Archive for radial velocity

Radial velocity discovery of Proxima Centauri b

Posted in astronomy with tags , on August 26, 2016 by Tim Kendall

The sky around Alpha Centauri and Proxima Centauri

“It is true. We are convinced that there is a planet orbiting Proxima now. The evidence goes as follows: a signal was spotted back in 2013 on previous surveys (UVES and HARPS). The preliminary detection was first done by Mikko Tuomi, our in-house applied mathematician and his Bayesian codes. However, the signal was not convincing as the data were really sparse and the period was ambiguous (other possible solutions at 20 and 40 days, plus a long period signal of unknown origin). We followed up Proxima in the next years but our two observing runs were 12 days, barely sufficient to secure a signal which ended up being 11.2 days. So the Pale Red Dot was designed with the sole purpose of confirming or refuting its strict periodicity, plus carefully monitor the star for activity induced variability. We got very lucky with the weather so we obtained 54 out of 60 observations. The photometric monitoring telescopes (ASH2 and several units of Las Cumbres Observatory Global Telescope network), worked flawlessly so we could see the effect of spots, flares and rotation of the star, which also had a footprint on the spectra. However, nothing indicated that spurious variability would be happening at 11.2 days. So that’s basically it: the Pale Red Dot campaign also detects the same period, and confirms that the signal has been in phase for the 16 years of accumulated observations. This is a requirement for a proper Keplerian orbit. Features like starspots are more short lived plus affect the velocities in the time-scales of the rotation of the star, which is now confirmed at ~83 days.”

Image [section, Proxima Centauri is the orange-red star in center of this image]: ESO press release. The alpha Centauri AB pair are off to the upper left of this image and of course exceedingly bright. Here is link to the Nature paper. Text: palereddot.org. Huge and heartfelt congratulations to the Pale Red Dot Team. Also, the website contains an interview by Guillem Anglada-Escudé, who led this work, with Didier Queloz, co-discoverer of 51 Peg b back in 1995, and it is well worth reading to compare the stories of the two discoveries, as well as everything else on palereddot.org, for a sense of the field.

High precision radial velocity planet searches in the near-infrared

Posted in astronomy with tags , , on April 23, 2016 by Tim Kendall

Spinvelocity-vs-mass-BetaPicb-and-solarsystemSystem along with the recently measured spin rate of the planet Beta Pictoris b. Credit: ESO/I. Snellen (Leiden University). Spin rates are a different matter, but even radial velocities (RV) are notoriously hard to measure for the majority of nearby stars, which are M dwarfs, and this extends to brown dwarfs as well. Very new instrumental developments are beginning to allow RV to be measured in the near-infrared, where the spectra of these stars are less crowded with lines, allowing line widths to be measured properly, with RV and spin rates also much easier to determine. A natural extension into the planet-seeking arena follows. A new paper by Jonathan Gagné, Peter Plavchan (who is a pioneer in this field) and many co-authors has been accepted to the Astrophysical Journal and appears on arXiv:

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 IRTF 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 as well as 18 nearby (< 25 pc) low-mass stars and achieved typical single-measurement precisions of 8-15 m/sec with a long-term stability of 15-50 m/sec. 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/sec at 2.3125 μm, thus constraining the effect of jitter at these wavelengths. We provide the first multi-wavelength confirmation of GJ 876 bc and independently retrieve orbital parameters consistent with previous studies. We recovered RV variability 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 3-5σ. Our method combined with the new iSHELL spectrograph will yield long-term RV precisions of 5 m/sec in the NIR, which will allow the detection of Super-Earths near the habitable zone of mid-M dwarfs.

The mass of Kepler-93b and the composition of terrestrial-type exoplanets

Posted in astronomy with tags , , , , on January 2, 2015 by Tim Kendall

kepler-planet-candidatesMore than three-quarters of the planet candidates discovered by NASA’s Kepler spacecraft have sizes ranging from that of Earth to that of Neptune, which is nearly four times as big as Earth. Such planets dominate the galactic census but are not represented in our own solar system. Image credit: NASA Ames

In a new preprint today Dressing et al. have measured a high density for this planet using archival radial velocity measurements to derive a precise mass. The paper is entitled “The Mass of Kepler-93b and The Composition of Terrestrial Planets” has been accepted to the Astrophysical Journal. Overall the Kepler data seem to suggest a rough cutoff around six Earth masses, above which much lower densities are derived, suggesting the presence of extensive hydrogen and helium envelopes. Above this approximate mass high density iron/silicate terrestrial-type planets appear rare. From the abstract:

Kepler-93b is a 1.478 +/- 0.019 Earth radius planet with a 4.7 day period around a bright (V=10.2), astroseismically-characterized host star with a mass of 0.911+/-0.033 solar masses and a radius of 0.919+/-0.011 solar radii. Based on 86 radial velocity observations obtained with the HARPS-N spectrograph on the Telescopio Nazionale Galileo and 32 archival Keck/HIRES observations, we present a precise mass estimate of 4.02+/-0.68 Earth masses. The corresponding high density of 6.88+/-1.18 g/cc is consistent with a rocky composition of primarily iron and magnesium silicate. We compare Kepler-93b to other dense planets with well-constrained parameters and find that between 1-6 Earth masses, all dense planets including the Earth and Venus are well-described by the same fixed ratio of iron to magnesium silicate. There are as of yet no examples of such planets with masses > 6 Earth masses: All known planets in this mass regime have lower densities requiring significant fractions of volatiles or H/He gas. We also constrain the mass and period of the outer companion in the Kepler-93 system from the long-term radial velocity trend and archival adaptive optics images. As the sample of dense planets with well-constrained masses and radii continues to grow, we will be able to test whether the fixed compositional model found for the seven dense planets considered in this paper extends to the full population of 1-6 Earth mass planets.