Abstract
Planets around young stars trace the early evolution of planetary systems. We report the discovery and validation of two planetary systems with ages ≲300 Myr from observations by the ...Transiting Exoplanet Survey Satellite (TESS). The
Myr old G star TOI-251 hosts a
mini-Neptune with a
day period. The
Myr old K star TOI-942 hosts a system of inflated Neptune-sized planets, with TOI-942b orbiting in a period of
days with a radius of
and TOI-942c orbiting in a period of
days with a radius of
. Though we cannot place either host star into a known stellar association or cluster, we can estimate their ages via their photometric and spectroscopic properties. Both stars exhibit significant photometric variability due to spot modulation, with measured rotation periods of ∼3.5 days. These stars also exhibit significant chromospheric activity, with age estimates from the chromospheric calcium emission lines and X-ray fluxes matching that estimated from gyrochronology. Both stars also exhibit significant lithium absorption, similar in equivalent width to well-characterized young cluster members. TESS has the potential to deliver a population of young planet-bearing field stars, contributing significantly to tracing the properties of planets as a function of their age.
Abstract
We report the discovery of two transiting brown dwarfs (BDs), TOI-811b and TOI-852b, from NASA’s Transiting Exoplanet Survey Satellite mission. These two transiting BDs have similar masses ...but very different radii and ages. Their host stars have similar masses, effective temperatures, and metallicities. The younger and larger transiting BD is TOI-811b at a mass of
M
b
= 59.9 ± 13.0
M
J
and radius of
R
b
= 1.26 ± 0.06
R
J
, and it orbits its host star in a period of
P
= 25.16551 ± 0.00004 days. We derive the host star’s age of
Myr from an application of gyrochronology. The youth of this system, rather than external heating from its host star, is why this BD’s radius is relatively large. This constraint on the youth of TOI-811b allows us to test substellar mass–radius evolutionary models at young ages where the radius of BDs changes rapidly. TOI-852b has a similar mass at
M
b
= 53.7 ± 1.4
M
J
but is much older (4 or 8 Gyr, based on bimodal isochrone results of the host star) and is also smaller with a radius of
R
b
= 0.83 ± 0.04
R
J
. TOI-852b’s orbital period is
P
= 4.94561 ± 0.00008 days. TOI-852b joins the likes of other old transiting BDs that trace out the oldest substellar mass–radius evolutionary models where contraction of the BD’s radius slows and approaches a constant value. Both host stars have a mass of
M
⋆
= 1.32
M
⊙
± 0.05 and differ in their radii,
T
eff
, and Fe/H, with TOI-811 having
R
⋆
= 1.27 ± 0.09
R
⊙
,
T
eff
= 6107 ± 77 K, and Fe/H = + 0.40 ± 0.09 and TOI-852 having
R
⋆
= 1.71 ± 0.04
R
⊙
,
T
eff
= 5768 ± 84 K, and Fe/H = + 0.33 ± 0.09. We take this opportunity to examine how TOI-811b and TOI-852b serve as test points for young and old substellar isochrones, respectively.
Planets around young stars trace the early evolution of planetary systems. We report the discovery and validation of two planetary systems with ages \(\lesssim 300\) Myr from observations by the ...Transiting Exoplanet Survey Satellite. TOI-251 is a 40-320 Myr old G star hosting a 2.74 +0.18/-0.18 REarth mini-Neptune with a 4.94 day period. TOI-942 is a 20-160 Myr old K star hosting a system of inflated Neptune-sized planets, with TOI-942b orbiting with a period of 4.32 days, with a radius of 4.81 +0.20/-0.20 REarth, and TOI-942c orbiting in a period of 10.16 days with a radius of 5.79 +0.19/-0.18 REarth. Though we cannot place either host star into a known stellar association or cluster, we can estimate their ages via their photometric and spectroscopic properties. Both stars exhibit significant photometric variability due to spot modulation, with measured rotation periods of \(\sim 3.5\) days. These stars also exhibit significant chromospheric activity, with age estimates from the chromospheric calcium emission lines and X-ray fluxes matching that estimated from gyrochronology. Both stars also exhibit significant lithium absorption, similar in equivalent width to well-characterized young cluster members. TESS has the potential to deliver a population of young planet-bearing field stars, contributing significantly to tracing the properties of planets as a function of their age.
Combinatorial covers of graphs were defined by Chung and Yau. Their main feature is that the spectra of the Combinatorial Laplacian of the base and the total space are related. We extend their ...definition to directed graphs. As an application, we compute the spectrum of the Combinatorial Laplacian of the homesick random walk $RW_{\mu}$ on the line. Using this calculation, we show that the heat kernel on the weighted line can be computed from the heat kernel of '$(1 + 1/\mu)$-regular' tree.
D. A. Kahzdan first put forth property (T) in relation to the study of
discrete subgroups of Lie groups of finite co-volume. Through a combinatorial
approach, we define an analogue of property (T) ...for regular graphs. We then
prove the basic combinatorial and metric properties of Kazhdan groups in this
context. In particular, we use our methods to construct infinite families of
expanders as in the classical case. Finally, we consider the combinatorial
analogue of the group theoretic property $(\tau)$ and prove its basic
properties.
D. A. Kahzdan first put forth property (T) in relation to the study of discrete subgroups of Lie groups of finite co-volume. Through a combinatorial approach, we define an analogue of property (T) ...for regular graphs. We then prove the basic combinatorial and metric properties of Kazhdan groups in this context. In particular, we use our methods to construct infinite families of expanders as in the classical case. Finally, we consider the combinatorial analogue of the group theoretic property \((\tau)\) and prove its basic properties.
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