arXiv subjects

Sydney Vach

Publications and source records attributed to Sydney Vach.

13 recordsLinked to original sources

arXiv2026

Preparing for the Early eVolution Explorer: Photometric Diagnostics of Magnetospheric Accretion Geometry in Young Stellar Objects

The inner disk truncation radius, $R_T$, plays a crucial role in the regulation of star-disk interaction and the early evolution of star-disk-planet systems; however, measuring this parameter is observationally challenging. We present a new method for determining $R_T$ in young accreting systems that hinges on the color dependence of the accretion shock emission in multi-band time-domain surveys. Based on the accretion simulations of Robinson et al. (2017, 2021), we produce synthetic color-magnitude diagrams at near-UV and optical wavelengths that predict the loci of accreting stars as a function of $R_T$. We test these model predictions on young stars with interferometric $R_T$ estimates, finding very good agreement in our results. We apply this novel technique to a pilot survey of 26 classical T Tauri stars in Taurus and Upper Scorpius. We find a predominance of sources with small truncation radii, $R_T < 4\ R_\star$, and an overall distribution of $R_T$ that is statistically similar to that inferred from interferometric studies, while differing from those inferred from emission line modeling. Finally, we discuss the application of this technique to NASA's mission concept EVE, with the goal to provide simultaneous measurements of inner disk truncation radii, corotation radii and mass accretion rates for hundreds of young stars across the Galaxy. The unprecedented survey of inner disk properties that the mission would produce would enable the first stringent test of angular momentum evolution theories in young stars and reveal the impact of the inner disk conditions on early planet architectures.

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arXiv2026

Companion architectures of sub-Saturns: Distinct migration pathways across the Neptunian landscape

Close-in sub-Saturns (4 - 8.5 R$_\oplus$) are depleted in the Neptunian desert, accumulate in a narrow overdensity near P = 3.2 - 5.7 d (the Neptunian ridge), and thin out into the more moderately populated savanna at longer periods. We test whether sub-Saturns have systematically different companion architectures, as predicted if desert and ridge planets arrived through high-eccentricity migration while savanna planets migrated quiescently. We compile 86 systems with both transit and RV data, construct completeness maps and combine them into detection probability surfaces for companions. The combined completeness maps are used to calculate companion occurrence rates across different companion types with a joint Poisson occurrence model. Companion architectures differ significantly across the landscape. $91.1_{-4.3}^{+3.2}\%$ of savanna sub-Saturns have nearby companions (P < 200 d) compared to only $35.8_{-13.6}^{+16.1}\%$ of desert and ridge sub-Saturns. This contrast is driven almost entirely by small ($M_\mathrm{c} < 20\,\mathrm{M}_\oplus$) companions, which accompany $88.2_{-5.5}^{+4.2}\%$ of savanna but only $29.2_{-14.1}^{+17.5}\%$ of desert and ridge sub-Saturns, while medium-mass and giant companions within 200\,d are rare ($\lesssim 13\%$) everywhere. Savanna sub-Saturns moreover typically reside in compact multi-planet systems. These contrasts are robust to crosscuts in sub-Saturn radius, bulk density, eccentricity, and host-star properties. Desert and ridge sub-Saturns reside in dynamically emptied systems whose nearby companion rates match those of hot Jupiters, while savanna sub-Saturns inhabit compact multi-planet systems resembling those of warm Jupiters. This parallel supports two migration channels operating within a single population: HEM delivering planets to the desert and ridge, and quiescent disk migration or in-situ formation populating the savanna.

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arXiv2026

A population view of transiting hot giant exoplanets: Tracing Fe and Ti chemistry with ESPRESSO and MAROON-X

Hot and ultra-hot Jupiters offer a unique laboratory to study atmospheric chemistry at the population level using ground-based high-resolution spectroscopy. Fe and Ti are key tracers of thermal and chemical structure, yet they exhibit different observational trends across the population. We present a homogeneous reanalysis of high-resolution transmission spectra of ten hot and ultra-hot Jupiters observed with VLT/ESPRESSO and Gemini-N/MAROON-X. We search for neutral Fe and Ti absorption and perform injection-recovery tests using models spanning a range of Ti-depletion levels and T-p profiles. For direct comparison across observations, we introduce the relative cross-correlation metric, $Δ$Ti-Fe. We detect Fe in 7 and Ti in 4 planets above 5$σ$. Across the population, $Δ$Ti-Fe decreases sharply towards lower equilibrium temperatures. Under the assumption of equal Ti depletion across planets, isothermal models fail to reproduce this trend, instead requiring a temperature-dependent depletion of Ti that increases toward cooler planets, consistent with cold-trapping processes in cooler atmospheres. Models with inverted T-p profiles naturally reproduce the decline without invoking temperature-dependent depletion. There, Ti is converted into TiO in deeper, cooler layers and then removed from the gas phase through condensation, leading to strong suppression of the observable atomic Ti signal. Nevertheless, even in the gradient models, overall depletion of Ti relative to Fe is required to match the hottest planets. Our results demonstrate that observable refractory chemistry is governed by the interplay of molecular partitioning, ionisation, condensation, and cold-trapping processes, as well as the vertical structure of ultra-hot Jupiter atmospheres. Additional observations will be necessary to distinguish between temperature-dependent cold-trapping and overall depletion.(abbrev.)

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arXiv2026

Stellar Obliquities of Young Systems, Atmospheres Undergoing Contraction and Escape (SOYSAUCE) II: a 135 Myr planet on an aligned orbit with transit timing variations

Young planets (<1 Gyr) provide opportunities to directly probe planet formation and evolution processes in action. However, due to heightened stellar activity, there is a lack of known transiting planets in adolescence (~100-500 Myr). Here we present the validation of TIC 150070085 b, a 3.6 R_E planet on a 10.47 day orbit, and report the candidate TIC 150070085 c, a 3.0 R_E planet on a 15.90 day orbit. While we are unable to validate the second signal, the proximity to mean motion resonance (3:2) and transit timing variations observed in the transits of TIC 150070085 b strongly suggest the signal is planetary. We confirm the host star as a member of Alessi 84 and combine the group's CMD, rotation, and variability properties to update the age to 135 +/- 10 Myr. We additionally use MAROON-X to observe the Rossiter-McLaughlin signal of TIC 150070085 b and measure the sky projected obliquity angle ($λ$). We find TIC 150070085 b is consistent with a near-aligned orbit with its host star (|$λ$| = 18 +/- 12$^\circ$), in line with similarly aged transiting planets with measured $λ$ values. Continued discovery and characterization of planets in this age regime are vital to link planetary infancy (<50 Myr) and maturity (>1 Gyr).

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arXiv2026

Preparing for the Early eVolution Explorer: Detecting the Primordial, Transiting Exoplanet Population

The close-in small planet population may be formed either with hydrogen/helium dominated envelopes or with water-rich interiors. Both scenarios reproduce the present day planet population in mass, radius, and periods, and are difficult to differentiate with the mature planet demographic. Hydrogen/Helium `gas-dwarfs' have low mean molecular weight atmospheres, while `water-worlds' have envelopes that are significantly heavier, and as such these two scenarios have different evolution tracks that diverge in the first ~50 Myr of their evolution. We show that a low Earth orbit multi-band photometric survey mission, within the scope of the NASA Small Explorers Program (SMEX), can determine the frequency of young close-in planets at the 5% level and definitively differentiate between the competing `gas-dwarf' and `water-world' hypotheses. We simulate a 2.5 year mission capable of simultaneous multi-band near-ultraviolet (NUV), optical, and near infrared (NIR) wide field photometry. Such a mission would perform a photometric survey of 30 different stare-fields selected to probe the young star population. The mission will yield ~100 transiting planets in young star clusters and associations with ages <50 Myr. In comparison, only 20 such planets are known from K2 and TESS today.

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arXiv2026

Exoplanet atmospheres and demographics in the 2040s

Direct observations of exoplanets probe the demographics and atmospheric composition of young self-luminous companions, yielding insight into their formation and early evolution history. In the near future, Gaia will reveal hundreds of nearby young exoplanets amenable to direct follow-up observations. Long-baseline interferometry with current and future facilities is best capable of exploiting this unique synergy which is poised to deliver a statistical sample of benchmark planets with precise dynamical masses and in-depth atmospheric characterization. This will enable tackling the longstanding question of how giant planets form from multiple angles simultaneously, shining light on the complex physical processes underlying planet formation.

astro-ph.IM↗

arXiv2025

Stellar Obliquities of Young Systems, Atmospheres Undergoing Contraction and Escape (SOYSAUCE): a likely aligned orbit for the 3 Myr planet TIDYE-1 b

Despite the wide range of planet-star (mis)alignments in the mature population of transiting exoplanets, the small number of known young transiting planets are nearly all aligned with the rotation axes of their host stars, as determined by the sky-projected obliquity angle. The small number of young systems with measured obliquities limits statistical conclusions. Here we determine the sky-projected obliquity ($λ$) of the 3 Myr transiting planet with a misaligned outer protoplanetary disk, TIDYE-1 b (IRAS 04125+2902 b), using the Rossiter-McLaughlin (RM) effect. Our dataset lacks a pre-transit baseline and ingress, complicating a blind RM fit. Instead, we use contemporaneous spectra and photometry from a mass-measurement campaign to model the stellar activity trend across the transit and provide an external prior on the velocity baseline. We determine $|λ|=11.8^{+5.9\,\circ}_{-5.0}$. Combined with the published rotational velocity of the star, we find a true three-dimensional obliquity of $ψ=15.2^{+7.3\,\circ}_{-5.7}$. Our result is consistent with an aligned orbit, suggesting the planet remains aligned to its star even though the outer disk is misaligned, though additional RM observations are needed to exclude the low-probability tail of misaligned ($>30^{\circ}$) scenarios present in our posterior.

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arXiv2025

A 16 Myr super-Neptune in Upper-Centaurus Lupus and a preliminary survey of transiting planets in Sco-Cen with TESS

Measuring the properties of planets younger than about 50 Myr helps to test different planetary formation and evolution models. NASA's Transiting Exoplanet Survey Satellite (TESS) has observed nearly the entire sky, including a wide range of star-forming regions and young stellar clusters, expanding our census of the newborn planet population. In this work, we present the discovery of the TIC 88785435 planetary system located in the Upper-Centaurus Lupus (UCL) region of the Scorpius-Centaurus OB association (Sco-Cen) and a preliminary survey of the planet population within Sco-Cen. TIC 88785435 is a pre-main sequence, K7V dwarf ($M_\star = 0.72M_\odot$, $R_\star = 0.91R_\odot$, $T_\mathrm{eff}$ = 3998K, V = 11.7 mag) located within the bounds of UCL. We investigate the distribution of rotation periods measured from the TESS long-cadence data and the Halpha and Li abundances from the spectra of TIC 88785435. TESS long-candence data reveal that TIC 88785435 hosts a transiting super-Neptune ($R_b = 5.03R_\oplus$, P = 10.51 days), TIC 88785435 b. Ground-based follow-up validates the planetary nature of TIC 88785435 b. Using the TESS data, we perform a preliminary survey to investigate how TIC 88785435 b compares to the population of newly born planets located within Sco-Cen.

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arXiv2025

A transiting multi-planet system in the 61 million year old association Theia 116

Observing and characterizing young planetary systems can aid in unveiling the evolutionary mechanisms that sculpt the mature exoplanet population. As an all-sky survey, NASA's Transiting Exoplanet Survey Satellite (TESS) has expanded the known young planet population as it has observed young comoving stellar populations. This work presents the discovery of a multiplanet system orbiting the 61 Myr old G4V star TIC 434398831 (M = 0.99 Msun, R = 0.91 Rsun, Teff = 5638 K, Tmag = 11.31) located in the Theia 116 comoving population. We estimate the population's age based on rotation periods measured from the TESS light curves, isochrone fitting, and measurements of lithium equivalent widths in the spectra of Theia 116 members. The TESS FFI light curves reveal a mini-Neptune (Rb = 3.51 Rearth, Pb = 3.69 days) and super-Neptune (Rc = 5.63 Rearth, Pc = 6.21 days) with an orbital period ratio slightly larger than 5:3. Follow-up observations from CHEOPS and ground-based telescopes confirm the transits of TIC 434398831 b and c, and constrain their transit times. We explore the potential mass-loss histories of the two planets in order to probe possible initial conditions of the planets immediately after formation.

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arXiv2024

The occurrence of small, short-period planets younger than 200 Myr with TESS

Within the first few hundreds of millions of years, many physical processes sculpt the eventual properties of young planets. NASA's TESS mission has surveyed young stellar associations across the entire sky for transiting planets providing glimpses into the various stages of planetary evolution. Using our own detection pipeline, we search a magnitude-limited sample of 7219 young stars ($\leq$200 Myr) observed in the first four years of TESS for small (2-8 R$_\oplus$), short period (1.6-20 days) transiting planets. The completeness of our survey is characterized by a series of injection and recovery simulations. Our analysis of TESS 2-minute cadence and Full Frame Image (FFI) light curves recover all known TOIs, as well as four new planet candidates not previously identified as TOIs. We derive an occurrence rate of $35^{+13}_{-10}$% for mini-Neptunes and $27^{+10}_{-8}$% for super-Neptunes from the 2-minute cadence data, and $22^{+8.6}_{-6.8}$% for mini-Neptunes and $13^{+3.9}_{-4.9}$% for super-Neptunes from FFI data. To independently validate our results, we compare our survey yield with the predicted planet yield assuming Kepler planet statistics. We consistently find a mild increase in the occurrence of super-Neptunes and a significant increase in the occurrence of Neptune-sized planets with orbital periods of 6.2-12 days when compared to their mature counterparts. The young planet distribution from our study is most consistent with evolution models describing the early contraction of hydrogen-dominated atmospheres undergoing atmospheric escape and inconsistent with heavier atmosphere models offering only mild radial contraction early on.

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arXiv2023

Spinning up a Daze: TESS Uncovers a Hot Jupiter orbiting the Rapid-Rotator TOI-778

NASA's Transiting Exoplanet Survey Satellite (TESS) mission, has been uncovering a growing number of exoplanets orbiting nearby, bright stars. Most exoplanets that have been discovered by TESS orbit narrow-line, slow-rotating stars, facilitating the confirmation and mass determination of these worlds. We present the discovery of a hot Jupiter orbiting a rapidly rotating ($v\sin{(i)}= 35.1\pm1.0$km/s) early F3V-dwarf, HD115447 (TOI-778). The transit signal taken from Sectors 10 and 37 of TESS's initial detection of the exoplanet is combined with follow-up ground-based photometry and velocity measurements taken from Minerva-Australis, TRES, CORALIE and CHIRON to confirm and characterise TOI-778b. A joint analysis of the light curves and the radial velocity measurements yield a mass, radius, and orbital period for TOI-778b of $2.76^{+0.24}_{-0.23}$Mjup, $1.370\pm0.043$Rjup and $\sim4.63$ days, respectively. The planet orbits a bright ($V = 9.1$mag) F3-dwarf with $M=1.40\pm0.05$Msun, $R=1.70\pm0.05$Rsun, and $\log g=4.05\pm0.17$. We observed a spectroscopic transit of TOI-778b, which allowed us to derive a sky-projected spin-orbit angle of $18^{\circ}\pm11^{\circ}$, consistent with an aligned planetary system. This discovery demonstrates the capability of smaller aperture telescopes such as Minerva-Australis to detect the radial velocity signals produced by planets orbiting broad-line, rapidly rotating stars.

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arXiv2021

TOI-712: a system of adolescent mini-Neptunes extending to the habitable zone

As an all-sky survey, NASA's $TESS$ mission is able to detect the brightest and rarest types of transiting planetary systems, including young planets that enable study of the evolutionary processes that occur within the first billion years. Here, we report the discovery of a young, multi-planet system orbiting the bright K4.5V star, TOI-712 ($V = 10.838$, $M_\star = 0.733_{-0.025}^{+0.026} M_\odot$, $R_\star = 0.674\pm0.016 R_\odot$, $T_{\rm eff} = 4622_{-60}^{+61}$ K). From the $TESS$ light curve, we measure a rotation period of 12.48 days, and derive an age between about $500$ Myr and 1.1 Gyr. The photometric observations reveal three transiting mini-Neptunes ($R_b = 2.049^{+0.12}_{-0.080} R_\oplus$, $R_c = 2.701^{+0.092}_{-0.082} R_\oplus$, $R_d = 2.474^{+0.090}_{-0.082} R_\oplus $), with orbital periods of $P_b = 9.531$ days, $P_c = 51.699$ days, and $P_d = 84.839$ days. After modeling the three-planet system, an additional Earth-sized candidate is identified, TOI-712.05 ($P = 4.32$ days, $R_P = 0.81 \pm 0.11 R_\oplus$). We calculate that the habitable zone falls between 0.339 and 0.844 au (82.7 and 325.3 days), placing TOI-712 d near its inner edge. Among planetary systems harboring temperate planets, TOI-712 ($T = 9.9$) stands out as a relatively young star bright enough to motivate further characterization.

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arXiv2021

TESS Delivers Five New Hot Giant Planets Orbiting Bright Stars from the Full Frame Images

We present the discovery and characterization of five hot and warm Jupiters -- TOI-628 b (TIC 281408474; HD 288842), TOI-640 b (TIC 147977348), TOI-1333 b (TIC 395171208, BD+47 3521A), TOI-1478 b (TIC 409794137), and TOI-1601 b (TIC 139375960) -- based on data from NASA's Transiting Exoplanet Survey Satellite (TESS). The five planets were identified from the full frame images and were confirmed through a series of photometric and spectroscopic follow-up observations by the $TESS$ Follow-up Observing Program (TFOP) Working Group. The planets are all Jovian size (R$_{\rm P}$ = 1.01-1.77 R$_{\rm J}$) and have masses that range from 0.85 to 6.33 M$_{\rm J}$. The host stars of these systems have F and G spectral types (5595 $\le$ T$_{\rm eff}$ $\le$ 6460 K) and are all relatively bright (9 $ 1.7R$_{\rm J}$, possibly a result of its host star's evolution) and resides on an orbit with a period longer than 5 days. TOI-628 b is the most massive hot Jupiter discovered to date by $TESS$ with a measured mass of $6.31^{+0.28}_{-0.30}$ M$_{\rm J}$ and a statistically significant, non-zero orbital eccentricity of e = $0.074^{+0.021}_{-0.022}$. This planet would not have had enough time to circularize through tidal forces from our analysis, suggesting that it might be remnant eccentricity from its migration. The longest period planet in this sample, TOI-1478 b (P = 10.18 days), is a warm Jupiter in a circular orbit around a near-Solar analogue. NASA's $TESS$ mission is continuing to increase the sample of well-characterized hot and warm Jupiters, complementing its primary mission goals.

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