25 research outputs found
Far-Ultraviolet Activity Levels of F, G, K, and M dwarf Exoplanet Host Stars
We present a survey of far-ultraviolet (FUV; 1150 - 1450 Ang) emission line
spectra from 71 planet-hosting and 33 non-planet-hosting F, G, K, and M dwarfs
with the goals of characterizing their range of FUV activity levels,
calibrating the FUV activity level to the 90 - 360 Ang extreme-ultraviolet
(EUV) stellar flux, and investigating the potential for FUV emission lines to
probe star-planet interactions (SPIs). We build this emission line sample from
a combination of new and archival observations with the Hubble Space
Telescope-COS and -STIS instruments, targeting the chromospheric and transition
region emission lines of Si III, N V, C II, and Si IV.
We find that the exoplanet host stars, on average, display factors of 5 - 10
lower UV activity levels compared with the non-planet hosting sample; this is
explained by a combination of observational and astrophysical biases in the
selection of stars for radial-velocity planet searches. We demonstrate that UV
activity-rotation relation in the full F - M star sample is characterized by a
power-law decline (with index ~ -1.1), starting at rotation periods
>~3.5 days. Using N V or Si IV spectra and a knowledge of the star's bolometric
flux, we present a new analytic relationship to estimate the intrinsic stellar
EUV irradiance in the 90 - 360 Ang band with an accuracy of roughly a factor of
~2. Finally, we study the correlation between SPI strength and UV activity in
the context of a principal component analysis that controls for the sample
biases. We find that SPIs are not a statistically significant contributor to
the observed UV activity levels.Comment: ApJS, accepted. 33 pages in emulateapj, 13 figures, 10 table
Probing UV-sensitive Pathways for CN and HCN Formation in Protoplanetary Disks with the Hubble Space Telescope
The UV radiation field is a critical regulator of gas-phase chemistry in surface layers of disks around young stars. In an effort to understand the relationship between photocatalyzing UV radiation fields and gas emission observed at infrared and submillimeter wavelengths, we present an analysis of new and archival Hubble Space Telescope (HST), Spitzer, ALMA, IRAM, and SMA data for five targets in the Lupus cloud complex and 14 systems in Taurus-Auriga. The HST spectra were used to measure Lyα and far-UV (FUV) continuum fluxes reaching the disk surface, which are responsible for dissociating relevant molecular species (e.g., HCN, N₂). Semi-forbidden C II] λ2325 and UV-fluorescent H₂ emission were also measured to constrain inner disk populations of C⁺ and vibrationally excited H2. We find a significant positive correlation between 14 μm HCN emission and fluxes from the FUV continuum and C II] λ2325, consistent with model predictions requiring N₂ photodissociation and carbon ionization to trigger the main CN/HCN formation pathways. We also report significant negative correlations between submillimeter CN emission and both C II] and FUV continuum fluxes, implying that CN is also more readily dissociated in disks with stronger FUV irradiation. No clear relationships are detected between either CN or HCN and Lyα or UV-H₂ emission. This is attributed to the spatial stratification of the various molecular species, which span several vertical layers and radii across the inner and outer disk. We expect that future observations with the James Webb Space Telescope will build on this work by enabling more sensitive IR surveys than were possible with Spitzer
First Detection and Modeling of Spatially Resolved Ly in TW Hya
Lyman- (Ly) is the strongest emission line in UV spectra from
T-Tauri stars. Due to its resonant nature, Ly emission carries
information about the physical properties of the H I medium via the scattering
process. This work presents spatially resolved Ly emission across a
protoplanetary disk in the iconic face-on T-Tauri star TW Hya, observed with
HST-STIS at spatial offsets 0, , and . To
comprehensively interpret these Ly spectra, we utilize a 3D Monte-Carlo
radiative transfer simulation in a wind-disk geometry. Successfully reproducing
the observed spectra requires scattering contributions from both the wind and
the H I disk. We constrain the properties of the wind, the H I column density
() and the outflow velocity (). To reproduce the observed spatial distribution of Ly, we find
that the wind must cover the H I disk when viewed face-on. Furthermore, to
explore the effect of Ly radiative transfer in T-Tauri stars, we
compute the radiation field within the scattering medium and reveal that the
wind reflection causes more Ly photons to penetrate the disk. We also
show the dependence between the disk inclination angle and the spatially
resolved Ly spectra. Understanding the role of Ly emission in
T-Tauri stars is pivotal for decoding the complex interactions between the
winds, protoplanetary disks, and surrounding environments, which can
significantly impact the chemistry in the protoplanetary disk. Our observation
and modeling of spatially resolved Ly show the necessity of spatially
resolved Ly observation of a broad range of targets.Comment: 17 pages, 18 figures, Submitted to MNRA
X-ray, Near-Ultraviolet, and Optical Flares Produced By Colliding Magnetospheres in The Young High-Eccentricity Binary DQ Tau
DQ Tau is a unique young high-eccentricity binary system that exhibits
regular magnetic reconnection flares and pulsed accretion near periastron. We
conducted NuSTAR, Swift, and Chandra observations during the July 30, 2022
periastron to characterize X-ray, near-ultraviolet (NUV), and optical flaring
emissions. Our findings confirm the presence of X-ray super-flares accompanied
by substantial NUV and optical flares, consistent with previous discoveries of
periastron flares in 2010 and 2021. These observations, supported by new
evidence, strongly establish the magnetosphere collision mechanism as the
primary driver of magnetic energy release during DQ Tau's periastron flares.
The energetics of the observed X-ray super-flares remain consistent across the
three periastrons, indicating recurring energy sources during each passage,
surpassing the capabilities of single stars. The observed flaring across
multiple bands supports the Adams et al. model for magnetosphere interaction in
eccentric binaries. Evidence from modeling and past and current observations
suggests that both the mm/X-ray periastron flares and tentatively, the magnetic
reconnection-related components of the optical/NUV emissions, conform to the
classical solar/stellar non-thermal thick-target model, except for the
distinctive magnetic energy source. However, our NuSTAR observations suffered
from high background levels, hindering the detection of anticipated non-thermal
hard X-rays. Furthermore, we report serendipitous discovery of X-ray
super-flares occurring away from periastron, potentially associated with
interacting magnetospheres. The current study is part of a broader
multi-wavelength campaign, which is planned to investigate the influence of DQ
Tau's stellar radiation on gas-phase ion chemistry within its circumbinary
disk.Comment: 27 pages, 9 figures, 3 tables. Accepted for publication in The
Astrophysical Journal, October 18, 202
Probing UV-Sensitive Pathways for CN and HCN Formation in Protoplanetary Disks with the Hubble Space Telescope
The UV radiation field is a critical regulator of gas-phase chemistry in
surface layers of disks around young stars. In an effort to understand the
relationship between photocatalyzing UV radiation fields and gas emission
observed at infrared and sub-mm wavelengths, we present an analysis of new and
archival HST, Spitzer, ALMA, IRAM, and SMA data for five targets in the Lupus
cloud complex and 14 systems in Taurus-Auriga. The HST spectra were used to
measure LyA and FUV continuum fluxes reaching the disk surface, which are
responsible for dissociating relevant molecular species (e.g. HCN, N2).
Semi-forbidden C II] 2325 and UV-fluorescent H2 emission were also measured to
constrain inner disk populations of C+ and vibrationally excited H2. We find a
significant positive correlation between 14 micron HCN emission and fluxes from
the FUV continuum and C II] 2325, consistent with model predictions requiring
N2 photodissociation and carbon ionization to trigger the main CN/HCN formation
pathways. We also report significant negative correlations between sub-mm CN
emission and both C II] and FUV continuum fluxes, implying that CN is also more
readily dissociated in disks with stronger FUV irradiation. No clear
relationships are detected between either CN or HCN and LyA or UV-H2 emission.
This is attributed to the spatial stratification of the various molecular
species, which span several vertical layers and radii across the inner and
outer disk. We expect that future observations with JWST will build on this
work by enabling more sensitive IR surveys than were possible with Spitzer.Comment: Accepted for publication in A
The Young Binary DQ Tau Produces Another X-Ray Flare Near Periastron
This work is part of a multi-wavelength program to study the effects of X-ray/UV/optical stellar radiation on the chemistry of the circumbinary disk around the young high-eccentricity binary DQ Tau. ALMA observations for near/around 2021 December 5 periastron were postponed due to bad weather, but supporting Swift-XRT-UVOT TOO observations were successful. These Swift observations along with previous X-ray-optical-mm data show that DQ Tau keeps exhibiting powerful flares near periastron, offering a unique laboratory for studies of flare effects on the gas-phase ion chemistry in protoplanetary disks
Probing UV-sensitive Pathways for CN and HCN Formation in Protoplanetary Disks with the Hubble Space Telescope
The UV radiation field is a critical regulator of gas-phase chemistry in surface layers of disks around young stars. In an effort to understand the relationship between photocatalyzing UV radiation fields and gas emission observed at infrared and submillimeter wavelengths, we present an analysis of new and archival Hubble Space Telescope (HST), Spitzer, ALMA, IRAM, and SMA data for five targets in the Lupus cloud complex and 14 systems in Taurus-Auriga. The HST spectra were used to measure Lyα and far-UV (FUV) continuum fluxes reaching the disk surface, which are responsible for dissociating relevant molecular species (e.g., HCN, N₂). Semi-forbidden C II] λ2325 and UV-fluorescent H₂ emission were also measured to constrain inner disk populations of C⁺ and vibrationally excited H2. We find a significant positive correlation between 14 μm HCN emission and fluxes from the FUV continuum and C II] λ2325, consistent with model predictions requiring N₂ photodissociation and carbon ionization to trigger the main CN/HCN formation pathways. We also report significant negative correlations between submillimeter CN emission and both C II] and FUV continuum fluxes, implying that CN is also more readily dissociated in disks with stronger FUV irradiation. No clear relationships are detected between either CN or HCN and Lyα or UV-H₂ emission. This is attributed to the spatial stratification of the various molecular species, which span several vertical layers and radii across the inner and outer disk. We expect that future observations with the James Webb Space Telescope will build on this work by enabling more sensitive IR surveys than were possible with Spitzer
Flares, Rotation, Activity Cycles and a Magnetic Star-Planet Interaction Hypothesis for the Far Ultraviolet Emission of GJ 436
Variability in the far ultraviolet (FUV) emission produced by stellar
activity affects photochemistry and heating in orbiting planetary atmospheres.
We present a comprehensive analysis of the FUV variability of GJ 436, a
field-age, M2.5V star ( d) orbited by a warm,
Neptune-size planet (, ,
d). Observations at three epochs from 2012 to 2018
span nearly a full activity cycle, sample two rotations of the star and two
orbital periods of the planet, and reveal a multitude of brief flares. Over
2012-2018, the star's yr activity cycle produced the largest
observed variations, % in the summed flux of major FUV emission lines.
In 2018, variability due to rotation was %. An additional %
scatter at 10 min cadence, treated as white noise in fits, likely has both
instrumental and astrophysical origins. Flares increased time-averaged emission
by 15% over the 0.88 d of cumulative exposure, peaking as high as 25
quiescence. We interpret these flare values as lower limits given that flares
too weak or too infrequent to have been observed likely exist. GJ 436's flare
frequency distribution (FFD) at FUV wavelengths is unusual compared to other
field-age M dwarfs, exhibiting a statistically-significant dearth of high
energy ( erg) events that we hypothesize to be the result of
a magnetic star-planet interaction (SPI) triggering premature flares. If an SPI
is present, GJ 436 b's magnetic field strength must be 100 G to
explain the statistically insignificant increase in orbit-phased FUV emission.Comment: 18 pages, 9 figures, accepted to the Astronomical Journa
The Radial Distribution and Excitation of H2 around Young Stars in the HST-ULLYSES Survey
The spatial distribution and evolution of gas in the inner 10 au of
protoplanetary disks form the basis for estimating the initial conditions of
planet formation. Among the most important constraints derived from
spectroscopic observations of the inner disk are the radial distributions of
the major gas phase constituents, how the properties of the gas change with
inner disk dust evolution, and how chemical abundances and excitation
conditions are influenced by the high-energy radiation from the central star.
We present a survey of the radial distribution, excitation, and evolution of
inner disk molecular hydrogen (H) obtained as part of the /ULLYSES
program. We analyze far-ultraviolet spectroscopy of 71 (63 accreting) pre-main
sequence systems in the ULLYSES DR5 release to characterize the H
emission lines, H dissociation continuum emission, and major
photochemical/disk evolution driving UV emissions (Ly, UV continuum,
and C IV). We use the widths of the H emission lines to show that most
fluorescent H arises between 0.1 - 1.4 au from the parent star, and show
positive correlations of the average emitting radius with the accretion
luminosity and with the dust disk mass. We find a strong correlation between
H dissociation emission and both the accretion-dominated Ly
luminosity and the inner disk dust clearing, painting a picture where water
molecules in the inner 3 au are exposed to and dissociated by strong Ly
emission as the opacity of the inner disk declines with time.Comment: 19 pages, 11 figures. Accepted to the Astronomical Journa