Detection of Ongoing Mass Loss from HD 63433c, a Young Mini Neptune. (arXiv:2106.05273v1 [astro-ph.EP])
<a href="http://arxiv.org/find/astro-ph/1/au:+Zhang_M/0/1/0/all/0/1">Michael Zhang</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Knutson_H/0/1/0/all/0/1">Heather A. Knutson</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Wang_L/0/1/0/all/0/1">Lile Wang</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Dai_F/0/1/0/all/0/1">Fei Dai</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Santos_L/0/1/0/all/0/1">Leonardo A. dos Santos</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Fossati_L/0/1/0/all/0/1">Luca Fossati</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Henry_G/0/1/0/all/0/1">Gregory W. Henry</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ehrenreich_D/0/1/0/all/0/1">David Ehrenreich</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Alibert_Y/0/1/0/all/0/1">Yann Alibert</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Hoyer_S/0/1/0/all/0/1">Sergio Hoyer</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Wilson_T/0/1/0/all/0/1">Thomas G. Wilson</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Bonfanti_A/0/1/0/all/0/1">Andrea Bonfanti</a>
We detect Lyman $alpha$ absorption from the escaping atmosphere of HD
63433c, a $R=2.67 R_oplus$, $P=20.5$ d mini Neptune orbiting a young (440 Myr)
solar analogue in the Ursa Major Moving Group. Using HST/STIS, we measure a
transit depth of $11.1 pm 1.5$% in the blue wing and $8 pm 3$% in the red.
This signal is unlikely to be due to stellar variability, but should be
confirmed by an upcoming second visit with HST. We do not detect Lyman $alpha$
absorption from the inner planet, a smaller $R=2.15 R_oplus$ mini Neptune on a
7.1 d orbit. We use Keck/NIRSPEC to place an upper limit of 0.5% on helium
absorption for both planets. We measure the host star’s X-ray spectrum and FUV
flux with XMM-Newton, and model the outflow from both planets using a 3D
hydrodynamic code. This model provides a reasonable match to the light curve in
the blue wing of the Lyman $alpha$ line and the helium non-detection for
planet c, although it does not explain the tentative red wing absorption or
reproduce the excess absorption spectrum in detail. Its predictions of strong
Lyman $alpha$ and helium absorption from b are ruled out by the observations.
This model predicts a much shorter mass loss timescale for planet b, suggesting
that b and c are fundamentally different: while the latter still retains its
hydrogen/helium envelope, the former has likely lost its primordial atmosphere.
We detect Lyman $alpha$ absorption from the escaping atmosphere of HD
63433c, a $R=2.67 R_oplus$, $P=20.5$ d mini Neptune orbiting a young (440 Myr)
solar analogue in the Ursa Major Moving Group. Using HST/STIS, we measure a
transit depth of $11.1 pm 1.5$% in the blue wing and $8 pm 3$% in the red.
This signal is unlikely to be due to stellar variability, but should be
confirmed by an upcoming second visit with HST. We do not detect Lyman $alpha$
absorption from the inner planet, a smaller $R=2.15 R_oplus$ mini Neptune on a
7.1 d orbit. We use Keck/NIRSPEC to place an upper limit of 0.5% on helium
absorption for both planets. We measure the host star’s X-ray spectrum and FUV
flux with XMM-Newton, and model the outflow from both planets using a 3D
hydrodynamic code. This model provides a reasonable match to the light curve in
the blue wing of the Lyman $alpha$ line and the helium non-detection for
planet c, although it does not explain the tentative red wing absorption or
reproduce the excess absorption spectrum in detail. Its predictions of strong
Lyman $alpha$ and helium absorption from b are ruled out by the observations.
This model predicts a much shorter mass loss timescale for planet b, suggesting
that b and c are fundamentally different: while the latter still retains its
hydrogen/helium envelope, the former has likely lost its primordial atmosphere.
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