The Camera of HPF

Introduction: Spectrometer Cameras

A spectrometer, as the name implies, records a ‘spectrum’ of an object. This spectrum, in its most basic form, is just a series of images of the instrument entrance aperture (whether it be a star, slit, optical fiber, or otherwise) that has been spatially separated based on the intrinsic color distribution of the source. The separation of these monochromatic (single-wavelength) images depends on the dispersion properties of the spectrometer.

Simulated star and wavelength calibration source spectra, as recorded by the HPF spectrometer. The calibration source spectrum (center channel), in this case a highly specialized laser source which only emits at discrete wavelengths, produces a spatially separated set of images of the spectrometer entrance slit. On the other hand, starlight (bottom channel), contains flux across a broad range of frequencies. This produces a continuous spectral image across each diffraction order.

The optic at the heart of the spectrometer, the high dispersion echelle grating, provides the chromatic dispersion required to separate the individual colors encoded in the incoming starlight, but it is the spectrometer camera that ultimately produces our final image of the stellar spectrum.

Cartoon overview of the HPF spectrometer optical train. Light from the telescope exits the fiber, gets dispersed by a pair of diffraction gratings (first the high dispersion Echelle grating, then the volume phase holographic grating), and is finally imaged onto our infrared detector array by the HPF camera. The camera assembly provides the needed focusing mechanism to deliver our final spectral image.

The spectral resolution of the spectrometer, which tells us how well we can separate individual colors, is entirely set by the properties of the entrance aperture and diffraction grating(s). However, the spectrometer camera must also have pristine image quality, or else our spectral resolution will suffer. Any blurring of the image of the spectrum due to uncorrected aberrations within the camera will degrade our ability to separate individual colors of the star.

Example of image quality degrading spectral resolution. On the left shows a near-diffraction-limited image of our spectrometer slit, while the right shows a highly aberrated image. The effective spectral resolution of the aberrated slit image (right) is nearly 5% lower.

The HPF Camera

Our spectrometer resolution directly affects how well we can measure minute stellar radial velocity shifts, so typical high resolution Doppler spectrometers require specially designed cameras that maintain excellent image quality across all of the diffraction orders. This high level of image quality is also needed to lower our sensitivity to illumination changes coming from our optical fiber delivery system.

Image quality of the HPF camera across the wavelengths that fall onto our infrared detector. The grey boxes represent individual detector pixels, magnified by a factor of 1000x. The colored points show the aberration distribution produced by the camera and upstream optics (a ‘perfect’ imaging system would have no geometric aberrations). As you can see, the expected aberrations in the system are, for the most part, far below what we’d be able to measure in single pixel. This means that the camera will produce a sharp, well-corrected image of our entire spectrum.

The HPF camera system, designed by team member Christian Schwab and fabricated by New England Optical Systems and Optimax Systems, was recently delivered to PSU and is now undergoing vacuum and cold testing.

The delivered HPF camera system! The camera is made of specialized lenses, which are coated with custom anti-reflective coatings. These complex coatings (appearing as green-yellow tints in the images) ensure that we capture every last photon from our targets. The mechanical assembly is unique in its own right; as the camera cools to the HPF operating temperature, the mounts compensate for thermal expansion differences between the various glasses and the aluminum housing. This allows us to test the camera both at room temperature and at the 180K operating temperature of HPF.

With the camera now delivered to Penn State, we’ve received almost all of the optical components for HPF. We’re now knee-deep, for the first time, in our alignment and testing of the entire optical system. Stay tuned!

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Stellar Activity in the Near-Infrared: We Need a New Ruler!

Introduction

We talk about stellar activity a lot on this blog.  Once HPF gets on sky, radial velocity noise from stellar activity will likely be the biggest impediment to finding exoplanets.  Thus, if we want HPF’s chief scientific mission of discovering low-mass exoplanets to be successful, we have to be prepared to deal with stellar noise from Day 1 of our upcoming survey.  In today’s post, we will look in detail at how the HPF team verified a set of near-infrared absorption lines from calcium as a first indicator of stellar magnetic activity for the HPF exoplanet survey.

Research led by HPF’s own Robert Marchwinski suggests that Doppler noise from stellar activity should actually be reduced at the near-infrared relative to the noise levels observed at visible-light wavelengths.  However, “less” does not mean “none,” and we certainly do not want to find any unpleasant surprises!  Following the examples of previous exoplanet surveys, we intend to track the magnetic activity of our target stars by monitoring the variability of atomic lines in the stellar spectra that change as magnetic activity increases and decreases on the stellar surface.

For HPF, this is potentially easier said than done.  The near-infrared wavelength coverage of our instrument, while ideal for targeting the smallest, coolest nearby stars, excludes all of the well-characterized atomic lines that previous Doppler surveys have relied on to provide information about stellar activity.  This is illustrated in the spectrum below.

A near-infrared spectrum of Gliese 411, an M2 dwarf that is the sixth closest hydrogen-burning star to the Sun. Wavelength ranges for HPF's coverage and the most commonly used activity indicators are shown. Adapted from Figure 2 of Kirkpatrick et al. (1993).

A near-infrared spectrum of Gliese 411, an M2 dwarf that is the sixth closest hydrogen-burning star to the Sun. Wavelength ranges for HPF’s coverage and the most commonly used activity indicators are shown. Adapted from Figure 2 of Kirkpatrick et al. (1993).

Therefore, if HPF is to confidently identify nearby exoplanets, we will have to establish at least one new atomic-line index to help us trace stellar magnetic activity.  It would be nice to have this done before HPF goes on sky, but such an experiment is potentially a major undertaking.  Ideally, one would like to use a high-resolution near-infrared spectrograph (of which there are few–hence we are building HPF!) to make many observations of one or more M dwarf stars (so we can see the activity changing).

Proxima to the Rescue

As it turns out, we can use some data that are already available to get started!  More specifically, many observations have already been taken of our nearest stellar neighbor, Proxima Centauri.  As we discussed on the last entry of the HPF blog, Proxima Centauri (Proxima for short) is a mid-M star that is about 4 light-years away from the Sun.  Recently, Proxima was found to host a low-mass planet in its liquid-water habitable zone, making it an exciting target for future efforts to characterize the atmospheres of potentially Earthlike planets and to search for signatures of extraterrestrial life.

Proxima is also relatively active for an M star of its age.  Indicators of magnetic activity in stars (brightness, spectral line variability, X-ray emission) show that Proxima varies on many timescales, from less than a day to several years.  A team of astronomers using the MOST spacecraft to search for a transit of Proxima b have come up empty so far, but in doing so found that Proxima exhibits stellar flares upwards of 60 times a day!  On the long-timescale end, other astronomers have found evidence that Proxima may have a long-term magnetic cycle like that of our own Sun.

In 2009, a team of astronomers used the high-resolution UVES spectrograph and the XMM-Newton spacecraft to observe Proxima intensely for three nights at both X-ray and optical/near-infrared wavelengths.  Their goal was to obtain information about Proxima’s atmosphere and magnetic field at different levels of activity.  Their data set includes hundreds of spectra of Proxima, and covers a significant amount of magnetic variability.

As it turns out, these observations are a great resource for us to study activity indicators for HPF!  In particular, the UVES observations were made with a special setup of the spectrograph, where the resulting spectra started at the hydrogen alpha (H-alpha or \textrm{H}\alpha for short) line, and extended redward into the blue half of HPF’s wavelength coverage.  As you may recall, \textrm{H}\alpha is a well known and characterized indicator of magnetic activity in M dwarf stars, and was instrumental in helping our team separate real exoplanets from activity-induced false positives in the Gliese 581 system.  So with these data, we can measure the variability of the \textrm{H}\alpha line as the magnetic activity stimulates its emission, and look at how the near-infrared lines respond to the same activity.  The most sensitive lines will be our activity tracers for HPF!  The results of this experiment were just published by the HPF team in a research article, but let’s take a look at the main points.

Results

The UVES spectra include 8 near-infrared atomic lines that previous studies have shown are at least somewhat sensitive to stellar activity.  These lines are produced by hydrogen, potassium, sodium, and calcium.  For each of the 562 spectra, we measured the flux in these lines, and compared them to the flux in the \textrm{H}\alpha line.

Examples of the near-infrared lines we considered as candidate indicators of stellar activity, with <img src='http://s.wordpress.com/latex.php?latex=%5Ctextrm%7BH%7D%5Calpha&bg=ffffff&fg=000000&s=0' alt='\textrm{H}\alpha' title='\textrm{H}\alpha' class='latex' /> at top left for comparison. Each line is shown in its average state, and at periods of high (red) and low (blue) activity. The more activity-sensitive lines show a greater difference between high- and low-activity states.

Examples of the near-infrared lines we considered as candidate indicators of stellar activity, with [latex]\textrm{H}\alpha[/latex] at top left for comparison. Each line is shown in its average state, and at periods of high (red) and low (blue) activity. The more activity-sensitive lines show a greater difference between high- and low-activity states.

In this experiment, the analysis was actually relatively simple.  Two of the three calcium lines, and both of the potassium lines, matched the behavior of \textrm{H}\alpha almost perfectly, while the sodium and hydrogen lines we considered were not nearly as useful.  The third calcium line appears to be as sensitive as the first two, but it is contaminated by a nearby iron line, making its measurement less reliable.

The two bluest calcium lines will be excellent activity indicators for HPF, and will allow us to start separating signals from planets and activity from the very beginning of our survey.  The potassium lines also look like great proxies for magnetic activity, but are just a bit too blue to be seen by HPF.  However, there are other instruments in development that use what is sometimes called the “red-optical” wavelength range to look for exoplanets around slightly hotter M stars.  These instruments include MAROON-X and Veloce, and the potassium lines may prove to be valuable tools for their surveys.

Concluding Thoughts

This study was an important reminder to keep an open mind when performing scientific experiments.  At the outset, we were convinced that the near-infrared sodium lines would turn out to be the best activity indicators for HPF.  The sodium lines are deep lines that are easily identified and measured for all M dwarfs, even in measurements of poor quality.  Furthermore, the sodium lines at optical wavelengths are extremely sensitive to activity for M stars, leading us to expect similar results for their near-infrared counterparts.  On the other hand, the near-infrared potassium lines are contaminated by absorption from Earth’s atmosphere, while two of the three calcium lines are much more difficult to identify than the other lines we considered.  We were thus unconvinced that those lines would be of much use.  As it turns out, the answer was exactly the opposite of what we expected!  In sports, one would say “that’s why they play the game.”  For us, perhaps the equivalent is “that’s why we do the experiment.”

There is certainly more work to be done for HPF.  Our wavelength coverage will include wavelengths considerably redder than UVES can access, and there are additional lines we must investigate for activity sensitivity.  Furthermore, it will be interesting to see how all of these candidate tracers behave for a variety of stars and magnetic phenomena.  Nevertheless, it is reassuring to know that our toolbox will be at least partially filled when the survey starts.

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The Plot Thickens: Habitable-Zone Exoplanets around Proxima Centauri and TRAPPIST-1

Introduction

As the time approaches to commission HPF on the Hobby-Eberly Telescope, we are learning that the spectrograph will be coming online in truly exciting times for exoplanet science!  The detection of habitable-zone exoplanets around two nearby M dwarf stars—including around the Sun’s nearest stellar neighbor—puts the science case for HPF into stark relief.  Let’s take a quick look at these systems, and discuss where HPF fits into the picture.

TRAPPIST-1: little star, big deal!

TRAPPIST is a robotic 60-centimeter telescope at La Silla Observatory in Chile.  It is entirely dedicated to observations of transiting exoplanets and minor bodies in the solar system.  One of the telescope’s first discoveries, announced in May, is TRAPPIST-1, a nearby M dwarf star with three transiting planets.  Each of the three planets have radii similar to that of Earth, and orbit near the star’s habitable zone.

 

Artist's impression of TRAPPIST-1 as seen from the outermost of its three transiting planets. Image credit: ESO/M. Kornmesser

Artist’s impression of TRAPPIST-1 as seen from the outermost of its three transiting planets. Image credit: ESO/M. Kornmesser

While we now know of exoplanets orbiting thousands of stars, the TRAPPIST-1 system is remarkable because of just how small and cool the star is.  TRAPPIST-1 is a member of a class of stars sometimes referred to as “ultracool dwarfs”. It has just 8 percent the mass of our Sun, and is only 5 ten-thousandths as luminous.  For comparison, if TRAPPIST-1 were as bright as a 100-watt incandescent light bulb, the Sun would be about as bright as a whole set of floodlights used at an outdoor sports complex.  TRAPPIST-1’s mass is near the minimum required to trigger hydrogen fusion in its core, making it just barely a star at all!

Ultracool dwarfs are so faint at visible wavelengths that they have almost never been targeted by exoplanet surveys.  Kepler observed almost none of them during its primary mission, and they are similarly underrepresented in Doppler planet searches to date.  Thus, the discovery of the TRAPPIST-1 planets is a real first.  To find that ultracool dwarfs host systems of multiple small planets like their more massive stellar siblings is an exciting step forward for the science of exoplanets around low-mass stars.

Proxima Centauri b: the habitable-zone planet next door

Proxima Centauri (Proxima for short) is part of the triple-star Alpha Centauri system, and the closest star to the Sun.  It is a small, distant companion to the binary stars Alpha Centauri A and B.  At a distance of about 4 light-years from the Sun, the Alpha Centauri system is so close that some people are already discussing sending robotic probes there.  From a more near-term perspective, stars as close as Proxima are prime targets for a huge number of astronomical instruments, including space-based telescopes such as the upcoming James Webb Space Telescope (JWST).  Thus, the time is now to identify and characterize any exoplanets around the Alpha Centauri stars.

Proxima has long been a target of exoplanet surveys.  HPF science team member Michael Endl was among the first astronomers to search for exoplanets around Proxima, observing it with the UVES spectrograph at the Very Large Telescope in Chile as far back as 2000.  As a result, we have known for some time that any planets orbiting close to Proxima must be small, or we would have found them by now.  This allowed for the tantalizing possibility that a small, terrestrial planet might reside in the star’s habitable zone.

It appears that this possibility has now proven to be a reality.  In January, a group of astronomers launched the Pale Red Dot program, a campaign to observe Proxima intensively with the HARPS spectrograph with the goal of discovering any terrestrial planets that might reside within its habitable zone.  The program also included near-simultaneous measurements of Proxima’s brightness as part of an effort to mitigate the effects of stellar magnetic activity, which we have discussed as a challenge for the discovery of low-mass exoplanets around M dwarf stars.

This week, the team announced they had in fact discovered a potentially Earthlike planet: Proxima b, which has a minimum mass just 27 percent greater than Earth’s and orbits in the very middle of the habitable zone.  At a distance of just 5 percent the separation of the Earth from the Sun, the planet completes an orbit every 11.2 days.  Proxima b has not yet been observed to transit its star, but it has not been ruled out, either.

Artist's impression of the Alpha Centauri stellar system as viewed from the surface of the habitable-zone planet Proxima b (Image credit: ESO/M. Kornmesser)

Artist’s impression of the Alpha Centauri stellar system as viewed from the surface of the habitable-zone planet Proxima b. Image credit: ESO/M. Kornmesser

Proxima b represents one of the most significant exoplanet discoveries to date.  Proxima Centauri is close enough to enable follow-up observations not feasible for many other stars.  If the planet does transit, JWST could potentially characterize its atmosphere.  If not, it may still be possible to detect molecules in its atmosphere with upcoming 30-meter-class telescopes.  Astronomers are a creative bunch, and it is likely that they will find ways to explore Proxima b that have not even been discussed yet!

How HPF fits in

It is important to keep in mind that the F in HPF stands for “Finder.” The primary objective of HPF is to discover new exoplanets around low-mass stars. However, the discovery of the TRAPPIST-1 system emphasizes another important role for HPF, which is to characterize planets that are found to transit nearby stars.  Experiments searching for transiting exoplanets such as TRAPPIST, Kepler (now operating as K2), and TESS will find many systems like TRAPPIST-1 in the coming years.  With its combination of near-infrared wavelength coverage and a large-diameter telescope, HPF is uniquely suited to provide Doppler follow-up on the coolest, dimmest nearby stars.

As for Proxima, the discovery of Proxima b reiterates the idea that there are many exciting worlds still waiting to be uncovered.  Proxima is a benchmark for the type of star HPF will observe most frequently, an idea we will discuss in more detail on this blog shortly.  The instrument is nearing completion, but the work is just beginning.

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