If you had a hot new instrument like the Habitable Zone Planet Finder (HPF) now mounted at the Hobby-Eberly Telescope (McDonald Observatory, University of Texas), how would you run it through its paces for fine-tuning and verification of its performance specs? The team behind HPF has chosen to deploy the instrument during its commissioning phase on a nearby target, Barnard’s Star, which for these purposes we can consider something of an M-dwarf standard.
Working at near-infrared wavelengths, HPF uses radial velocity methods to identify low-mass planets around nearby M-dwarf stars. The choice of wavelength is determined by the mission: M-dwarfs (also known as ‘red dwarfs’) are prey to substantial magnetic activity that shows up as spots and flares that disrupt instruments working in visible light, not to mention the fact that they are small to begin with and thus faint on the sky. In the near-infrared, close to but not in the visible spectrum, this category of star appears brighter and its surface activity more muted.
I mentioned Barnard’s Star as a kind of standard because it precisely suits astronomers’ needs for calibrating such an instrument. Here let me quote from a Penn State blog on HPF (Penn State built the instrument), which lays out the ideal for commissioning:
While the ultimate goal of any Doppler spectrograph is to find lots of exoplanets, boring is better during the commissioning phase. The only way to test the stability and precision of your end-to-end measurement system–from the telescope, through the fiber optics, and ultimately the optics and detector of the spectrograph–is to make repeated measurements of a star with little or no variability. That way, any variability seen in the measurements must be caused by the instrument, rather than the star itself. In other words, the less variability we measure in observations of our stable “standard star,” the better the instrument is performing.
Barnard’s Star fits the bill beautifully. For one thing, it’s close by, at about 6 light years, making it the second-closest system to the Sun. At 14 percent of the Sun’s mass, it’s also typical of the kind of stars HPF will survey. But the real value lies in its age, for Barnard’s Star is thought to be extremely old, possibly as old as the Milky Way itself. The star rotates slowly and shows little stellar activity of the kind that would mask the radial velocity signal in other M-dwarfs.
Image: The new Penn State-led Habitable Zone Planet Finder (HPF) provides the highest precision measurements to date of infrared signals from nearby stars. Pictured: The HPF instrument during installation in its clean-room enclosure in the Hobby Eberly Telescope at McDonald Observatory. Credit: Guðmundur Stefánssonn, Penn State.
To increase precision at the HPF, Penn State has added a laser frequency comb (LFC) to the mix. Custom-built by the National Institute of Standards and Technology (NIST), the comb is a kind of ‘ruler’ that is used to calibrate the near-infrared signal from other stars. Work like this demands a calibration source because a spectrum from the observed star will ‘drift’ slightly, a movement that must be corrected when astronomers are looking for signals in the area of 1 meter per second to identify a small planet in the habitable zone of an M-dwarf. This is a kind of false Doppler effect likely due to physical issues in the instrument itself. Measuring the spectra of two sources at once — one of them being the stable frequency comb — allows the correction to be made, letting the true Doppler effect induced by planets around the star be observed.
Atomic emission lamps have been used for such calibration in the past, but laser frequency combs produce spectra with finely calibrated emission lines that are stable and of uniform brightness. Adding a laser comb to HPF ensures maximum performance, says Suvrath Mahadevan (Penn State), who is principal Investigator of the HPF project:
“The laser comb…separates individual wavelengths of light into separate lines, like the teeth of a comb, and is used like a ruler to calibrate the near-infrared energy from the stars. This combination of technologies has allowed us to demonstrate unprecedented near-infrared radial velocity precision with observations of Barnard’s Star, one of the closest stars to the Sun.”
Image: An example comparison of calibration spectra for astronomical spectrographs. Credit: HPF / Penn State.
Mahadevan adds that the technical challenges of reaching this level of precision are substantial. The instrument is highly sensitive to any infrared light emitted at room temperature, which means operations must take place at extremely cold temperatures. Thus far, the results speak for themselves, as discussed in a paper in Optica that describes the Barnard’s Star work (citation below).
The current data series on Barnard’s Star shows a stability of about 1.5 meters per second, which tops anything achieved by an infrared instrument. This is actually close to the best earlier measurements of the star, which have come from the renowned HARPS spectrograph working at visible wavelengths (378 nm – 691 nm); these come in at 1.2 meters per second. The HPF goal is 1 meter per second, not yet attained, though the team continues to refine its numbers while searching for possible instrumental issues that may play a role. From the blog:
We would be remiss if we did not emphasize that working all of the kinks out of an ultra-precise Doppler spectrograph is a years-long process, and we are far from done making improvements to the instrument and our analysis techniques. With that said, our early observations of Barnard’s star are extremely promising!
Can HPF confirm the Pale Red Dot project’s super-Earth around Barnard’s Star? Not yet. Although the instrument has the precision to see Barnard’s Star b, a problem remains:
As it turns out, cosmic coincidence prevents us from having much information on Barnard b at this point. The orbit of the proposed planet is eccentric, which means the Doppler signal is more pronounced at some phases of its orbit than others. Through nothing but luck, our HPF-LFC observations completely missed the most dynamic section of the Barnard b phase curve. Thus, while our HPF measurements do not rule out the proposed planet, they cannot yet confirm it, either. This is just one of many examples of how exoplanet detection is a data-intensive process!
Image: The orbital model of Barnard b (blue), with HPF measurements (gold) folded to the orbital phase. Our measurements have not yet covered the maximum of the eccentric orbit. Credit: HPF team / Penn State.
The paper on applying laser frequency comb techniques to the HPF in studies of Barnard’s Star is Metcalf et al., “Stellar spectroscopy in the near-infrared with a laser frequency comb,” Optica Vol. 6, No. 2 (2019), pp. 233-239 (abstract).