Redshifted Sodium Transient near Exoplanet Transit

• Main Authors: Oza, Apurva V, Seidel, Julia V, Hoeijmakers, H. Jens, Unni, Athira, Kesseli, Aurora Y, Schmidt, Carl A, Thirupathi, Sivarani, Bello-Arufe, Aaron, Gebek, Andrea, Westram, Moritz Meyer zu, Sousa, Sérgio G, Lopes, Rosaly M. C, Hu, Renyu, de Kleer, Katherine, Fisher, Chloe, Charnoz, Sébastien, Baker, Ashley D, Halverson, Samuel P, Schneider, Nicholas M, Psaridi, Angelica, Wyttenbach, Aurélien, Torres, Santiago, Bhatnagar, Ishita, Johnson, Robert E
• Format: Journal Article
• Language: English
• Published: 29-09-2024
• Subjects: Physics - Earth and Planetary Astrophysics
• DOI: 10.48550/arxiv.2409.19844

Neutral sodium (Na I) is an alkali metal with a favorable absorption cross
section such that tenuous gases are easily illuminated at select transiting
exoplanet systems. We examine both the time-averaged and time-series alkali
spectral flux individually, over 4 nights at a hot Saturn system on a $\sim$
2.8 day orbit about a Sun-like star WASP-49 A. Very Large Telescope/ESPRESSO
observations are analyzed, providing new constraints. We recover the previously
confirmed residual sodium flux uniquely when averaged, whereas night-to-night
Na I varies by more than an order of magnitude. On HARPS/3.6-m Epoch II, we
report a Doppler redshift at $v_{ \Gamma, \mathrm{NaD}} =$ +9.7 $\pm$ 1.6 km/s
with respect to the planet's rest frame. Upon examining the lightcurves, we
confirm night-to-night variability, on the order of $\sim$ 1-4 % in NaD rarely
coinciding with exoplanet transit, not readily explained by stellar activity,
starspots, tellurics, or the interstellar medium. Coincident with the $\sim$+10
km/s Doppler redshift, we detect a transient sodium absorption event
dF$_{\mathrm{NaD}}$/F$_{\star}$ = 3.6 $\pm$ 1 % at a relative difference of
$\Delta F_{\mathrm{NaD}} (t) \sim$ 4.4 $\pm$ 1 %, enduring $\Delta
t_{\mathrm{NaD}} \gtrsim$ 40 minutes. Since exoplanetary alkali signatures are
blueshifted due to the natural vector of radiation pressure, estimated here at
roughly $\sim$ -5.7 km/s, the radial velocity is rather at +15.4 km/s, far
larger than any known exoplanet system. Given that the redshift magnitude
v$_{\Gamma}$ is in between the Roche limit and dynamically stable satellite
orbits, the transient sodium may be a putative indication of a natural
satellite orbiting WASP-49 A b.