Browsing by Author "Stefansson, Gudmundur"

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    Gaia-4b and 5b: Radial Velocity Confirmation of Gaia Astrometric Orbital Solutions Reveal a Massive Planet and a Brown Dwarf Orbiting Low-mass Stars
    (IOP Publishing Ltd, 2025) Stefansson, Gudmundur; Mahadevan, Suvrath; Winn, Joshua N.; Marcussen, Marcus L.; Kanodia, Shubham; Albrecht, Simon; Fitzmaurice, Evan; Mikulskyte, One; Canas, Caleb I.; Espinoza Retamal, Juan Ignacio; Zwart, Yiri; Krolikowski, Daniel M.; Hotnisky, Andrew; Robertson, Paul; Alvarado-Montes, Jaime A.; Bender, Chad F.; Blake, Cullen H.; Callingham, J. R.; Cochran, William D.; Delamer, Megan; Diddams, Scott A.; Dong, Jiayin; Fernandes, Rachel B.; Giovinazzi, Mark R.; Halverson, Samuel; Libby-Roberts, Jessica; Logsdon, Sarah E.; Mcelwain, Michael W.; Ninan, Joe P.; Rajagopal, Jayadev; Reji, Varghese; Roy, Arpita; Schwab, Christian; Wright, Jason T.
    Gaia astrometry of nearby stars is precise enough to detect the tiny displacements induced by substellar companions, but radial velocity (RV) data are needed for definitive confirmation. Here we present RV follow-up observations of 28 M and K stars with candidate astrometric substellar companions, which led to the confirmation of two systems, Gaia-4b and Gaia-5b, identification of five systems that are single lined but require additional data to confirm as substellar companions, and the refutation of 21 systems as stellar binaries. Gaia-4b is a massive planet (M = 11.8 +/- 0.7 MJ) in a P = 571.3 +/- 1.4 day orbit with a projected semimajor axis a0 = 0.312 +/- 0.040 mas orbiting a 0.644 +/- 0.02M circle dot star. Gaia-5b is a brown dwarf (M = 20.9 +/- 0.5MJ) in a P = 358.62 +/- 0.20 days eccentric e = 0.6423 +/- 0.0026 orbit with a projected angular semimajor axis of a0 = 0.947 +/- 0.038 mas around a 0.34 +/- 0.03M circle dot star. Gaia-4b is one of the first exoplanets discovered via the astrometric technique, and is one of the most massive planets known to orbit a low-mass star.
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    HATS-38 b and WASP-139 b Join a Growing Group of Hot Neptunes on Polar Orbits
    (2024) Espinoza-Retamal, Juan I.; Stefansson, Gudmundur; Petrovich, Cristobal; Brahm, Rafael; Jordan, Andres; Sedaghati, Elyar; Lucero, Jennifer P.; Pinto, Marcelo Tala; Munoz, Diego J.; Boyle, Gavin; Leiva, Rodrigo; Suc, Vincent
    We constrain the sky-projected obliquities of two low-density hot Neptune planets, HATS-38 b and WASP-139 b, orbiting nearby G and K stars using Rossiter-McLaughlin (RM) observations with VLT/ESPRESSO, yielding lambda=-108(-16)(+11) deg and -85.6(-4.2)(+7.7) deg, respectively. To model the RM effect, we use a new publicly available code, ironman, which is capable of jointly fitting transit photometry, Keplerian radial velocities, and RM effects. WASP-139 b has a residual eccentricity e=0.103(-0.041)(+0.050) while HATS-38 b has an eccentricity of e=0.112(-0.070)(+0.072), which is compatible with a circular orbit given our data. Using the obliquity constraints, we show that they join a growing group of hot and low-density Neptunes on polar orbits. We use long-term radial velocities to rule out companions with masses similar to 0.3-50 M-J within similar to 10 au. We show that the orbital architectures of the two Neptunes can be explained with high-eccentricity migration from greater than or similar to 2 au driven by an unseen distant companion. If HATS-38b has no residual eccentricity, its polar and circular orbit can also be consistent with a primordial misalignment. Finally, we performed a hierarchical Bayesian modeling of the true obliquity distribution of Neptunes and found suggestive evidence for a higher preponderance of polar orbits of hot Neptunes compared to Jupiters. However, we note that the exact distribution is sensitive to the choice of priors, highlighting the need for additional obliquity measurements of Neptunes to robustly compare the hot Neptune obliquity distribution to Jupiters.
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    NEID Reveals That the Young Warm Neptune TOI-2076 b Has a Low Obliquity
    (2023) Frazier, Robert C.; Stefansson, Gudmundur; Mahadevan, Suvrath; Yee, Samuel W.; Canas, Caleb I.; Winn, Joshua N.; Luhn, Jacob; Dai, Fei; Doyle, Lauren; Cegla, Heather; Kanodia, Shubham; Robertson, Paul; Wisniewski, John; Bender, Chad F.; Dong, Jiayin; Gupta, Arvind F.; Halverson, Samuel; Hawley, Suzanne; Hebb, Leslie; Holcomb, Rae; Kowalski, Adam; Libby-Roberts, Jessica; Lin, Andrea S. J.; McElwain, Michael W.; Ninan, Joe P.; Petrovich, Cristobal; Roy, Arpita; Schwab, Christian; Terrien, Ryan C.; Wright, Jason T.
    TOI-2076 b is a sub-Neptune-sized planet (R = 2.39 + 0.10 R-circle plus) that transits a young (204 + 50 MYr) bright (V= 9.2) K-dwarf hosting a system of three transiting planets. Using spectroscopic observations obtained with the NEID spectrograph on the WIYN 3.5 m Telescope, we model the Rossiter-McLaughlin effect of TOI-2076 b, and derive a sky-projected obliquity of lambda = - 3 (-15) (+16) . Using the size of the star (R = 0.775 + 0.015 R-?), and the stellar 16 rotation period (P-rot = 7.27 + 0.23 days), we estimate an obliquity of y =18(-9) (+10) (psi < 34 degrees at 95% confidence), 10 demonstrating that TOI-2076 b is in a well-aligned orbit. Simultaneous diffuser-assisted photometry from the 3.5 m telescope at Apache Point Observatory rules out flares during the transit. TOI-2076 b joins a small but growing sample of young planets in compact multi-planet systems with well-aligned orbits, and is the fourth planet with an age ?300 Myr in a multi-transiting system with an obliquity measurement. The low obliquity of TOI-2076 b and the presence of transit timing variations in the system suggest the TOI-2076 system likely formed via convergent disk migration in an initially well-aligned disk.
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    The Warm Neptune GJ 3470b Has a Polar Orbit
    (2022) Stefansson, Gudmundur; Mahadevan, Suvrath; Petrovich, Cristobal; Winn, Joshua N.; Kanodia, Shubham; Millholland, Sarah C.; Maney, Marissa; Canas, Caleb, I; Wisniewski, John; Robertson, Paul; Ninan, Joe P.; Ford, Eric B.; Bender, Chad F.; Blake, Cullen H.; Cegla, Heather; Cochran, William D.; Diddams, Scott A.; Dong, Jiayin; Endl, Michael; Fredrick, Connor; Halverson, Samuel; Hearty, Fred; Hebb, Leslie; Hirano, Teruyuki; Lin, Andrea S. J.; Logsdon, Sarah E.; Lubar, Emily; McElwain, Michael W.; Metcalf, Andrew J.; Monson, Andrew; Rajagopal, Jayadev; Ramsey, Lawrence W.; Roy, Arpita; Schwab, Christian; Schweiker, Heidi; Terrien, Ryan C.; Wright, Jason T.
    The warm Neptune GJ 3470b transits a nearby (d = 29 pc) bright slowly rotating M1.5-dwarf star. Using spectroscopic observations during two transits with the newly commissioned NEID spectrometer on the WIYN 3.5 m Telescope at Kitt Peak Observatory, we model the classical Rossiter-McLaughlin effect, yielding a sky-projected obliquity of lambda = 98(-12)(+15)degrees and a v sin i = 0.85(-033)(+0.27) km s(-1). Leveraging information about the rotation period and size of the host star, our analysis yields a true obliquity of psi = 95(-8)(+9)degrees, revealing that GJ 3470b is on a polar orbit. Using radial velocities from HIRES, HARPS, and the Habitable-zone Planet Finder, we show that the data are compatible with a long-term radial velocity (RV) slope of gamma = -0.0022 +/- 0.0011 m s(-1) day(-1) over a baseline of 12.9 yr. If the RV slope is due to acceleration from another companion in the system, we show that such a companion is capable of explaining the polar and mildly eccentric orbit of GJ 3470b using two different secular excitation models. The existence of an outer companion can be further constrained with additional RV observations, Gaia astrometry, and future high-contrast imaging observations. Lastly, we show that tidal heating from GJ 3470b's mild eccentricity has most likely inflated the radius of GJ 3470b by a factor of similar to 1.5-1.7, which could help account for its evaporating atmosphere.