Star-planet interactions and selection effects from planet detection methods. (arXiv:1303.0307v1 [astro-ph.SR]):
Planets may have effects on their host stars by tidal or magnetic
interaction. Such star-planet interactions are thought to enhance the activity
level of the host star. However, stellar activity also affects the sensitivity
of planet detection methods. Samples of planet-hosting stars which are
investigated for such star-planet interactions are therefore subject to strong
selection effects which need to be taken into account.
RKS Note: Discusses X-ray implications
Showing posts with label Planets. Show all posts
Showing posts with label Planets. Show all posts
Sunday, March 10, 2013
Sunday, February 17, 2013
A coordinated optical and X-ray spectroscopic campaign on HD179949: searching for planet-induced chromospheric and coronal activity. (arXiv:1301.7748v1 [astro-ph.SR])
A coordinated optical and X-ray spectroscopic campaign on HD179949: searching for planet-induced chromospheric and coronal activity. (arXiv:1301.7748v1 [astro-ph.SR]):
HD179949 is an F8V star, orbited by a close-in giant planet with a period of
~3 days. Previous studies suggested that the planet enhances the magnetic
activity of the parent star, producing a chromospheric hot spot which rotates
in phase with the planet orbit. However, this phenomenon is intermittent since
it was observed in several but not all seasons. A long-term monitoring of the
magnetic activity of HD179949 is required to study the amplitude and time
scales of star-planet interactions. In 2009 we performed a simultaneous optical
and X-ray spectroscopic campaign to monitor the magnetic activity of HD179949
during ~5 orbital periods and ~2 stellar rotations. We analyzed the CaII H&K
lines as a proxy for chromospheric activity, and we studied the X-ray emission
in search of flux modulations and to determine basic properties of the coronal
plasma. A detailed analysis of the flux in the cores of the CaII H&K lines and
a similar study of the X-ray photometry shows evidence of source variability,
including one flare. The analysis of the the time series of chromospheric data
indicates a modulation with a ~11 days period, compatible with the stellar
rotation period at high latitudes. Instead, the X-ray light curve suggests a
signal with a period of ~4 days, consistent with the presence of two active
regions on opposite hemispheres. The observed variability can be explained,
most likely, as due to rotational modulation and to intrinsic evolution of
chromospheric and coronal activity. There is no clear signature related to the
orbital motion of the planet, but the possibility that just a fraction of the
chromospheric and coronal variability is modulated with the orbital period of
the planet, or the stellar-planet beat period, cannot be excluded. We conclude
that any effect due to the presence of the planet is difficult to disentangle.
HD179949 is an F8V star, orbited by a close-in giant planet with a period of
~3 days. Previous studies suggested that the planet enhances the magnetic
activity of the parent star, producing a chromospheric hot spot which rotates
in phase with the planet orbit. However, this phenomenon is intermittent since
it was observed in several but not all seasons. A long-term monitoring of the
magnetic activity of HD179949 is required to study the amplitude and time
scales of star-planet interactions. In 2009 we performed a simultaneous optical
and X-ray spectroscopic campaign to monitor the magnetic activity of HD179949
during ~5 orbital periods and ~2 stellar rotations. We analyzed the CaII H&K
lines as a proxy for chromospheric activity, and we studied the X-ray emission
in search of flux modulations and to determine basic properties of the coronal
plasma. A detailed analysis of the flux in the cores of the CaII H&K lines and
a similar study of the X-ray photometry shows evidence of source variability,
including one flare. The analysis of the the time series of chromospheric data
indicates a modulation with a ~11 days period, compatible with the stellar
rotation period at high latitudes. Instead, the X-ray light curve suggests a
signal with a period of ~4 days, consistent with the presence of two active
regions on opposite hemispheres. The observed variability can be explained,
most likely, as due to rotational modulation and to intrinsic evolution of
chromospheric and coronal activity. There is no clear signature related to the
orbital motion of the planet, but the possibility that just a fraction of the
chromospheric and coronal variability is modulated with the orbital period of
the planet, or the stellar-planet beat period, cannot be excluded. We conclude
that any effect due to the presence of the planet is difficult to disentangle.
Monday, February 11, 2013
X-ray Irradiation of the LkCa 15 Protoplanetary Disk. (arXiv:1302.2111v1 [astro-ph.SR])
X-ray Irradiation of the LkCa 15 Protoplanetary Disk. (arXiv:1302.2111v1 [astro-ph.SR]):
LkCa 15 in the Taurus star-forming region has recently gained attention as
the first accreting T Tauri star likely to host a young protoplanet. High
spatial resolution infrared observations have detected the suspected
protoplanet within a dust-depleted inner gap of the LkCa 15 transition disk at
a distance of 15 AU from the star. If this object's status as a protoplanet is
confirmed, LkCa 15 will serve as a unique laboratory for constraining physical
conditions within a planet-forming disk. Previous models of the LkCa 15 disk
have accounted for disk heating by the stellar photosphere but have ignored the
potential importance of X-ray ionization and heating. We report here the
detection of LkCa 15 as a bright X-ray source with Chandra. The X-ray emission
is characterized by a cool heavily-absorbed plasma component at kT_cool ~0.3
keV and a harder component at kT_hot ~5 keV. We use the observed X-ray
properties to provide initial estimates of the X-ray ionization and heating
rates within the tenuous inner disk. These estimates and the observed X-ray
properties of LkCa 15 can be used as a starting point for developing more
realistic disk models of this benchmark system.
LkCa 15 in the Taurus star-forming region has recently gained attention as
the first accreting T Tauri star likely to host a young protoplanet. High
spatial resolution infrared observations have detected the suspected
protoplanet within a dust-depleted inner gap of the LkCa 15 transition disk at
a distance of 15 AU from the star. If this object's status as a protoplanet is
confirmed, LkCa 15 will serve as a unique laboratory for constraining physical
conditions within a planet-forming disk. Previous models of the LkCa 15 disk
have accounted for disk heating by the stellar photosphere but have ignored the
potential importance of X-ray ionization and heating. We report here the
detection of LkCa 15 as a bright X-ray source with Chandra. The X-ray emission
is characterized by a cool heavily-absorbed plasma component at kT_cool ~0.3
keV and a harder component at kT_hot ~5 keV. We use the observed X-ray
properties to provide initial estimates of the X-ray ionization and heating
rates within the tenuous inner disk. These estimates and the observed X-ray
properties of LkCa 15 can be used as a starting point for developing more
realistic disk models of this benchmark system.
Wednesday, January 23, 2013
The interplay between X-ray photoevaporation and planet formation. (arXiv:1301.3015v1 [astro-ph.SR])
The interplay between X-ray photoevaporation and planet formation. (arXiv:1301.3015v1 [astro-ph.SR]):
We assess the potential of planet formation instigating the early formation
of a photoevaporation driven gap, up to radii larger than typical for
photoevaporation alone. For our investigation we make use of hydrodynamics
models of photoevaporating discs with a giant planet embedded. We find that, by
reducing the mass accretion flow onto the star, discs that form giant planets
will be dispersed at earlier times than discs without planets by X-ray
photoevaporation. By clearing the portion of the disc inner of the planet
orbital radius, planet formation induced photoevaporation (PIPE) is able to
produce transition disc that for a given mass accretion rate have larger holes
when compared to standard X-ray photoevaporation. This constitutes a possible
route for the formation of the observed class of accreting transition discs
with large holes, which are otherwise difficult to explain by planet formation
or photoevaporation alone. Moreover, assuming that a planet is able to filter
dust completely, PIPE produces a transition disc with a large hole and may
provide a mechanism to quickly shut down accretion. This process appears to be
too slow however to explain the observed desert in the population of transition
disc with large holes and low mass accretion rates.
We assess the potential of planet formation instigating the early formation
of a photoevaporation driven gap, up to radii larger than typical for
photoevaporation alone. For our investigation we make use of hydrodynamics
models of photoevaporating discs with a giant planet embedded. We find that, by
reducing the mass accretion flow onto the star, discs that form giant planets
will be dispersed at earlier times than discs without planets by X-ray
photoevaporation. By clearing the portion of the disc inner of the planet
orbital radius, planet formation induced photoevaporation (PIPE) is able to
produce transition disc that for a given mass accretion rate have larger holes
when compared to standard X-ray photoevaporation. This constitutes a possible
route for the formation of the observed class of accreting transition discs
with large holes, which are otherwise difficult to explain by planet formation
or photoevaporation alone. Moreover, assuming that a planet is able to filter
dust completely, PIPE produces a transition disc with a large hole and may
provide a mechanism to quickly shut down accretion. This process appears to be
too slow however to explain the observed desert in the population of transition
disc with large holes and low mass accretion rates.
Wednesday, September 5, 2012
FUV and X-ray irradiated protoplanetary disks: a grid of models II - Gas diagnostic line emission. (arXiv:1209.0591v1 [astro-ph.SR])
FUV and X-ray irradiated protoplanetary disks: a grid of models II - Gas diagnostic line emission. (arXiv:1209.0591v1 [astro-ph.SR]):
Most of the mass in protoplanetary disks is in the form of gas. The study of
the gas and its diagnostics is of fundamental importance in order to achieve a
detailed description of the thermal and chemical structure of the disk. The
radiation from the central star (from optical to X-ray wavelengths) and viscous
accretion are the main source of energy and dominates the disk physics and
chemistry in its early stages. This is the environment in which the first
phases of planet formation will proceed. We investigate how stellar and disk
parameters impact the fine-structure cooling lines [NeII], [ArII], [OI], [CII]
and H2O rotational lines in the disk. These lines are potentially powerful
diagnostics of the disk structure and their modelling permits a thorough
interpretation of the observations carried out with instrumental facilities
such as Spitzer and Herschel. Following Aresu et al. (2011), we computed a grid
of 240 disk models, in which the X-ray luminosity, UV-excess luminosity,
minimum dust grain size, dust size distribution power law and surface density
distribution power law, are systematically varied. We solve self-consistently
for the disk vertical hydrostatic structure in every model and apply detailed
line radiative transfer to calculate line fluxes and profiles for a series of
well known mid- and far-infrared cooling lines. The [OI] 63 micron line flux
increases with increasing FUV luminosity when Lx < 1e30 erg/s, and with
increasing X-ray luminosity when LX > 1e30 erg/s. [CII] 157 micron is mainly
driven by FUV luminosity via C+ production, X-rays affect the line flux to a
lesser extent. [NeII] 12.8 micron correlates with X-rays; the line profile
emitted from the disk atmosphere shows a double-peaked component, caused by
emission in the static disk atmosphere, next to a high velocity double-peaked
component, caused by emission in the very inner rim. (abridged)
Most of the mass in protoplanetary disks is in the form of gas. The study of
the gas and its diagnostics is of fundamental importance in order to achieve a
detailed description of the thermal and chemical structure of the disk. The
radiation from the central star (from optical to X-ray wavelengths) and viscous
accretion are the main source of energy and dominates the disk physics and
chemistry in its early stages. This is the environment in which the first
phases of planet formation will proceed. We investigate how stellar and disk
parameters impact the fine-structure cooling lines [NeII], [ArII], [OI], [CII]
and H2O rotational lines in the disk. These lines are potentially powerful
diagnostics of the disk structure and their modelling permits a thorough
interpretation of the observations carried out with instrumental facilities
such as Spitzer and Herschel. Following Aresu et al. (2011), we computed a grid
of 240 disk models, in which the X-ray luminosity, UV-excess luminosity,
minimum dust grain size, dust size distribution power law and surface density
distribution power law, are systematically varied. We solve self-consistently
for the disk vertical hydrostatic structure in every model and apply detailed
line radiative transfer to calculate line fluxes and profiles for a series of
well known mid- and far-infrared cooling lines. The [OI] 63 micron line flux
increases with increasing FUV luminosity when Lx < 1e30 erg/s, and with
increasing X-ray luminosity when LX > 1e30 erg/s. [CII] 157 micron is mainly
driven by FUV luminosity via C+ production, X-rays affect the line flux to a
lesser extent. [NeII] 12.8 micron correlates with X-rays; the line profile
emitted from the disk atmosphere shows a double-peaked component, caused by
emission in the static disk atmosphere, next to a high velocity double-peaked
component, caused by emission in the very inner rim. (abridged)
Monday, August 27, 2012
Background X-ray Radiation Fields Produced by Young Embedded Star Clusters
Background X-ray Radiation Fields Produced by Young Embedded Star Clusters
Most star formation in our galaxy occurs within embedded clusters, and these background environments can affect the star and planet formation processes occurring within them. In turn, young stellar members can shape the background environment and thereby provide a feedback mechanism. This work explores one aspect of stellar feedback by quantifying the background X-ray radiation fields produced by young stellar objects. Specifically, the distributions of X-ray luminosities and X-ray fluxes produced by cluster environments are constructed as a function of cluster membership size $N$. Composite flux distributions, for given distributions of cluster sizes $N$, are also constructed. The resulting distributions are wide and the X-ray radiation fields are moderately intense, with the expected flux levels exceeding the cosmic and galactic X-ray backgrounds by factors of $\sim10-1000$ (for energies 0.2 -- 15 keV). For circumstellar disks that are geometrically thin and optically thick, the X-ray flux from the background cluster dominates that provided by a typical central star in the outer disk where $r \ga 9 - 14$ AU. In addition, the expectation value of the ionization rate provided by the cluster X-ray background is $\zeta_X\sim8\times10^{-17}$ s$^{-1}$, about 4 -- 8 times larger than the canonical value of the ionization rate from cosmic rays. These elevated flux levels in clusters indicate that X-rays can affect ionization, chemistry, and heating in circumstellar disks and in the material between young stellar objects.
Most star formation in our galaxy occurs within embedded clusters, and these background environments can affect the star and planet formation processes occurring within them. In turn, young stellar members can shape the background environment and thereby provide a feedback mechanism. This work explores one aspect of stellar feedback by quantifying the background X-ray radiation fields produced by young stellar objects. Specifically, the distributions of X-ray luminosities and X-ray fluxes produced by cluster environments are constructed as a function of cluster membership size $N$. Composite flux distributions, for given distributions of cluster sizes $N$, are also constructed. The resulting distributions are wide and the X-ray radiation fields are moderately intense, with the expected flux levels exceeding the cosmic and galactic X-ray backgrounds by factors of $\sim10-1000$ (for energies 0.2 -- 15 keV). For circumstellar disks that are geometrically thin and optically thick, the X-ray flux from the background cluster dominates that provided by a typical central star in the outer disk where $r \ga 9 - 14$ AU. In addition, the expectation value of the ionization rate provided by the cluster X-ray background is $\zeta_X\sim8\times10^{-17}$ s$^{-1}$, about 4 -- 8 times larger than the canonical value of the ionization rate from cosmic rays. These elevated flux levels in clusters indicate that X-rays can affect ionization, chemistry, and heating in circumstellar disks and in the material between young stellar objects.
FUV and X-ray irradiated protoplanetary disks: a grid of models I. The disk structure. (arXiv:1208.4959v1 [astro-ph.SR])
FUV and X-ray irradiated protoplanetary disks: a grid of models I. The disk structure. (arXiv:1208.4959v1 [astro-ph.SR]):
Context. Planets are thought to eventually form from the mostly gaseous (~99%
of the mass) disks around young stars. The density structure and chemical
composition of protoplanetary disks are affected by the incident radiation
field at optical, FUV, and X-ray wavelengths, as well as by the dust
properties.
Aims. The effect of FUV and X-rays on the disk structure and the gas chemical
composition are investigated. This work forms the basis of a second paper,
which discusses the impact on diagnostic lines of, e.g., C+, O, H2O, and Ne+
observed with facilities such as Spitzer and Herschel.
Methods. A grid of 240 models is computed in which the X-ray and FUV
luminosity, minimum grain size, dust size distribution, and surface density
distribution are varied in a systematic way. The hydrostatic structure and the
thermo-chemical structure are calculated using ProDiMo.
Results. The abundance structure of neutral oxygen is stable to changes in
the X-ray and FUV luminosity, and the emission lines will thus be useful
tracers of the disk mass and temperature. The C+ abundance distribution is
sensitive to both X-rays and FUV. The radial column density profile shows two
peaks, one at the inner rim and a second one at a radius r=5-10 AU. Ne+ and
other heavy elements have a very strong response to X-rays, and the column
density in the inner disk increases by two orders of magnitude from the lowest
(LX = 1e29 erg/s) to the highest considered X-ray flux (LX = 1e32 erg/s). FUV
confines the Ne+ ionized region to areas closer to the star at low X-ray
luminosities (LX = 1e29 erg/s). H2O abundances are enhanced by X-rays due to
higher temperatures in the inner disk and higher ionization fractions in the
outer disk. The line fluxes and profiles are affected by the effects on these
species, thus providing diagnostic value in the study of FUV and X-ray
irradiated disks around T Tauri stars. (abridged)
Context. Planets are thought to eventually form from the mostly gaseous (~99%
of the mass) disks around young stars. The density structure and chemical
composition of protoplanetary disks are affected by the incident radiation
field at optical, FUV, and X-ray wavelengths, as well as by the dust
properties.
Aims. The effect of FUV and X-rays on the disk structure and the gas chemical
composition are investigated. This work forms the basis of a second paper,
which discusses the impact on diagnostic lines of, e.g., C+, O, H2O, and Ne+
observed with facilities such as Spitzer and Herschel.
Methods. A grid of 240 models is computed in which the X-ray and FUV
luminosity, minimum grain size, dust size distribution, and surface density
distribution are varied in a systematic way. The hydrostatic structure and the
thermo-chemical structure are calculated using ProDiMo.
Results. The abundance structure of neutral oxygen is stable to changes in
the X-ray and FUV luminosity, and the emission lines will thus be useful
tracers of the disk mass and temperature. The C+ abundance distribution is
sensitive to both X-rays and FUV. The radial column density profile shows two
peaks, one at the inner rim and a second one at a radius r=5-10 AU. Ne+ and
other heavy elements have a very strong response to X-rays, and the column
density in the inner disk increases by two orders of magnitude from the lowest
(LX = 1e29 erg/s) to the highest considered X-ray flux (LX = 1e32 erg/s). FUV
confines the Ne+ ionized region to areas closer to the star at low X-ray
luminosities (LX = 1e29 erg/s). H2O abundances are enhanced by X-rays due to
higher temperatures in the inner disk and higher ionization fractions in the
outer disk. The line fluxes and profiles are affected by the effects on these
species, thus providing diagnostic value in the study of FUV and X-ray
irradiated disks around T Tauri stars. (abridged)
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