Juliette C. Becker, John Asher Johnson, Andrew Vanderburg, Timothy D. Morton
4 min
High-precision radial velocity (RV) measurements have traditionally been limited to slow-rotating, Sun-like stars (F, G, K, M types) because their spectra contain numerous sharp lines. Massive A- and B-type stars, however, rotate rapidly, causing their spectral lines to broaden significantly and making standard RV extraction techniques ineffective. This paper addresses the challenge of extracting precise RVs from these "featureless" stars using archival data from the California Planet Survey (CPS).
The authors developed a forward-modeling technique that fits the radial velocity shift and the echelle blaze function simultaneously across all echelle orders. Because rapidly rotating stars have broad lines that can span a significant fraction of an echelle order, the researchers modeled the continuum shape as a continuous function of both order and pixel position. For relative RVs, they used the first observed spectrum of a star as a template; for absolute RVs, they utilized PHOENIX model stellar spectra. They also implemented a method to extrapolate the wavelength solution from the iodine-rich green chip to the blue chip, where most hydrogen and helium lines reside.
By applying this technique to archival HIRES spectra, the authors achieved an RV precision of 0.5–2.0 km/s per epoch for over 100 A- and B-type stars. This precision was sufficient to detect orbital motion in several systems. Specifically, the study identified two previously unknown spectroscopic binaries and confirmed one known astrometric system. The results demonstrate that archival calibration spectra, often overlooked, contain valuable scientific information for studying stellar multiplicity and Galactic space motions.
This technique enables the use of massive, rapidly rotating stars as targets for RV surveys, which was previously considered impractical. It provides a pathway to measure or constrain the masses of sub-stellar companions found by transit surveys around these stars. Furthermore, it facilitates future spectroscopic binarity surveys and studies of Galactic dynamics, as these stars serve as excellent tracers of stellar populations and cluster kinematics.
We present a technique to extract radial velocity (RV) measurements from echelle spectrograph observations of rapidly rotating stars ( V i sin 50 km s -1 ). This type of measurement is difficult because the line widths of such stars are often comparable to the width of a single echelle order. To compensate for the scarcity of lines and Doppler information content, we have developed a process that forward-models the observations, fitting the RV shift of the star for all echelle orders simultaneously with the echelle blaze function. We use our technique to extract RV measurements from a sample of rapidly rotating A-and B-type stars used as calibrator stars observed by the California Planet Survey observations. We measure absolute RVs with a precision ranging from 0.5-2.0 km s -1 per epoch for more than 100 A-and B-type stars. In our sample of 10 well-sampled stars with RV scatter in excess of their measurement uncertainties, three of these are single-lined binaries with long observational baselines. From this subsample, we present detections of two previously unknown spectroscopic binaries and one known astrometric system. Our technique will be useful in measuring or placing upper limits on the masses of sub-stellar companions discovered by wide-field transit surveys, and conducting future spectroscopic binarity surveys and Galactic space-motion studies of massive and/or young, rapidly rotating stars.
Sam: That's the right question to ask. The team used theoretical templates — mathematical predictions of what these stars' light should look like. Those templates aren't perfect matches in every fine detail. But they don't need to be. The broad smeared shape, which is what carries the speed information, is captured well enough to get reliable measurements. The researchers found their precision was generally within one to two kilometers per second — which, for stars that were previously considered impossible to measure, is a meaningful result.
Alex: So you accept some imprecision in the fine details in exchange for being able to measure something you couldn't measure at all before.
Sam: That's the trade-off, yes. And by applying this method to archived data from the California Planet Survey — observations that already existed and had never been fully used — the team was able to study more than a hundred massive stars that had simply been skipped over. They even identified two new binary star systems: pairs of stars orbiting each other that no one had recognized before.
Alex: All from data that was already sitting in an archive.
Sam: Without a single additional night on a telescope. That's what makes this approach worth paying attention to. There are large collections of astronomical data that contain observations of these fast-rotating stars, recorded over years, that have never been properly analyzed. This method opens those archives up.
Alex: It's a useful reminder that in science, the value of data often depends entirely on the tools you bring to it. The information was always there — the technique just hadn't caught up yet.
Sam: That's a fair summary. Sometimes the most productive path forward isn't building a bigger telescope. It's building a better way to read what you've already collected. Thanks for listening to ResearchPod.