My work as astronomer, a bit more in detail

Even though I am not working as an astronomer that much any more, I like to believe I did some pretty cool science, contributing to our understanding of exoplanets and their atmospheres. Below you can find a selected list of publications. Feel free to read them, and I dare you not to fall asleep! :)

My CV, including a list of publications, can be accessed through this link. In addition, my publications can be found under this NASA/ADS link.

Synchronizing with the clocks aboard TESS

SAC assistant professor Carolina von Essen is main author of a paper titeled "TESS Data for Asteroseismology: Timing verification" accepted by Astrophysical Journal, and now available on arxiv.

In order to reliably study stars, TESS data have to be very accurate in time. In consequence, to check that the time on board of TESS does not have an offset or a drift, we carried out from the ground and from space contemporaneous observations of eclipses of binary stars. Comparing the timings at which the eclipses took place in a complex study that is fully detailed in the publication, we determined that both clocks (the one on board of TESS and the one corresponding to the ground-based stations) were actually synchronized.

The work was done through an international collaboration of researchers based in Denmark, in the United States, and in Argentina. The PI, Carolina von Essen, is assistant professor at SAC.

The press release can be found here.

SAC has characterized the atmosphere of an exoplanet using TESS data

In the study, the researchers find that WASP-33b's host star, of delta Scuti type, shows non-radial pulsations in the milli-magnitude regime, with periods comparable to that of the primary transit. Analyzing TESS photometry the researchers found 29 pulsation frequencies, that were used to clean the light curve and study the planet. A secondary eclipse depth of 305.8 +/- 35.5 parts-per-million (ppm) is reported in the paper, along with an amplitude of the phase curve of 100.4 +/- 13.1 ppm and a corresponding westward offset between the region of maximum brightness and the substellar point of 28.7 +/- 7.1 degrees, making WASP-33b one of the few planets with such an offset found so far.

The researchers also determined the Bond albedo, heat recirculation efficiency, and day and night side brightness temperatures. From the detection of photometric variations due to gravitational interactions, they estimated a planet mass of M_P = 2.81 +/- 0.53 M_Jup, consistent with literature values. Analyzing the stellar pulsations in the frame of the planetary orbit, they found no signals of star-planet interactions. The article can be found on arxiv, and has been accepted in Astronomy & Astrophysics.

The press release can be found here.

Artist’s impression of a hot Jupiter (left) and its host star. Credit: NASA.

HST/STIS transmission spectrum of the ultra-hot Jupiter WASP-76 b confirms the presence of sodium in its atmosphere

We present an atmospheric transmission spectrum of the ultra-hot Jupiter WASP-76 b by analyzing archival data obtained with the Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope (HST). The dataset spans three transits, two with a wavelength coverage between 2900 and 5700 Å, and the third one between 5250 and 10300 Å. From the one-dimensional, time dependent spectra we constructed white and chromatic light curves, the latter with typical integration band widths of∼200 Å. We computed the wavelength dependent planet-to-star radii ratios taking into consideration WASP-76’s companion. The resulting transmission spectrum of WASP-76 b is dominated by a spectral slope of increasing opacity towards shorter wavelengths of amplitude of about three scale heights under the assumption of planetary equilibrium temperature. If the slope is caused by Rayleigh scattering, we derive a lower limit to the temperature of∼870 K. Following-up on previous detection of atomic sodium derived from highresolution spectra, we re-analyzed HST data using narrower bands centered around sodium. From an atmospheric retrieval of thistransmission spectrum, we report evidence of sodium at 2.9σ significance. In this case, the retrieved temperature at the top of theatmosphere (10−5bar) is 2300+412−392K. We also find marginal evidence for titanium hydride. However, additional high resolutionground-based data are required to confirm this discovery.

The paper can be found under the followin arxiv link.

The potassium absorption on HD189733b and HD209458b

Ever since the earliest theoretical predictions 20 years ago, the chemical elements potassium and sodium were expected to be detectable in atmospheres of “hot Jupiters”, gaseous planets with temperatures of a few thousand Kelvin that orbit closely around far-away stars. While sodium was detected with high resolution observations already early on, potassium was not, which created a puzzle for atmospheric chemistry and physics.

A team of astronomers led by AIP PhD student Engin Keles detected the chemical element potassium in the atmosphere of an exoplanet, for the first time with overwhelming significance and applying high-resolution spectroscopy. The Potsdam Echelle Polarimetric and Spectroscopic Instrument (PEPSI) at the Large Binocular Telescope (LBT) in Arizona was used to study the atmosphere on the Jupiter-like exoplanet HD189733b.

Artist’s impression of a hot Jupiter (right) and its cool host star. Credit: AIP/Kristin Riebe.

Aluminium oxide found in an ultra-hot Jupiter

WASP-33, the planet-star system that is the subject of this research work, lies some 380 light years away from Earth. The host star is the first Delta Scuti star known to be orbited by a hot Jupiter. The pronounced stellar pulsations, showing periods comparable to the primary transit duration, interfere with transit modeling. Thus, it is extremely challenging to carry out a proper characterization of the physical properties of the exoplanet without addressing the variability of the star.

The study, published in the journal Astronomy & Astrophysics, analyses the chemical composition of its atmosphere. This is important because "The current models of exoplanetary atmospheres predict that the ultra-hot Jupiters should be free of clouds, and present a range of oxides in the visible spectrum, such as vanadium oxide, titanium oxide, and aluminium oxide," explains Carolina von Essen, of the University of Aarhus (Denmark), the principal investigator of this study. "But there is a limited number of exoplanets for which these molecules have been detected with a high significance, which makes us question the models." The paper can also be found under the following arxiv link.

The press release can be found here.


My work as a post-doctoral researcher, both at Goettingen University and at Aarhus University, has been mainly focused into two areas: the characterization of the atmospheres of exoplanets, and the determination of their dynamical masses through transit timing variations. I have dedicated my first years as postdoctoral researcher at SAC and my posterior years as assistant professor to deepen my knowledge about exo-atmospheric characterization. While my ultimate, personal goal is to find biomarkers in the atmospheres of alien worlds that resembles our Earth, due to technological limitations hot Jupiters are most frequently targeted. Even though it is extremely unlikely that life can develop on such extreme worlds they are still worth to study, as they can serve to contrast current models of atmospheres with observational data of good quality. Through low resolution transmission spectroscopy I have investigated macro-features in a wide wavelength range, such as the presence of clouds or hazes. Here, I have intensively contributed to the field, finding for instance the first indication of aluminium oxide in an exoplanet atmosphere, and one of the first attempts at carrying out comparative studies between two exoplanets. I have also developed a novel model for transmission spectroscopy. Owing to the extremely high quality of the data, I could construct and test a detailed model including variations in the intensity of the light from the star. This demonstrated that the position of the planet over the stellar disk impacts the stellar energy budget and thereby the interaction with the exo-atmosphere. I found that the planetary spectrum moved significantly relative to the stellar spectrum, modulating the absorption signal. In addition, I have found evidences for hydrogen Roche Lobe overflow (paper submitted), and I have determined the temperature-versus-altitude profile obtained from fitting the exo-atmospheric Na D2 absorption line in the core, wings and continuum.

Orbital dynamics, and particularly orbital perturbations, have also occupied a significant amount of my research time. During my 6-month stay at the Institute for Astrophysics Goettingen, I built and since then lead the Kepler Object of Interest Network (KOINet) an international network of researchers with access to ground-based telescopes organized by me to carry out the photometric follow-up of Kepler planets showing Transit Timing Variations that required more data than Kepler's to properly determine their masses. After introducing the network and our milestones, we predicted transits of Kepler-9c to disappear in 2050, and we found a non-transiting exoplanet in the Kepler-82 system. Other TTV efforst include the study of Neptune-sized exoplanets.

I have dedicated my PhD to the detailed study of transiting hot Jupiters. I have investigated several aspects of these extreme worlds, including not only the determination of the properties of the planets, but those of the host stars as well. During my three years of
PhD study I gained experience in several areas, such as transit fitting techniques, orbital dynamics, asteroseismology, stellar polarization, stellar activity, the use and development of modern statistical tools, and reduction and analysis techniques for photometric, spectroscopic, and polarimetric data. I quickly learned how to become an independent researcher, developing my own collaborations. During my PhD I focused my research mainly in the detailed characterization of two exoplanets, Qatar-1b and WASP-33b. For Qatar-1b I carried out a photometric follow-up of the system during transits. Over two years I collected up to 50 transit light curves that I used to investigate the feasibility of transit timing variations. For WASP-33b, I carried out a photometric follow-up of the system for two and a half years. I collected around 650 hours of in-and-out of transit photometry, that I used to characterize the pulsation spectrum of the host star and, with it, the transit parameters of WASP-33b free of pulsations.

As part of a collaborative effort between the National University of Buenos Aires and the National University of La Plata, I assembled a 0.4 meter telescope, for the express purpose of carrying out observations of exoplanet primary transits. The telescope, now available to the Argentinian astronomical community, is located at El Leoncito, Argentina's largest observatory. The telescope, and my work connected to it, were pioneer efforts to carry out exoplanet research in Argentina.