, the IP, adjustments to the water and methane abundances in the atmospheric model, telluric spectrum Doppler shift, and stellar Doppler shift. Tests of the method based on the analysis of hundreds of spectra obtained for late-M dwarfs over 6 months demonstrate that precisions of similar to 5 m s(-1) are obtainable over long timescales, and precisions of better than 3 m s(-1) can be obtained over timescales up to a week. The obtained precision is comparable to the predicted photon-limited errors, but primarily limited over long timescales by the imperfect modeling of the telluric lines. (Less)
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author
organization
publishing date
type
Contribution to journal
publication status
published
subject
keywords
stars: individual (GJ551, GJ 699, GJ, instrumentation: spectrographs, GJ 442B), 406, stars: low-mass, techniques: radial velocities
in
Astrophysical Journal
volume
713
issue
1
pages
410 - 422
publisher
University of Chicago Press
external identifiers
  • wos:000275918500035
  • scopus:77950249163
ISSN
0004-637X
DOI
10.1088/0004-637X/713/1/410
language
English
LU publication?
yes
id
c323a2be-8404-4cc1-b0ae-a71e224ee4a8 (old id 1587874)
date added to LUP
2010-04-22 14:28:47
date last changed
2018-06-03 04:13:49
@article{c323a2be-8404-4cc1-b0ae-a71e224ee4a8,
  abstract     = {Radial velocities measured from near-infrared (NIR) spectra are a potentially powerful tool to search for planets around cool stars and sub-stellar objects. However, no technique currently exists that yields NIR radial velocity precision comparable to that routinely obtained in the visible. We are carrying out an NIR radial velocity planet search program targeting a sample of the lowest-mass M dwarfs using the CRIRES instrument on the Very Large Telescope. In this first paper in a planned series about the project, we describe a method for measuring high-precision relative radial velocities of these stars from K-band spectra. The method makes use of a glass cell filled with ammonia gas to calibrate the spectrograph response similar to the "iodine cell" technique that has been used very successfully in the visible. Stellar spectra are obtained through the ammonia cell and modeled as the product of a Doppler-shifted template spectrum of the object and a spectrum of the cell, convolved with a variable instrumental profile (IP) model. A complicating factor is that a significant number of telluric absorption lines are present in the spectral regions containing useful stellar and ammonia lines. The telluric lines are modeled simultaneously as well using spectrum synthesis with a time-resolved model of the atmosphere over the observatory. The free parameters in the complete model are the wavelength scale of the spectrum, the IP, adjustments to the water and methane abundances in the atmospheric model, telluric spectrum Doppler shift, and stellar Doppler shift. Tests of the method based on the analysis of hundreds of spectra obtained for late-M dwarfs over 6 months demonstrate that precisions of similar to 5 m s(-1) are obtainable over long timescales, and precisions of better than 3 m s(-1) can be obtained over timescales up to a week. The obtained precision is comparable to the predicted photon-limited errors, but primarily limited over long timescales by the imperfect modeling of the telluric lines.},
  author       = {Bean, Jacob L. and Seifahrt, Andreas and Hartman, Henrik and Nilsson, Hampus and Wiedemann, Guenter and Reiners, Ansgar and Dreizler, Stefan and Henry, Todd J.},
  issn         = {0004-637X},
  keyword      = {stars: individual (GJ551,GJ 699,GJ,instrumentation: spectrographs,GJ 442B),406,stars: low-mass,techniques: radial velocities},
  language     = {eng},
  number       = {1},
  pages        = {410--422},
  publisher    = {University of Chicago Press},
  series       = {Astrophysical Journal},
  title        = {The CRIRES search for planets around the lowest-mass stars. I. High-precision near-infrared radial velocities with an ammonia gas cell},
  url          = {http://dx.doi.org/10.1088/0004-637X/713/1/410},
  volume       = {713},
  year         = {2010},
}