Eternal spotshine of the spinning red suns

Introduction

Much of our time on sky with HPF is dedicated to discovering exoplanets, and measuring their detailed properties.  To do this, we concentrate our efforts on older, magnetically quiet stars to minimize noise and make the planets easier to detect.  But M dwarf stars are astrophysically interesting in their own right, and we have reserved a small amount of our observing time to studying younger, more active M stars.  In this post, we’ll look at a new result our team has just published, from an experiment in which we looked at some very active stars in an attempt to understand differences in Doppler signals originating from exoplanets and magnetic activity.

“An Easy Target”

Back in 2018, when we were just getting HPF on sky, a team of astronomers analyzing archival radial velocity data from the Keck/HIRES and HARPS spectrometers announced a fascinating discovery.  Examining years’ worth of Doppler measurements of AD Leonis–a very young and active M dwarf star–they found a periodic signal at around 2.2 days, which matches the star’s rotation period.  Ordinarily, one would dismiss a Doppler signal at the rotation period because it is almost certainly caused by magnetic starspots rotating across the stellar surface.  However, the HIRES and HARPS data appeared to show a signal that was consistent for years at a time, which is not expected of starspots.  Spots should disappear within a few rotations of the star, so the hypothesis presented for AD Leo was that this signal represented a giant planet whose orbit was coupled to the rotation of the star.  If true, this would be a rare and exciting discovery!  But the specter of starspots left this possibility stuck in “candidate planet” limbo.

This type of situation is exactly what HPF, with its near-infrared wavelength coverage, was built for.  We expect starspot signals to be chromatic: that is, the size of the signal will change depending on the wavelengths (or colors) you observe them with.  This is due to the fact that the color contrast between a cool starspot and the surrounding stellar atmosphere is greater at visible wavelengths than in the infrared.

SDO Solar colors

Multi-wavelength images of the Sun, as captured by NASA’s Solar Dynamics Observatory (SDO). Various features of the Sun’s atmosphere appear more prominently at certain wavelengths. Image Credit: NASA Goddard Space Flight Center

On the other hand, a genuine exoplanet should show no such wavelength dependence.  So if the signal for AD Leo is due to a real planet, it should look exactly the same for HPF as it did for the optical-wavelength HIRES and HARPS.  The nice thing about the AD Leo signal is that it is so massive as to be easily detected by HPF, even if it didn’t hit its full measurement precision goals straight out of the gate.  Given the timing of the announcement of the planet candidate, it seemed like an obvious choice for HPF’s first science project.  Once HPF was set up and running, we immediately started collecting observations of AD Leo.

Too much of a good thing

In 2018, we watched AD Leo until it set for the season, and were extremely encouraged.  Our data showed a signal that matched extremely well to the HIRES and HARPS data, as we would expect if the signal were caused by a planet.  We were confident enough in the status of the planet that we wrote a complete draft of a research journal paper titled “Near-infrared radial velocities confirm AD Leonis b,” and were on the verge of submitting it for publication.

But then we looked at our next target, and pulled the emergency brake.  Because we saw the same thing again.

G 227-22 is another young, rapidly-rotating M star.  The MEarth team, who are looking for planets that transit nearby M dwarfs, showed that it completes a rotation every 6 hours, which is remarkably fast.  It was another early science target for HPF, because the TESS satellite is collecting an enormous amount of data for it, which is especially valuable for studying stars and their planets.  What complicated our interpretation of AD Leo is that HPF radial velocities of G 227-22 also showed a signal at the stellar rotation period that did not change or disappear, even over many stellar rotations.

To see two such similar rotation-period signals among HPF’s early set of targets could not be coincidence.  We saw two possible interpretations for this result, neither of which seemed especially intuitive:

  1. Both AD Leo and G 227-22 have giant exoplanets in orbits coupled to those stars’ rotation.  But for us to find two such objects among our first two targets would suggest that nearly all young M dwarfs must have this type of planet!  This seemed inconsistent with previous studies of M stars.
  2. These signals are caused by magnetic starspots.  However, in order for them to produce the Doppler signals we saw, they would have to behave quite differently from what we see on the Sun.

It seemed impossible to convincingly distinguish between these possibilities without more data.  So our results for AD Leo were frustratingly put on hold.

A multi-color Doppler campaign

HET and Keck

The Hobby-Eberly (left) and Keck I and II (right) telescopes.

Either of the two possibilities above are quite interesting from an astrophysics perspective, so we were determined to settle the debate.  The approach we decided to take was to mount an intensive, multi-wavelength Doppler observing campaign for AD Leo, G 227-22, and two more similar stars (GJ 1245B and GJ 3959).  We set out to get both visible and near-infrared radial velocity measurements, which involved enlisting two 10-meter telescopes: in addition to HET, which houses HPF, we enlisted the HIRES spectrometer on the Keck I Telescope in Hawaii.  The idea was that if any (or all!) of these stars hosted spin-orbit coupled planets, their visible and infrared Doppler signals should look the same.  Our experiment benefited from TESS observing these stars at around the same time, so our targets received quite a bit of attention during the 2019 observing season!

Spots that won’t quit

Once we completed our observations, it became apparent that none of our targets seem likely to host a planet orbiting at the period of the stars’ rotation.  The signals appear to shift in phase, and typically have smaller amplitudes in the infrared wavelengths, as we would expect from starspots.  Even the AD Leo signal finally gave way in 2019–the infrared signal was much smaller than we saw in 2018, and another team has confirmed that it was also smaller at optical wavelengths.

AD Leo Seasons

HPF radial velocities of AD Leo from 2018 (left) and 2019 (right). The shrinking amplitude confirms the signal is caused by stellar magnetic activity, not a planet.

Still, it’s interesting to look in more detail at what caused us and other teams to suspect planets in the first place.  These spots do not act like what we observe on the Sun!  What we found is that the signals from these spots stay constant for 6 months to a year, typically.  But since these stars complete a rotation every few days or less, that means these signals persist for hundreds–or even thousands–of stellar rotations.  For comparison, Sunspots typically disappear within one or two rotations. Below, we show the brightness of G 227-22, as measured by TESS.  The consistency of the brightness variations suggests the spot patterns on the star stay fixed over time.

Phase curves for G 227-22

Brightness measurements of G 227-22, as measured by TESS. Moving from bottom to top, we see how the star dims as regions with more starspots rotate into view. From left to right, we see that the brightness distribution does not change over time, suggesting the spot pattern is fixed over many stellar rotations.

This adds to the challenge of finding exoplanets with the Doppler technique, because if we can’t rely on starspot signals to degrade over short periods of time, then we need more information to help separate real planets from false positives.  Once again, we learn that there’s no such thing as an “easy target,” especially when dealing with M dwarfs.

Really, no planets?

Probably not!  But we’ll leave you with a little cliffhanger, just for fun.  Remember in the previous section where we said that the observed starspot signals “typically” had smaller amplitudes in the infrared?  Well, there was one target–GJ 3959– for which the optical and infrared signals match up surprisingly well.  Take a look:

GJ 3959 RVs

Optical Keck/HIRES (blue) and near-infrared HPF (red) radial velocities of GJ 3959, shown at the phase of the star’s rotation. The optical and infrared signals match in phase and amplitude.

We were almost tempted to label this signal a planet candidate.  However, our colleagues on the CARMENES team have also looked at this star, and observed evidence of wavelength-dependent Doppler shifts.  Thus, we concluded that this signal is most likely caused by starspots, and the spots may have changed size between the times we observed with HIRES and HPF, leading to the signals coincidentally matching.  However, there’s one more chance to determine whether this might be the real deal: TESS is observing this star as we write, and the first data are scheduled to come down any day now.  If there is a planet here, it has a high probability of transiting, and thus being detected by TESS!  If that happens, you’ll see us frantically updating this post.

Where can I learn more?

A research article describing this experiment in full detail has been accepted for publication in the Astrophysical Journal.  Prior to publication, you can read the pre-print on arXiv.

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Measuring Exoplanet Atmospheres with HPF

Introduction: Transmission Spectroscopy

As we have covered in previous posts, HPF was designed and built for the goal of detecting exoplanets through the Doppler motion of their host stars.  Of course, there are other astrophysics experiments that can take advantage of HPF!  There are plenty of applications for a stable, high-resolution near-infrared spectrometer.  In today’s post, we will discuss one such application that allows us to learn more about nearby exoplanets: transmission spectroscopy.

Normally, it is extremely difficult to learn anything about the atmosphere of an exoplanet.  The planet itself is lost in the glare of its host star, and for a planet like Earth, the atmosphere is a tiny component of its overall radius–if Earth were the size of an apple, the atmosphere would only be as thick as a normal apple’s skin.  One special situation in which we can catch a glimpse of an exoplanet’s atmosphere is when the planet transits (or eclipses) its star.  When a transit occurs, a tiny (but measurable!) fraction of the star’s light will filter through the planet’s atmosphere, where atoms and molecules in that atmosphere will absorb light at very specific wavelengths.  Thus, by comparing the normal spectrum of the star with the spectrum as observed during transit, we can determine which chemical species are absorbing light in the planet’s atmosphere.  With the right contextual information, we may also be able to probe properties like the atmospheric temperature, pressure, and more.

Transmission spectroscopy examines starlight filtered through an exoplanet’s atmosphere to determine the composition and other properties of the planet’s atmosphere (Image credit: Christine Daniloff/MIT, Julien de Wit).

Planets orbiting very close to their host stars get so hot that their atmospheres begin to escape into space, trailing behind the planet like a giant comet tail.  Recent simulations of these escaping atmospheres suggest that absorption of near-infrared light by helium atoms should create a large signal for transmission spectroscopy.  The wavelength of this helium absorption is right in the middle of HPF’s waveband, so as an early science program for HPF, we attempted to detect this effect for an exoplanet transiting an M dwarf star.

The target: GJ 3470b

We have mentioned the M dwarf star GJ 3470 on this blog before–it was actually the star chosen for HPF’s first light image.  The star hosts a planet about the size of Neptune orbiting very close, which is the best type of planet for atmospheric characterization.  Other studies have already suggested that the planet should have a blue sky!  Based on what was already known about the planet’s size, density, and stellar radiation exposure, we predicted it should exhibit significant absorption from helium.  So we set out to determine whether HPF could measure it.

The experiment

When you need a planet to be in front of its star in order to make a measurement, the timing is crucial.  Careful planning is needed to observe the star at a time when a transit occurs at night from your observatory, and the weather has to be good.  In the case of GJ 3470b, we had to observe three such events in order to develop enough signal from the helium absorption.  In the meantime, we also observed the star when the planet wasn’t transiting, in order to compare the in- and out-of-transit cases.  All said, it took us about 5 months to get all of the observations required to do this experiment.

So did we see helium absorption?

To date, GJ 3470b is the smallest planet for which atmospheric helium absorption has been measured.  As a result, the helium signal is relatively small, and we had to be careful with how we merged all the observations into a single spectrum.  We also had to convince ourselves that the signal was not created by changes in the HPF instrument, or in Earth’s atmosphere.

An additional complication comes from the fact that the atmosphere of GJ 3470b is so hot that the temperature causes helium atoms to fly out of its atmosphere into space at high velocity, in a process known as a Parker wind.  This motion of the helium gas distorts the absorption feature we would otherwise expect to see, so we had to carefully model the effects of the Parker wind in order to make sure our data were consistent with the all of the physics involved in the interaction of the planet’s atmosphere with the intense stellar radiation.

After accounting for all of these effects, we determined that the HPF spectra show evidence for helium absorption caused by the atmosphere of the planet.  In the image below, we show the comparison between the spectrum of the star with and without the planet passing in front.  The spectrum shows a little more absorption at the wavelength of the helium feature during transit, in approximately the shape we expect from the wind-shifted motions of the atmosphere.  The signal is weak, but our careful analysis of the data in combination with the wind model suggests it is real.  Our detection is also supported by observations of the same planet by another team using the CARMENES spectrometer.

GJ 3470b Transmission Spectrum

Spectra of GJ 3470 during (top) and outside of (bottom) the transit of its planet. The red curve shows our model of the expected absorption from helium in the planet’s atmosphere. Only during transits does the star’s spectrum show the expected absorption!

In short: the technique works!  This detection is an enticing preview of what’s possible in terms of atmospheric characterization of exoplanets using HPF.

Curious case of blue-shifted broad absorption in GJ3470b

While infrared helium absorption has been detected earlier in the atmospheres of larger planets, the most puzzling aspect of our new detection in the first warm Neptune was its unusually broad absorption profile.  This is especially true on the blue-wavelength side (leftwards in the image above), since the red side of the profile has more noise due to contamination from Earth’s atmosphere.

Line of Sight Velocoty of exosphere around GJ3470b

The line-of-sight velocity field of upper atmosphere around GJ 3470b. An Earth-based observer is towards the left side of the plot, and the host star GJ 3470 is towards the right side.  Bluer colors indicate more rapid motions towards Earth.

The reason turned out to be simple orbital dynamics. The orbit of GJ 3470b is not perfectly circular, and so its motion along the line connecting the star and Earth (the so-called “line of sight velocity”) is actually slowing down during the transit. Just like a slingshot, helium atoms escaping the gravitational pull of the planet continue to move towards us at the velocity they had the moment they escaped from the planet. Once you incorporate this velocity change into the modeled absorption profile from the helium atoms, it matches well with the observation!

What did we learn about the planet?

These measurements allowed us to estimate properties like the density, temperature, and velocity of helium atoms in the planet’s atmosphere.  In the image below, we have actually mapped out the distribution of helium surrounding the planet itself.

GJ 3470b model

Scale model of the star GJ 3470 (large black circle) with its planet (inner white circle) in transit. The red-yellow color scale indicates the density of helium atoms surrounding the planet, based on our transmission spectrum.

With powerful instruments and enough observing time, we can learn a lot more about a planet’s atmosphere.  Similar observations can reveal different chemical species, probe deeper into a planet’s atmosphere, and monitor changes over time.  Eventually, we will use this technique to look for changes in planetary atmospheres that could be caused by life!  So while this first step for HPF is exciting, it is only scratching the surface of an aspect of exoplanet study that will become increasingly important in the coming years.

Where can I find out more?

Details of this experiment were recently published in the AAS Journals in an article led by HPF team member Joe Ninan.  We encourage you to go there for more information!

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G 9-40b: HPF’s first planet validation

HPF has validated its first planet called G 9-40b, a sub-Neptune sized planet (2 Earth radii) orbiting a nearby low mass M-dwarf star with an orbital period of 6 days. The paper has been published in the Astronomical Journal and is available freely on the arXiv preprint server here.

It takes a Village to Validate a Planet

What does it take to say that a planet candidate is a bone-fide planet? Even for promising planet candidates around nearby stars, it takes a number of follow-up observations to say that a planet candidate is really a planet, and not something else, such as an eclipsing binary star system or other astrophysical false positives that can imitate planet transits.

G 9-40 was originally presented to have a candidate transiting planet in a paper by Yu et al. 2018 using photometric data from the K2 mission. After a closer look at this candidate planet, we saw it looked interesting for many reasons.

First, at a distance of only ~90 light years, G 9-40b is among the 20 closest transiting planetary systems known (see Figure 1), and is currently the second closest transiting planets discovered by the K2 mission to date. Second, the host star is a nearby bright M-dwarf star—but being bright makes it easier for us both to get a good radial velocity precision on this target with HPF, and to observe additional transits of it from the ground at high precision. Further, the brightness of the host star along with the large transit depth of the planet (~0.35%, see Figure 2) makes G 9-40b one of the most favorable sub-Neptune-sized planets orbiting an M-dwarf for transmission spectroscopy with the James Webb Space Telescope (JWST) in the future—more on that below.

Figure 1: Planet radius as a function of the distance of the planet system from Earth in parsecs (1 parsec is about 3.2 light years). G 9-40b (highlighted in the bright red) is among the closest transiting systems known to date. Red points show planets around low mass stars (M-dwarfs) and blue points show planets orbiting more massive hotter stars. Figure 12 from the paper.

Given these reasons, we decided to conduct more observations of this planet candidate and characterize it better to decide if this was a planet or not. To do so we used Adaptive Optics (AO) imaging observations, further precision transit follow-up observations from the ground, and precision radial velocities with HPF. Lets dive into each of these observations below.

Figure 2: Transits observed in the light curve of G 9-40 from the K2 mission. The transits are deep, making it a compelling candidate target for transmission spectroscopy in the future with the James Webb Space Telescope. Figure 7c from the paper.

Adaptive Optics Imaging

To check if G 9-40 is blended with another star—which could confuse us on which star is the true source of the transit, or indicate that the system would be an eclipsing binary system rather than a true planet—we used high contrast adaptive optics imaging observations from the ShaneAO system at Lick Observatory. These adaptive optics observations are often conducted by shooting a high power laser into the air (Figure 3) to generate a so-called laser guide star. This laser guide star is then used to correct for the distortion in the atmosphere by rapidly changing the shape of an ‘adaptive’ mirror in the optical system, allowing us to obtain extremely high contrast images of the star. From these observations we see no evidence of blending or other stellar companions nearby the star G 9-40, further suggesting that G 9-40 is the true source of the transits we see from K2 and in our ground-based data (see next section).

Figure 3: Adaptive optics image (left inset) and resulting contrast curve as observed with the ShaneAO Adaptive Optics (AO) System on the 3m Shane Telescope at Lick Observatory (right). The contrast curve on the left shows the brightness of the inset image (in the near-infrared K-band) as a function of the distance from the center of the star. We see no evidence of a background star in the image that could dilute and/or be the source of the transits we see in the K2 and/or ground-based data. Left: Figure 1 from the paper. Right image credit: Laurie Hatch.

Ground-based Transit Photometry using Engineered Diffusers

To constrain the planet parameters and further update the orbital ephemeris of the system, we used high-precision diffuser-assisted photometry using the ARCTIC imager on the 3.5m Telescope at Apache Point Observatory. We observed the transit using a custom-built narrow-band filter made by Semrock / AVR Optics  that operates in a band with little-to-no water absorption. We often call this filter the ‘cloud-killer’. If you are interested, you can actually buy a very similar filter from Semrock off-the-shelf here!

The plot below shows our ground-based transit, which agrees well with the transits observed by K2 further suggesting that our planet candidate is indeed a planet. Further, these observations allow us to pinpoint the timing of the transit extremely well, allowing us to predict midpoints at the start of the JWST era (~2021-2023) with less than 2minutes of error. Such small errors are important to enable easy scheduling for in-transit spectroscopic follow-up in the future.

Figure 4: Transit observations of G 9-40b. Left: Ground-based transit measurements—showing the flux of the star as a function of time, showing a clear transit dip—obtained using the ARCTIC imager and the Engineered Diffuser on the 3.5m Telescope at Apache Point Observatory (right). These observations allow us to further characterize key parameters of the planet, including its period and its radius. Image credit: Figure 7e from the paper (left), Gudmundur Stefansson (right).

But what is the diffuser-assisted photometry? Excellent question. This means that we used recent technology called Engineered Diffusers—developed by RPC Photonics—to help conduct the observations. Engineered Diffusers are nano-fabricated devices that you can easily place in a telescope beam-path to mold the focal plane image of a star to a stabilized top-hat shape (Figure 5). In doing so allows you to keep the image of the star stable throughout the night, minimizing photometric errors due to the atmosphere. Further, in spreading out the light over many pixels opens up the possibility to observe bright targets on large telescopes—especially useful in observing G 9-40—with the 3.5m Telescope at Apache Point Observatory. If you are interested in reading more about Engineered Diffusers for astronomy you can read more about them in a past press release here or here, or in our previous paper here.

Figure 5: An Engineered Diffuser is capable of molding the image of a star into a broad and stable top-hat shape. b) We used an Engineered Diffuser in the telescope beam path of the 3.5m Telescope at Apache Point Observatory to stabilize the image of the star and allow us to obtain extremely high photometric precisions allowing us to precisely characterize the parameters of G 9-40b. Image credit: RPC Photonics (left), Gudmundur Stefansson (right).

Precision Radial Velocities from HPF

Last but not least, this paper includes some of the first precision near-infrared (NIR) radial velocities (RVs) from the Habitable-zone Planet Finder. In a previous paper led by AJ Metcalf, we showed that HPF is capable of 1.5m/s RV precision in the NIR (see blog post here), enabled by the precise near-infrared HPF Laser Frequency Comb (LFC) also discussed in detail in our previous blog post here. Using precision NIR RVs from HPF enabled by the HPF LFC, we placed an upper limit on the mass of G 9-40b of 12 MEarth. We hope to obtain a robust mass measurement with future RV measurements with HPF in the future. In doing so will allow us to constrain the composition of the planet, and to inform future atmospheric follow-up efforts with JWST.

Figure 6: Precise radial velocities from HPF (left) on the 10m Hobby-Eberly Telescope (right) allowed us to place an upper limit on the mass of the planet of 12 Earth masses. We hope to get a further precise mass constraint by continuing to observe G 9-40 in the future. Image credit: Figure 11a from the paper (left), Gudmundur Stefansson (right).

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