25 research outputs found
Precision near-infrared radial velocity instrumentation II: Non-Circular Core Fiber Scrambler
We have built and commissioned a prototype agitated non-circular core fiber
scrambler for precision spectroscopic radial velocity measurements in the
near-infrared H band. We have collected the first on-sky performance and modal
noise tests of these novel fibers in the near-infrared at H and K bands using
the CSHELL spectrograph at the NASA InfraRed Telescope Facility (IRTF). We
discuss the design behind our novel reverse injection of a red laser for
co-alignment of star-light with the fiber tip via a corner cube and visible
camera. We summarize the practical details involved in the construction of the
fiber scrambler, and the mechanical agitation of the fiber at the telescope. We
present radial velocity measurements of a bright standard star taken with and
without the fiber scrambler to quantify the relative improvement in the
obtainable blaze function stability, the line spread function stability, and
the resulting radial velocity precision. We assess the feasibility of applying
this illumination stabilization technique to the next generation of
near-infrared spectrographs such as iSHELL on IRTF and an upgraded NIRSPEC at
Keck. Our results may also be applied in the visible for smaller core diameter
fibers where fiber modal noise is a significant factor, such as behind an
adaptive optics system or on a small < 1 meter class telescope such as is being
pursued by the MINERVA and LCOGT collaborations.Comment: Proceedings of the SPIE Optics and Photonics Conference "Techniques
and Instrumentation for Detection of Exoplanets VI" held in San Diego, CA,
August 25-29, 201
Retrieval of Precise Radial Velocities from Near-Infrared High Resolution Spectra of Low Mass Stars
Given that low-mass stars have intrinsically low luminosities at optical
wavelengths and a propensity for stellar activity, it is advantageous for
radial velocity (RV) surveys of these objects to use near-infrared (NIR)
wavelengths. In this work we describe and test a novel RV extraction pipeline
dedicated to retrieving RVs from low mass stars using NIR spectra taken by the
CSHELL spectrograph at the NASA Infrared Telescope Facility, where a methane
isotopologue gas cell is used for wavelength calibration. The pipeline
minimizes the residuals between the observations and a spectral model composed
of templates for the target star, the gas cell, and atmospheric telluric
absorption; models of the line spread function, continuum curvature, and
sinusoidal fringing; and a parameterization of the wavelength solution. The
stellar template is derived iteratively from the science observations
themselves without a need for separate observations dedicated to retrieving it.
Despite limitations from CSHELL's narrow wavelength range and instrumental
systematics, we are able to (1) obtain an RV precision of 35 m/s for the RV
standard star GJ 15 A over a time baseline of 817 days, reaching the photon
noise limit for our attained SNR, (2) achieve ~3 m/s RV precision for the M
giant SV Peg over a baseline of several days and confirm its long-term RV trend
due to stellar pulsations, as well as obtain nightly noise floors of ~2 - 6
m/s, and (3) show that our data are consistent with the known masses, periods,
and orbital eccentricities of the two most massive planets orbiting GJ 876.
Future applications of our pipeline to RV surveys using the next generation of
NIR spectrographs, such as iSHELL, will enable the potential detection of
Super-Earths and Mini-Neptunes in the habitable zones of M dwarfs.Comment: 64 pages, 28 figures, 5 tables. Accepted for publication in PAS
Design and Construction of Absorption Cells for Precision Radial Velocities in the K Band using Methane Isotopologues
We present a method to optimize absorption cells for precise wavelength
calibration in the near-infrared. We apply it to design and optimize methane
isotopologue cells for precision radial velocity measurements in the K band. We
also describe the construction and installation of two such cells for the
CSHELL spectrograph at NASA's IRTF. We have obtained their high-resolution
laboratory spectra, which we can then use in precision radial velocity
measurements and which can also have other applications. In terms of obtainable
RV precision methane should out-perform other proposed cells, such as the
ammonia cell (NH) recently demonstrated on CRIRES/VLT. The
laboratory spectra of Ammonia and the Methane cells show strong absorption
features in the H band that could also be exploited for precision Doppler
measurements. We present spectra and preliminary radial velocity measurements
obtained during our first-light run. These initial results show that a
precision down to 20-30 m s can be obtained using a wavelength interval
of only 5 nm in the K band and S/N150. This supports the prediction that
a precision down to a few m s can be achieved on late M dwarfs using the
new generation of NIR spectrographs, thus enabling the detection of terrestrial
planets in their habitable zones. Doppler measurements in the NIR can also be
used to mitigate the radial velocity jitter due to stellar activity enabling
more efficient surveys on young active stars.Comment: accepted PASP, Apr 2012 (in press). Preprint version with 36 pages, 9
Figures, 2 Table
A planet within the debris disk around the pre-main-sequence star AU Microscopii
AU Microscopii (AU Mic) is the second closest pre main sequence star, at a
distance of 9.79 parsecs and with an age of 22 million years. AU Mic possesses
a relatively rare and spatially resolved3 edge-on debris disk extending from
about 35 to 210 astronomical units from the star, and with clumps exhibiting
non-Keplerian motion. Detection of newly formed planets around such a star is
challenged by the presence of spots, plage, flares and other manifestations of
magnetic activity on the star. Here we report observations of a planet
transiting AU Mic. The transiting planet, AU Mic b, has an orbital period of
8.46 days, an orbital distance of 0.07 astronomical units, a radius of 0.4
Jupiter radii, and a mass of less than 0.18 Jupiter masses at 3 sigma
confidence. Our observations of a planet co-existing with a debris disk offer
the opportunity to test the predictions of current models of planet formation
and evolution.Comment: Nature, published June 24th [author spelling name fix
A high-precision near-infrared survey for radial velocity variable low-mass stars using CSHELL and a methane gas cell
We present the results of a precise near-infrared (NIR) radial velocity (RV) survey of 32 low-mass stars with spectral types K2-M4 using CSHELL at the NASA InfraRed Telescope Facility in the K band with an isotopologue methane gas cell to achieve wavelength calibration and a novel, iterative RV extraction method. We surveyed 14 members of young (≈25-150 Myr) moving groups, the young field star ϵ Eridani, and 18 nearby (<25 pc) low-mass stars and achieved typical single-measurement precisions of 8-15 m s-1with a long-term stability of 15-50 m s-1 over longer baselines. We obtain the best NIR RV constraints to date on 27 targets in our sample, 19 of which were never followed by high-precision RV surveys. Our results indicate that very active stars can display long-term RV variations as low as ∼25-50 m s-1 at ≈2.3125 μm, thus constraining the effect of jitter at these wavelengths. We provide the first multiwavelength confirmation of GJ 876 bc and independently retrieve orbital parameters consistent with previous studies. We recovered RV variabilities for HD 160934 AB and GJ 725 AB that are consistent with their known binary orbits, and nine other targets are candidate RV variables with a statistical significance of 3σ-5σ. Our method, combined with the new iSHELL spectrograph, will yield long-term RV precisions of ≲5 m s-1 in the NIR, which will allow the detection of super-Earths near the habitable zone of mid-M dwarfs