arXiv subjects

Karan Molaverdikhani

Publications and source records attributed to Karan Molaverdikhani.

53 records · Page 3Linked to original sources

arXiv2022

A transiting, temperate mini-Neptune orbiting the M dwarf TOI-1759 unveiled by TESS

We report the discovery and characterization of TOI-1759~b, a temperate (400 K) sub-Neptune-sized exoplanet orbiting the M~dwarf TOI-1759 (TIC 408636441). TOI-1759 b was observed by TESS to transit on sectors 16, 17 and 24, with only one transit observed per sector, creating an ambiguity on the orbital period of the planet candidate. Ground-based photometric observations, combined with radial-velocity measurements obtained with the CARMENES spectrograph, confirm an actual period of $18.85019 \pm 0.00014$ d. A joint analysis of all available photometry and radial velocities reveal a radius of $3.17 \pm 0.10\,R_\oplus$ and a mass of $10.8 \pm 1.5\,M_\oplus$. Combining this with the stellar properties derived for TOI-1759 ($R_\star = 0.597 \pm 0.015\,R_\odot$; $M_\star = 0.606 \pm 0.020\,M_\odot$; $T_{\textrm{eff}} = 4065 \pm 51$ K), we compute a transmission spectroscopic metric (TSM) value of over 80 for the planet, making it a good target for transmission spectroscopy studies. TOI-1759 b is among the top five temperate, small exoplanets ($T_\textrm{eq} < 500$ K, $R_p < 4 \,R_\oplus$) with the highest TSM discovered to date. Two additional signals with periods of 80 d and $>$ 200 d seem to be present in our radial velocities. While our data suggest both could arise from stellar activity, the later signal's source and periodicity are hard to pinpoint given the $\sim 200$ d baseline of our radial-velocity campaign with CARMENES. Longer baseline radial-velocity campaigns should be performed in order to unveil the true nature of this long period signal.

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arXiv2022

Towards RNA life on Early Earth: From atmospheric HCN to biomolecule production in warm little ponds

The origin of life on Earth involves the early appearance of an information-containing molecule such as RNA. The basic building blocks of RNA could have been delivered by carbon-rich meteorites, or produced in situ by processes beginning with the synthesis of hydrogen cyanide (HCN) in the early Earth's atmosphere. Here, we construct a robust physical and non-equilibrium chemical model of the early Earth atmosphere. The atmosphere is supplied with hydrogen from impact degassing of meteorites, sourced with water evaporated from the oceans, carbon dioxide from volcanoes, and methane from undersea hydrothermal vents, and in which lightning and external UV-driven chemistry produce HCN. This allows us to calculate the rain-out of HCN into warm little ponds (WLPs). We then use a comprehensive sources and sinks numerical model to compute the resulting abundances of nucleobases, ribose, and nucleotide precursors such as 2-aminooxazole resulting from aqueous and UV-driven chemistry within them. We find that at 4.4 bya (billion years ago) the limits of adenine concentrations in ponds for habitable surfaces is 0.05$μ$M in the absence of seepage. These concentrations can be maintained for over 100 Myr. Meteorite delivery of adenine to WLPs can provide boosts in concentration by 2-3 orders of magnitude, but these boosts deplete within months by UV photodissociation, seepage, and hydrolysis. The early evolution of the atmosphere is dominated by the decrease of hydrogen due to falling impact rates and atmospheric escape, and the rise of oxygenated species such as OH from H2O photolysis. Our work points to an early origin of RNA on Earth within ~200 Myr of the Moon-forming impact.

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arXiv2021

Final Report for SAG 22: A Target Star Archive for Exoplanet Science

Present and upcoming NASA missions will be intensively observing a selected, partially overlapping set of stars for exoplanet studies. Key physical and chemical information about these stars and their systems is needed for planning observations and interpreting the results. A target star archive of such data would benefit a wide cross-section of the exoplanet community by enhancing the chances of mission success and improving the efficiency of mission observatories. It would also provide a common, accessible resource for scientific analysis based on standardized assumptions, while revealing gaps or deficiencies in existing knowledge of stellar properties necessary for exoplanetary system characterization.

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arXiv2021

Moderately misaligned orbit of the warm sub-Saturn HD 332231 b

Measurements of exoplanetary orbital obliquity angles for different classes of planets are an essential tool in testing various planet formation theories. Measurements for those transiting planets on relatively large orbital periods ($P$\,$>$\,10\,d) present a rather difficult observational challenge. Here we present the obliquity measurement for the warm sub-Saturn planet HD\,332231\,b, which was discovered through Transiting Exoplanet Survey Satellite (TESS) photometry of sectors 14 and 15, on a relatively large orbital period (18.7\,d). Through a joint analysis of previously obtained spectroscopic data and our newly obtained CARMENES transit observations, we estimated the spin-orbit misalignment angle, \textlambda, to be $-42.0^{+11.3}_{-10.6}$\,$°$, which challenges Laplacian ideals of planet formation. Through the addition of these new radial velocity (RV) data points obtained with CARMENES, we also derived marginal improvements on other orbital and bulk parameters for the planet, as compared to previously published values. We showed the robustness of the obliquity measurement through model comparison with an aligned orbit. Finally, we demonstrated the inability of the obtained data to probe any possible extended atmosphere of the planet, due to a lack of precision, and place the atmosphere in the context of a parameter detection space.

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arXiv2020

A Highly Eccentric Warm Jupiter Orbiting TIC 237913194

The orbital parameters of warm Jupiters serve as a record of their formation history, providing constraints on formation scenarios for giant planets on close and intermediate orbits. Here, we report the discovery of TIC 237913194b, detected in full frame images from Sectors 1 and 2 of TESS, ground-based photometry (CHAT, LCOGT), and FEROS radial velocity time series. We constrain its mass to $M_\mathrm{P} = 1.942_{-0.091}^{+0.091}\,{\rm M_{J}} $, and its radius to $R_\mathrm{P} = 1.117_{-0.047}^{+0.054}\,{\rm R_J}$, implying a bulk density similar to Neptune's. It orbits a G-type star (${\rm M}_{\star} = 1.026_{-0.055}^{+0.057}\,{\rm M}_{\odot}$, $V = 12.1$ mag) with a period of $15.17\,$d on one of the most eccentric orbits of all known warm giants ($e \approx 0.58$). This extreme dynamical state points to a past interaction with an additional, undetected massive companion. A tidal evolution analysis showed a large tidal dissipation timescale, suggesting that the planet is not a progenitor for a hot Jupiter caught during its high-eccentricity migration. TIC 237913194b further represents an attractive opportunity to study the energy deposition and redistribution in the atmosphere of a warm Jupiter with high eccentricity.

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arXiv2020

HCN production in Titan's Atmosphere: Coupling quantum chemistry and disequilibrium atmospheric modeling

Hydrogen cyanide (HCN) is a critical reactive source of nitrogen for building key biomolecules relevant for the origin of life. Still, many HCN reactions remain uncharacterized by experiments and theory, and the complete picture of HCN production in planetary atmospheres is not fully understood. To improve this situation, we develop a novel technique making use of computational quantum chemistry, experimental data, and atmospheric numerical simulations. First, we use quantum chemistry simulations to explore the entire field of possible reactions for a list of primary species in N2-, CH4-, and H2-dominated atmospheres. In this process, we discover 33 new reactions with no previously known rate coefficients. From here, we develop a consistent reduced atmospheric hybrid chemical network (CRAHCN) containing experimental values when available, and our calculated rate coefficients otherwise. Next, we couple CRAHCN to a 1D chemical kinetic model (ChemKM) to compute the HCN abundance as a function of atmospheric depth on Titan. Our simulated atmospheric HCN profile agrees very well with the Cassini observations. CRAHCN contains 104 reactions however nearly all of the simulated atmospheric HCN profile can be obtained using a scaled down network of only 19 dominant reactions. From here, we form a complete picture of HCN chemistry in Titan's atmosphere, from the dissociation of the main atmospheric species, down to the direct production of HCN along 4 major channels. One of these channels was first discovered and characterized in Pearce et al. (2019) and this work.

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arXiv2020

The role of clouds on the depletion of methane and water dominance in the transmission spectra of irradiated exoplanets

Observations suggest an abundance of water and paucity of methane in the majority of observed exoplanetary atmospheres. We isolate the effect of atmospheric processes to investigate possible causes. Previously, we studied the effect of effective temperature, surface gravity, metallicity, carbon-to-oxygen ratio, and stellar type assuming cloud-free thermochemical equilibrium and disequilibrium chemistry. However, under these assumptions, methane remains a persisting spectral feature in the transmission spectra of exoplanets over a certain parameter space, the Methane Valley. In this work we investigate the role of clouds on this domain and we find that clouds change the spectral appearance of methane in two direct ways: 1) by heating-up the photosphere of colder planets, and 2) by obscuring molecular features. The presence of clouds also affects methane features indirectly: 1) cloud heating results in more evaporation of condensates and hence releases additional oxygen, causing water dominated spectra of colder carbon-poor exoplanets, and 2) HCN/CO production results in a suppression of depleted methane features by these molecules. The presence of HCN/CO and a lack of methane could be an indication of cloud formation on hot exoplanets. Cloud heating can also deplete ammonia. Therefore, a simultaneous depletion of methane and ammonia is not unique to photochemical processes. We propose that the best targets for methane detection are likely to be massive but smaller planets with a temperature around 1450 K orbiting colder stars. We also construct Spitzer synthetic color-maps and find that clouds can explain some of the high contrast observations by IRAC's channel 1 and 2.

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arXiv2020

EDEN: Sensitivity Analysis and Transiting Planet Detection Limits for Nearby Late Red Dwarfs

Small planets are common around late-M dwarfs and can be detected through highly precise photometry by the transit method. Planets orbiting nearby stars are particularly important as they are often the best-suited for future follow-up studies. We present observations of three nearby M-dwarfs referred to as EIC-1, EIC-2, and EIC-3, and use them to search for transits and set limits on the presence of planets. On most nights our observations are sensitive to Earth-sized transiting planets, and photometric precision is similar to or better than TESS for faint late-M dwarfs of the same magnitude (I=15 mag). We present our photometry and transit search pipeline, which utilizes simple median detrending in combination with transit least squares based transit detection (Hippke & Heller 2019).For these targets, and transiting planets between one and two Earth radii, we achieve an average transit detection probability of 60% between periods of 0.5 and 2 days, 30% between 2 and 5 days,and 10% between 5 and 10 days. These sensitivities are conservative compared to visual searches.

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arXiv2020

Understanding the atmospheric properties and chemical composition of the ultra-hot Jupiter HAT-P-7b II. Mapping the effects of gas kinetics

The atmospheres of ultra-hot Jupiters are commonly considered to be at thermochemical equilibrium. We aim to provide disequilibrium chemistry maps for a global understanding of the chemistry in HAT-P-7b's atmosphere and assess the importance of disequilibrium chemistry on UHJs. We apply a hierarchical modelling approach utilising 97 1D atmospheric profiles from 3D GCM of HAT-P-7b. For each 1D profile, we evaluate our kinetic cloud formation model consistently with the local gas-phase composition in chemical equilibrium. We then evaluate quenching results from a zeroth-order approximation in comparison to a kinetic gas-phase approach. We find that the zeroth-order approach of estimating quenching points agrees well with the full gas-kinetic modeling results. Chemical disequilibrium has the greatest effect on the nightside and morning abundance of species such as H, H$_2$O, CH$_4$, CO$_2$, HCN, and all C$_n$H$_m$ molecules; heavier C$_n$H$_m$ molecules are more affected by disequilibrium processes. CO abundance, however, is affected only marginally. While dayside abundances also notably change, those around the evening terminator of HAT-P-7b are the least affected by disequilibrium processes. The latter finding may partially explain the consistency of observed transmission spectra of UHJs with atmospheres in thermochemical equilibrium. Photochemistry only negligibly affects molecular abundances and quenching levels. In general, the quenching points of HAT-P-7b's atmosphere are at much lower pressures in comparison to the cooler hot-jupiters. We propose several avenues to look for the effect of disequilibrium processes on UHJs that are, in general, based on abundance and opacity measurements at different local times. It remains a challenge to completely disentangle this from the chemical effects of clouds and that of a primordial non-solar abundance.

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arXiv2019

From cold to hot irradiated gaseous exoplanets: Fingerprints of chemical disequilibrium in atmospheric spectra

Almost all planetary atmospheres are affected by disequilibrium chemical processes. In this paper we introduce our recently developed Chemical Kinetic Model (\texttt{ChemKM}). We show that the results of our HD189733b model are in good agreement with previously published results, except at $μ$bar regime, where molecular diffusion and photochemistry are the dominant processes. We thus recommend careful consideration of these processes when abundances at the top of the atmosphere are desired. We also propose a new metric for a quantitative measure of quenching levels. By applying this metric, we find that quenching pressure decreases with the effective temperature of planets, but it also varies significantly with other atmospheric parameters such as [Fe/H], log(g), and C/O. In addition, we find that the "Methane Valley", a region between 800 and 1500K where above a certain C/O threshold value a greater chance of CH$_4$ detection is expected, still exists after including the vertical mixing. The first robust CH$_4$ detection on an irradiated planet (HD102195b) places this object within this region; supporting our prediction. We also investigate the detectability of disequilibrium spectral fingerprints by JWST, and suggest focusing on the targets with T$_{eff}$ between 1000 and 1800K, orbiting around M-dwarfs, having low surface gravity but high metallicity and a C/O ratio value around unity. Finally, constructing Spitzer color-maps suggests that the main two color-populations are largely insensitive to the vertical mixing. Therefore any deviation of observational points from these populations are likely due to the presence of clouds and not disequilibrium processes. However, some cold planets (T$_{eff}<$900K) with very low C/O ratios ($<$0.25) show significant deviations; making these planets interesting cases for further investigation.

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arXiv2019

The Need for Laboratory Measurements and Ab Initio Studies to Aid Understanding of Exoplanetary Atmospheres

We are now on a clear trajectory for improvements in exoplanet observations that will revolutionize our ability to characterize their atmospheric structure, composition, and circulation, from gas giants to rocky planets. However, exoplanet atmospheric models capable of interpreting the upcoming observations are often limited by insufficiencies in the laboratory and theoretical data that serve as critical inputs to atmospheric physical and chemical tools. Here we provide an up-to-date and condensed description of areas where laboratory and/or ab initio investigations could fill critical gaps in our ability to model exoplanet atmospheric opacities, clouds, and chemistry, building off a larger 2016 white paper, and endorsed by the NAS Exoplanet Science Strategy report. Now is the ideal time for progress in these areas, but this progress requires better access to, understanding of, and training in the production of spectroscopic data as well as a better insight into chemical reaction kinetics both thermal and radiation-induced at a broad range of temperatures. Given that most published efforts have emphasized relatively Earth-like conditions, we can expect significant and enlightening discoveries as emphasis moves to the exotic atmospheres of exoplanets.

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arXiv2019

Constraining Stellar Photospheres as an Essential Step for Transmission Spectroscopy of Small Exoplanets

Transmission spectra probe the atmospheres of transiting exoplanets, but these observations are also subject to signals introduced by magnetic active regions on host stars. Here we outline scientific opportunities in the next decade for providing useful constraints on stellar photospheres and inform interpretations of transmission spectra of the smallest ($R<4\,R_{\odot}$) exoplanets. We identify and discuss four primary opportunities: (1) refining stellar magnetic active region properties through exoplanet crossing events; (2) spectral decomposition of active exoplanet host stars; (3) joint retrievals of stellar photospheric and planetary atmospheric properties with studies of transmission spectra; and (4) continued visual transmission spectroscopy studies to complement longer-wavelength studies from $\textit{JWST}$. We make five recommendations to the Astro2020 Decadal Survey Committee: (1) identify the transit light source (TLS) effect as a challenge to precise exoplanet transmission spectroscopy and an opportunity ripe for scientific advancement in the coming decade; (2) include characterization of host star photospheric heterogeneity as part of a comprehensive research strategy for studying transiting exoplanets; (3) support the construction of ground-based extremely large telescopes (ELTs); (4) support multi-disciplinary research teams that bring together the heliophysics, stellar physics, and exoplanet communities to further exploit transiting exoplanets as spatial probes of stellar photospheres; and (5) support visual transmission spectroscopy efforts as complements to longer-wavelength observational campaigns with $\textit{JWST}$.

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arXiv2019

Stellar Astrophysics and Exoplanet Science with the Maunakea Spectroscopic Explorer (MSE)

The Maunakea Spectroscopic Explorer (MSE) is a planned 11.25-m aperture facility with a 1.5 square degree field of view that will be fully dedicated to multi-object spectroscopy. A rebirth of the 3.6m Canada-France-Hawaii Telescope on Maunakea, MSE will use 4332 fibers operating at three different resolving powers (R ~ 2500, 6000, 40000) across a wavelength range of 0.36-1.8mum, with dynamical fiber positioning that allows fibers to match the exposure times of individual objects. MSE will enable spectroscopic surveys with unprecedented scale and sensitivity by collecting millions of spectra per year down to limiting magnitudes of g ~ 20-24 mag, with a nominal velocity precision of ~100 m/s in high-resolution mode. This white paper describes science cases for stellar astrophysics and exoplanet science using MSE, including the discovery and atmospheric characterization of exoplanets and substellar objects, stellar physics with star clusters, asteroseismology of solar-like oscillators and opacity-driven pulsators, studies of stellar rotation, activity, and multiplicity, as well as the chemical characterization of AGB and extremely metal-poor stars.

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arXiv2019

From cold to hot irradiated gaseous exoplanets: Toward an observation-based classification scheme

A carbon-to-oxygen ratio (C/O) of around unity is believed to act as a natural separator of water- and methane-dominated spectra when characterizing exoplanet atmospheres. In this paper we quantify the C/O ratios at which this separation occurs by calculating a large self-consistent grid of cloud-free atmospheric models in chemical equilibrium, using the latest version of petitCODE. Our study covers a broad range of parameter space: 400 K$<$T$_{eff}<$2600 K, 2.0$<$log(g)$<$5.0, -1.0$<$[Fe/H]$<$2.0, 0.25$<$C/O$<$1.25, and stellar types from M to F. We make the synthetic transmission and emission spectra, as well as the temperature structures publicly available. We find that the transition C/O ratio depends on many parameters such as effective temperature, surface gravity, metallicity and spectral type of the host star, and could have values less, equal, or higher than unity. By mapping all the transition C/O ratios we propose a "four-class" classification scheme for irradiated planets in this temperature range. We find a parameter space where methane always remains the cause of dominant spectral features. CH$_4$ detection in this region, or the lack of it, provides a diagnostic tool to identify the prevalence of cloud formation and non-equilibrium chemistry. As another diagnostic tool, we construct synthetic Spitzer IRAC color-diagrams showing two distinguishable populations of planets. Since most of the exoplanet atmospheres appear cloudy when studied in transmission, we regard this study as a starting point of how such a C/O-sensitive observation-based classification scheme should be constructed. This preparatory work will have to be refined by future cloudy and non-equilibrium modeling, to further investigate the existence and exact location of the classes, as well as the color-diagram analysis.

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arXiv2018

Ground-based detection of an extended helium atmosphere in the Saturn-mass exoplanet WASP-69b

Hot gas giant exoplanets can lose part of their atmosphere due to strong stellar irradiation, affecting their physical and chemical evolution. Studies of atmospheric escape from exoplanets have mostly relied on space-based observations of the hydrogen Lyman-α line in the far ultraviolet which is strongly affected by interstellar absorption. Using ground-based high-resolution spectroscopy we detect excess absorption in the helium triplet at 1083 nm during the transit of the Saturn-mass exoplanet WASP-69b, at a signal-to-noise ratio of 18. We measure line blue shifts of several km/s and post transit absorption, which we interpret as the escape of part of the atmosphere trailing behind the planet in comet-like form. [Additional notes by authors: Furthermore, we provide upper limits for helium signals in the atmospheres of the exoplanets HD 209458b, KELT-9b, and GJ 436b. We investigate the host stars of all planets with detected helium signals and those of the three planets we derive upper limits for. In each case we calculate the X-ray and extreme ultraviolet flux received by these planets. We find that helium is detected in the atmospheres of planets (orbiting the more active stars and) receiving the larger amount of irradiation from their host stars.]

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arXiv2010

Common Radial Velocity vs. Rare Microlensing: Difficulties and Futures

In this paper, effective factors for success of Microlensing and Radial Velocity methods were choose. A semi-Delphi process applied on the factors to evaluating them and finding the most important factors for present situation of ML and RV, with help from about 100 experts, in or related exoplanets detection. I found the public definition on "success of exoplanets detection methods" is not correct and we should change it, as some experts did it, in the form of fundamental questions in planetary science. Also, the views of "Special Experts" are different from other experts that help us to choose the right way in evaluating. The next step was choosing the best strategy for future and finally, from SWOT landscape and with a new objective of ML method (New Game Board Strategy) I suggested four critical future strategies for completing current strategic directions.

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arXiv2009

Mapping the Probability of Microlensing Detection of Extra-Solar Planets

The growing rate of increase in the number of the discovered extra-solar planets which has consequently raised the enthusiasm to explore the universe in hope of finding earth-like planets has resulted in the wide use of Gravitational Microlensing as a planet detection method. However, until November 2009, only 9 out of the overall 405 discovered exoplanets have been detected through Microlensing, a fact which shows that this method is relatively new in the detection of extra-solar planets. Therefore, preparing a map of the sky which pinpoints the regions with higher probability of planet detection by this method and is drawn based on the available equipments and other regional factors would, indeed, help speed up the discovery of exoplanets. This paper provides calculations and reasoning to suggest looking toward two distinct regions in constellations Centaurus and Sagittarius in addition to the customary Galactic Bulge in the search for other habitable worlds.

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