Does it rain on vB 10?

Posted on November 25, 2021 by Shubham Kanodia

The primary science goal of HPF is to find planets around mid-to-late M dwarfs using the radial velocity method. The largest of these are about a third the size of the Sun, and about 2/3rd as hot (~ 3000 K vs 5777 K for the Sun). To achieve this, we conduct routine and intensive observations of a small (carefully selected) group of such stars, one of which is the ‘ultracool-dwarf’ vB 10.

So what is vB 10?

Discovered in 1944 by van Biesbroeck (hence the ‘vB’ in vB 10), it was the faintest star known back then and was observed using photographic plates. Interestingly enough, vB 10 was discovered using observations from 2.1 m telescope at McDonald Observatory, which now also hosts the 10 m Hobby-Eberly Telescope where HPF is situated. It’s about a tenth the radius of the Sun, (in fact is closer to Jupiter in size), while being about half the temperature of the Sun at 2800 K. An M8 dwarf, it belongs to the category of objects referred to as ‘ultracool dwarfs’, and is pretty much the smallest star that can still fuse Hydrogen (1H) to Helium (stars less massive than M8 dwarfs are typically called brown dwarfs; Figure 1). Over the years, there have been numerous claims and dismissals (here and here) of planetary detection around vB 10, which pretty much rule out giant gaseous Jovian type planets around this star.

Figure 1: Size comparison of the Sun, low mass stars, brown dwarfs, and planetary bodies. Image Credit: STScI

What did we find with HPF?

With HPF’s near-infrared wavelength coverage, stable instrument, and HET’s large mirror, we are trying to find smaller planets around vB 10, which have so far been out of reach of the previous generations of planet-hunting instruments. Aside from obtaining precise radial velocity measurements of the star, these observations are also useful for serendipitous observations of transient phenomena.  HPF observed vB 10 on 2019 August 20th at 05:40 (UTC), as part of its regular queue operations; subsequent analysis a couple months after  revealed a huge flux increase (emission) in some of the atomic lines present in the near-infrared spectra (Figure 2). These are indicative of a stellar flare on vB 10, a phenomena that has been observed in the past (here, here and here).

Figure 2: The top panel shows the excess flux at ~ 8500 Angstroms, which is a Calcium atomic transition, compared to the template (average) spectrum in the middle panel. The bottom panel shows the difference between the two, highlighting the excess flux that was measured on this date.

What are flares?

Stellar flares are short lived intense eruptions of energy on stellar surfaces, with their exact provenance not being completely understood. The current best hypothesis is that when a magnetic field lines (think of them as a tense and taut rubber band) on stellar surfaces rupture and reconnect, they release a LOT of magnetic potential energy. Some of this is converted to thermal energy, and generates very high temperatures, while the rest accelerates ions and electrons on the star to very fast speeds. Some of the gas at the site of magnetic reconnection rushes towards the surface of the star, and the rest is shot above the flare away from the stellar surface (Figure 4).

Figure 3: A schematic of a flare showing the magnetic field lines in green, and energy outflows. The red circles represent over-densities of plasma or hot gas caused by the release of energy at the reconnection site. Image Credit: Liu et al. (2008)

 

A Red Excess?

Apart from the Calcium lines, we observe a few different atoms that were excited enough (in a high energy state), that they displayed emission peaks.  Specifically, with HPF we are able to observe the three atomic transitions (triplet) of Helium at about 10830 A (the same transitions which we use to detect escaping planetary atmospheres). These observations at wavelengths > 9000 A, are typically inaccessible for traditional optical, CCD based spectrographs. For example, we see that the Helium atomic lines (Figure 3), show excess flux similar to Calcium, Hydrogen, etc. However, we also find a red excess which cannot be explained by the atomic transitions.

Figure 4. Emission flux observed in Helium, showing the positions of the triplet, as well as the peaks. When we fit a model to the Helium lines (shown in solid blue, red and purple), we see that it cannot reproduce an excess towards the redder wavelengths (dotted red).

What is coronal rain?

This excess has about 30% the flux of the main Helium lines, and is offset in wavelength by ~ 3 A, which corresponds to a velocity shift of 70 km/s (in other words, the Helium atoms that emitted the red feature were falling with a velocity of about 70 km/s towards the surface of the star). It is indicative of a phenomenon called coronal rain, which has been observed for the Sun (video below) for many decades.

While a fraction of the hot gas that is shot away from the star after the reconnection escapes the gravity of the star and is ejected out as a coronal mass ejections (CME), not all of it is hot enough (and hence moving fast enough) to escape the star. Some of it falls back to the star during the gradual phase, which occurs a while after the initial reconnection event (typically referred to as the impulse phase). This gas falling towards the surface of the star, is moving away from us (ignoring projection effects; this gas  in our line of sight is between us and the star), and is therefore red-shifted compare to other atoms present in the stellar atmosphere. If it is hot and dense enough, it can emit light while falling towards the surface of the star. This is coronal rain is what we think is responsible for the red asymmetry we see in our Helium observations during the vB 10 flare — hot, dense blobs of gas that are falling towards the surface of the star, from high in the stellar atmosphere, after they got there typically due to the excess energy emitted during stellar flares. In the video below we see an actual resolved video of the Solar surface showing coronal rain with the hot blobs of gas falling towards the Sun along the curved magnetic lines.

Since we cannot resolve the surfaces of distant stars, studying these asymmetries in the atomic lines tells us about the movement of the atoms that were emitting the photons we detect. It helps us understand not just the composition of stars, but also their structure and velocity fields. While there have been previous observations of some early M dwarfs which allude red asymmetries to coronal rain, this is the first quantitative analysis for such a cool, late-type star, and also the first using the Helium atom as the indicator.  All the clues suggest that it does rain on vB 10… (though not the kind of pleasant H20 rain we’re used to, but in the form of extremely hot blobs of gas at ~ 10,000 Kelvin).

Additionally we’re able to use images from the guide camera accompanying HPF observations to place limits on the total amount of energy emitted by the star during this flare, and also place limits on its duration and frequency. Thereby this work presents a unique combination of high resolution spectroscopy + photometry to study the interesting phenomenon of stellar flares, and the features accompanying them. You can read more about this in this recent research article by HPF team member Shubham Kanodia.

 

 

 

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A Search for Planetary Helium Absorption in the V1298 Tau System

Posted on August 13, 2021 by Gudmundur Kari Stefansson

Introduction

Understanding how planets form and evolve is one of the fundamental topics of research in exoplanet science. How do planets and their atmospheres form? What are their atmospheres composed of, and how do they evolve with time?

The current understanding is that planets likely form in clouds of gas and dust, and planets similar in mass to Neptune or Jupiter are capable of retaining substantial gaseous atmospheres of hydrogen and helium. Throughout the evolution of a planetary system, the atmospheres of planets evolve as well, where substantial atmospheric mass loss is expected to occur in the first million years of their lives. During this time, the host star is active and releases considerable amounts of high energy radiation that is capable of eroding away exoplanetary atmospheres.

However, most planetary systems known orbit old stars—billions of years old—where the bulk of the atmospheric mass loss has already occurred, making it difficult to understand how the atmosphere evolved. With recent space-based missions such as the K2 mission and the TESS mission, a number of planets orbiting young stars have been detected. Enter the V1298 Tau planetary system: a promising young system for studies of atmospheric evaporation.

V1298 Tau

Figure 1: The young V1298 Tau system is composed of four planets known to transit its host star. The planets are expected to still be contracting and the high energy irradiation from the young star could be eroding significant amounts of their atmospheres away. Image credit: AIP/J. Fohlmeister.

V1298 Tau is a young K star (slightly cooler than the Sun) in the constellation of Taurus at a distance of 354 light years from Earth, recently discovered to host at least four transiting planets. The planets have orbital periods of 8.25, 12.4, 24.1, and >36 days. At an age of 23 million years, the system is one of the youngest planetary systems known to host multiple transiting planets. Being a young star, V1298 Tau is active and flares, bombarding the planets with high energy radiation. The planets are all observed to be large, and could still be contracting and cooling from their original formation condition. The young age of V1298 Tau, together with the expected extended atmospheres of the planets, makes the system exciting for studying evaporating atmospheres. Do the planets show signatures of ongoing atmospheric erosion? If so, how much?

These questions were the focus of a new paper accepted for publication in the Astronomical Journal led by graduate student Shreyas Vissapragada at Caltech and HPF team member Guðmundur Stefánsson. The paper is available on arXiv, and the key results are summarized below.

Measuring Evaporating Atmospheres in the Near-Infrared with the helium 1083-nanometer line

To probe for signatures of atmospheric erosion in the V1298 Tau system, we used the helium 1083-nanometer line. This line of helium is a relatively new way to measure the signatures of evaporating atmospheres. We have previously used HPF and the He 1083 nm line to detect the atmospheric evaporation of the M-dwarf planet GJ 3470b, which marked the first detection of He 1083 nm absorption in a planet orbiting an M dwarf. The line is produced by a short-lived transition of helium from an excited energy state to a lower non-excited state. Populating the excited state of helium in exoplanet atmospheres requires high energy X-ray/UV radiation, making planets orbiting young active stars particularly promising targets for such observations.

To probe if the V1298 Tau planets show signatures of evaporating atmospheres, we used two ways to investigate the He 1083 nm line: a) we used HPF on the 10m Hobby-Eberly Telescope, and b) we used specialized 1083 nm narrow-band filter observations on the 200″ Hale Telescope at Palomar Observatory.

Telescopes

Figure 2: Telescopes used in this work: The 10m Hobby-Eberly Telescope at McDonald Observatory (left), the 200″ Hale Telescope at Palomar Observatory (middle), and the 3.5m Astrophysical Research Consortium 3.5m Telescope at Apache Point Observatory (bottom right). Figure credits: University of Texas at Austin,  Palomar/Caltech, and Apache Point Observatory, respectively.

Transit Observations of V1298 Tau c with HPF and the ARC 3.5m Telescope

First, we used HPF’s coverage of the He 1083 nm line to look for evidence of helium absorption during the transit of the innermost planet V1298 c.

Figure 3a shows in-transit observations of V1298 Tau c using an Engineered Diffuser—nanofabricated pieces of optics enabling some of the highest precision photometric observations from the ground—on the 3.5m Telescope at Apache Point Observatory. We see a clear transit, along with a flare happening about an hour before the center of the transit. Simultaneously with these photometric observations we obtained HPF observations, where the timing of the HPF observations is denoted with the black triangles in Figure 3a. It is not often that you get spectroscopic observations in-transit and right after a flare!

The black spectra in Figure 3c show the combined spectroscopic observations from HPF during the in-transit observations, while the red, blue, and green spectra show HPF observations on different nights outside of transit. We see that the He 1083 nm line (three orange lines in Figure 3) is deepest during the transit: could that mean that we saw the helium absorption during the transit? It could be. However, we also see that the depth and shape of the line is highly variable, making it difficult to rule out the possibility that we are seeing intrinsic variations in the line from the star. Although non-conclusive, these observations give a direct constraint on the line behavior in-transit and outside of transit.

V1298 Tau

Figure 3: In-transit investigations of V1298 Tau c with HPF and ARCTIC on the 3.5m Telescope at Apache Point Observatory (APO). a) Transit from APO, showing a clear transit and a flare happening ~1 hour before the transit midpoint. b) Residuals from the transit and flare fit. c) HPF observations of the He 1083 nm line (denoted by the three vertical orange lines). In-transit observations are shown in black, while the red, blue and green spectra show out-of-transit comparison observations. We see that the line is the deepest during the in-transit observations. This could suggest atmospheric absorption, but we also see clear variability of the line in the other nights. As such, it is unclear if the variability is due to the planet or due to the activity of the star.  Click image for full-size version.

Narrow-band diffuser-assisted observations of V1298 Tau c, d and b with the Palomar 200″ Telescope

Second, we used a specialized narrow-band filter centered on the He 1083 nm line developed by Shreyas Vissapragada for exactly these types of studies on the WIRC (Wide Field Infrared Camera) instrument on the 200″ Telescope at Palomar Observatory. You can read more on the specialized instrumental setup in a recent paper by Shreyas Vissapragada here.

Figure 4 shows the observations from the Palomar 200″ of V1298 Tau c, d and b. Planet c orbits closest to the host star, whereas planet b orbits furthest out (planet b was discovered first, hence the offset labeling). To see if we see signatures of excess absorption, we compare the transit depth we observe in the narrowband 1083 nm filter (shown in solid lines in Figure 4) to the known transit depths in broad-band filters (dashed lines). In Figure 4, we see that planets b and c show no clear evidence for increased transit depths. However, planet d shows a clear increased depth—a telltale signature of atmospheric absorption.

With planet d being the only planet showing a clear excess absorption, we discuss in the paper the possibility that planet d might be in a sweet spot for atmospheric erosion, whereas planets c and b might potentially be too close-in/far-out to create an absorption signal. Another possibility is that planet d is the lightest of the three planets, so it has the least amount of gravity to hold onto its atmosphere. We note that observations of planet d were performed on two different nights, so we urge additional observations that ideally cover a full transit in a single night to further confirm these observations and to look for  variability in the transit depth from transit-to-transit.

V1298 Tau observations from palomar

Figure 4: V1298 Tau observations of the transits of V1298 Tau c, d and b from the Palomar 200″ telescope using a purpose-built He 1083 nm narrow-band filter. Here we compare the expected transit depth in the He 1083 nm (solid lines) filter to previously known transit depth measurements (dashed lines). For planets c and b, we see transit depths that are consistent with previously measured transit depths. However, for planet d, we see evidence of a significantly larger transit depth.

With new young systems being discovered with increased frequency, we are excited to continue using HPF and these other instruments to study atmospheric evolution in different exoplanet systems to gain further insights into what drives atmospheric escape that ultimately sculpts the atmospheres we see for older more mature planets. For further information on this work, we invite you to take a look at the paper available on arXiv.

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Scrambling the pupil of the Hobby Eberly Telescope

Posted on June 24, 2021 by Shubham Kanodia

Detection of small Earth like planets in the habitable-zones (HZ) of M dwarfs requires precise radial velocity measurements (RV) at the meter-per-second level. These extremely precise measurements are really hard! As the schematic below illustrates, they necessitate each step in the measurement process to be well understood and calibrated — the Earth’s atmosphere, the telescope, the fibers, the instrument, etc.

The multi-faceted nature of precise RV measurements.

 

Achieving the exquisite RV precision that we have demonstrated on sky with HPF requires each of these sub-systems to work seamlessly. In this blog, we have discussed the performance of the some of these, with the detector, the extensive environmental control and stability,  the optics, the calibrators. However, these systems together do not suffice to guarantee the level of RV precision required for HPF. One big factor external to the instrument is the telescope, the Hobby Eberly Telescope (HET).

Quick intro to the Hobby Eberly Telescope (HET)

The HET is one of the world’s largest optical telescopes, with a mirror that is 11 meters across. It consists of 91 hexagonal segments, each about 1 meter in size. The telescope was conceptualized by Larry Ramsey, an astronomer at Penn State (also a part of the HPF team) in the 1980s. However, unlike conventional observatories (which track objects in both altitude and azimuth), the HET primary mirror is fixed in altitude at about 55 degrees, and can only move in azimuth. Therefore, to track the object as the Earth rotates, the secondary mirror and corrective optics (situated in the prime focus instrument package; PFIP) move across the field of view of the primary.

HET’s hexagonal mirror

 

Varying telescope pupil

The telescope focuses light into the HPF telescope fiber, as it tracks the star across the sky. However, the effective pupil projected by the combination of the telescope primary and PFIP on to the HPF fiber changes with time. In the movie clip below we show a simulation of the HET pupil changing across an observation (referred to as a track),  where the circular pupil (green) moves across the hexagonal telescope primary (grey) and causes the centroid (red cross) to drift. The pupil of a telescope is the virtual image it forms, akin to the distribution in angular space. (Minor Aside: The pupil of a telescope is like the beam from a flash light that is incident on a far away surface. The nice circular angular distribution that is seen, is analogous to the pupil here.)

 

A change in the centroid coordinates with time can cause an artificial RV shift for HPF, which would correlate with the centroid coordinates, pupil area, and the object altitude. Therefore, the HPF team has developed a scrambling system which uses a 2 mm glass ball lens. This ball scrambler effectively decouples the spectrograph from input illumination changes, without losing too many precious photons. Read more about it in this paper.

In addition to the changing pupil area and centroid shown above, while observing certain objects in the sky, the telescope pupil is also obstructed by the a 28 meter tower, at azimuth ~ 68 degrees. This tower houses the center of curvature alignment sensor (CCAS), which is used to align the individual segments of the primary mirror into a perfect spherical mirror.

Left: An image of HET showing the dome housing the telescope, as well as the CCAS tower to the left of the dome. Credit: Marty Harris/McDonald Observatory. Right: Snapshots of the pupil viewing camera under afternoon sky illumination. The central obscuration with the PFIP, as well as the support trusses. Also visible are the fiber bundles on both sides of the central obscuration. The CCAS tower is visible in the top right image obscuring part of the pupil.

The combination of the central obscuration with the PFIP, support trusses, and the CCAS tower change the pupil as shown below.

 

But how does this affect the RVs?

In order to measure the effect of this changing illumination on the RVs, we need to track the RVs as a function of these telescope parameters that are changing with time. This requires a star that is bright enough to afford short exposure times. Luckily, one of the brightest M dwarf targets in the sky is visible from HET — GJ 411.

GJ 411 is the brightest M dwarf in the Northern Hemisphere, and is situated just about 2.5 parsecs (8 light years) from Earth. For comparison, the closest star Proxima Centauri is about 1.3 parsecs from us. This bright M dwarf is routinely observed by HPF, as part of its long term monitoring and engineering programs, with about 30 one-minute exposures during every visit. Across 43 such visits, we have about 1200 exposures on this object, which allow us to sample the RVs across a range of different centroid positions, pupil areas, and airmass. The plots below show how the centroid, and pupil area change with time and object altitude.

Change in pupil parameters with time

We then search for correlations between these GJ 411 RVs and these parameters, by combining the RVs across different visits. For example, the plot below shows the change in RVs as a function of altitude after combining the visits (for reference,the exposures across a single visit are shown in red).

Change in RVs as a function of Altitude

So what do we find?

We look at the RVs as a function of different parameters, namely the centroid position, altitude, and also some environmental factors such as ambient temperature, humidity, pressure, seeing and sky brightness, and find no dependence.  This places an upper limit on the scrambling performance of the ball scrambler used in HPF. We use the same ball scrambler system in HPF’s sister spectrograph – NEID, and also demonstrate the viability of this scrambling system for the upcoming generation of extreme precision radial velocity spectrographs aiming to find an Earth analogue in its Habitable zone.

You can read more about this work in this recent research article by HPF team member Shubham Kanodia.

 

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