43 research outputs found
Ca II H and K Chromospheric Emission Lines in Late K and M Dwarfs
We have measured the profiles of the Ca II H and K chromospheric emission
lines in 147 main sequence stars of spectral type M5-K7 (0.30-0.55 solar
masses) using multiple high resolution spectra obtained during six years with
the HIRES spectrometer on the Keck 1 telescope. Remarkably, the average FWHM,
equivalent widths, and line luminosities of Ca II H and K increase by a factor
of 3 with increasing stellar mass over this small range of stellar masses. We
fit the H and K lines with a double Gaussian model to represent both the
chromospheric emission and the non-LTE central absorption. Most of the sample
stars display a central absorption that is typically redshifted by ~0.1 km/s
relative to the emission, but the nature of this velocity gradient remains
unknown. The FWHM of the H and K lines increase with stellar luminosity,
reminiscent of the Wilson-Bappu effect in FGK-type stars. Both the equivalent
widths and FWHM exhibit modest temporal variability in individual stars. At a
given value of M_v, stars exhibit a spread in both the equivalent width and
FWHM of Ca II H and K, due both to a spread in fundamental stellar parameters
including rotation rate, age, and possibly metallicity, and to the spread in
stellar mass at a given M_v. The K line is consistently wider than the H line,
as expected, and its central absorption is more redshifted, indicating that the
H and K lines form at slightly different heights in the chromosphere where the
velocities are slightly different. The equivalent width of H-alpha correlates
with Ca II H and K only for stars having Ca II equivalent widths above ~2
angstroms, suggesting the existence of a magnetic threshold above which the
lower and upper chromospheres become thermally coupled.Comment: 40 pages including 12 figures and 17 pages of tables, accepted for
publication in PAS
Recommended from our members
CCN measurements at the Princess Elisabeth Antarctica research station during three austral summers
For three austral summer seasons (2013-2016, each from December to February) aerosol particles arriving at the Belgian Antarctic research station Princess Elisabeth (PE) in Dronning Maud Land in East Antarctica were characterized. This included number concentrations of total aerosol particles (N CN ) and cloud condensation nuclei (N CCN ), the particle number size distribution (PNSD), the aerosol particle hygroscopicity, and the influence of the air mass origin on N CN and N CCN . In general N CN was found to range from 40 to 6700cm -3 , with a median of 333cm -3 , while N CCN was found to cover a range between less than 10 and 1300cm-3 for supersaturations (SSs) between 0.1% and 0.7%. It is shown that the aerosol is dominated by the Aitken mode, being characterized by a significant amount of small, and therefore likely secondarily formed, aerosol particles, with 94% and 36% of the aerosol particles smaller than 90 and â35nm, respectively. Measurements of the basic meteorological parameters as well as the history of the air masses arriving at the measurement station indicate that the station is influenced by both marine air masses originating from the Southern Ocean and coastal areas around Antarctica (marine events - MEs) and continental air masses (continental events - CEs). CEs, which were defined as instances when the air masses spent at least 90% of the time over the Antarctic continent during the last 10 days prior to arrival at the measurements station, occurred during 61% of the time during which measurements were done. CEs came along with rather constant N CN and N CCN values, which we denote as Antarctic continental background concentrations. MEs, however, cause large fluctuations in N CN and N CCN , with low concentrations likely caused by scavenging due to precipitation and high concentrations likely originating from new particle formation (NPF) based on marine precursors. The application of HYSPLIT back trajectories in form of the potential source contribution function (PSCF) analysis indicate that the region of the Southern Ocean is a potential source of Aitken mode particles. On the basis of PNSDs, together with N CCN measured at an SS of 0.1%, median values for the critical diameter for cloud droplet activation and the aerosol particle hygroscopicity parameter ° were determined to be 110nm and 1, respectively. For particles larger than Äâ°110nm the Southern Ocean together with parts of the Antarctic ice shelf regions were found to be potential source regions. While the former may contribute sea spray particles directly, the contribution of the latter may be due to the emission of sea salt aerosol particles, released from snow particles from surface snow layers, e.g., during periods of high wind speed, leading to drifting or blowing snow. The region of the Antarctic inland plateau, however, was not found to feature a significant source region for aerosol particles in general or page276 for cloud condensation nuclei measured at the PE station in the austral summer
A photometric and astrometric investigation of the brown dwarfs in Blanco 1
We present the results of a photometric and astrometric study of the low mass
stellar and substellar population of the young open cluster Blanco 1. We have
exploited J band data, obtained recently with the Wide Field Camera (WFCAM) on
the United Kingdom InfraRed Telescope (UKIRT), and 10 year old I and z band
optical imaging from CFH12k and Canada France Hawaii Telescope (CFHT), to
identify 44 candidate low mass stellar and substellar members, in an area of 2
sq. degrees, on the basis of their colours and proper motions. This sample
includes five sources which are newly discovered. We also confirm the lowest
mass candidate member of Blanco 1 unearthed so far (29MJup). We determine the
cluster mass function to have a slope of alpha=+0.93, assuming it to have a
power law form. This is high, but nearly consistent with previous studies of
the cluster (to within the errors), and also that of its much better studied
northern hemisphere analogue, the Pleiades.Comment: 8 Pages, 5 Figures, 2 Tables and 1 Appendix. Accepted for publication
in MNRA
The Implications of M Dwarf Flares on the Detection and Characterization of Exoplanets at Infrared Wavelengths
We present the results of an observational campaign which obtained high time
cadence, high precision, simultaneous optical and IR photometric observations
of three M dwarf flare stars for 47 hours. The campaign was designed to
characterize the behavior of energetic flare events, which routinely occur on M
dwarfs, at IR wavelengths to milli-magnitude precision, and quantify to what
extent such events might influence current and future efforts to detect and
characterize extrasolar planets surrounding these stars. We detected and
characterized four highly energetic optical flares having U-band total energies
of ~7.8x10^30 to ~1.3x10^32 ergs, and found no corresponding response in the J,
H, or Ks bandpasses at the precision of our data. For active dM3e stars, we
find that a ~1.3x10^32 erg U-band flare (delta Umax ~1.5 mag) will induce <8.3
(J), <8.5 (H), and <11.7 (Ks) milli-mags of a response. A flare of this energy
or greater should occur less than once per 18 hours. For active dM4.5e stars,
we find that a ~5.1x10^31 erg U-band flare (delta Umax ~1.6 mag) will induce
<7.8 (J), <8.8 (H), and <5.1 (Ks) milli-mags of a response. A flare of this
energy or greater should occur less than once per 10 hours. No evidence of
stellar variability not associated with discrete flare events was observed at
the level of ~3.9 milli-mags over 1 hour time-scales and at the level of ~5.6
milli-mags over 7.5 hour time-scales. We therefore demonstrate that most M
dwarf stellar activity and flares will not influence IR detection and
characterization studies of M dwarf exoplanets above the level of ~5-11
milli-mags, depending on the filter and spectral type. We speculate that the
most energetic megaflares on M dwarfs, which occur at rates of once per month,
are likely to be easily detected in IR observations with sensitivity of tens of
milli-mags.Comment: Accepted in Astronomical Journal, 17 pages, 6 figure
Debris Disks of Members of the Blanco 1 Open Cluster
We have used the Spitzer Space Telescope to obtain Multiband Imaging
Photometer for Spitzer (MIPS) 24 um photometry for 37 members of the ~100 Myr
old open cluster Blanco 1. For the brightest 25 of these stars (where we have
3sigma uncertainties less than 15%), we find significant mid-IR excesses for
eight stars, corresponding to a debris disk detection frequency of about 32%.
The stars with excesses include two A stars, four F dwarfs and two G dwarfs.
The most significant linkage between 24 um excess and any other stellar
property for our Blanco 1 sample of stars is with binarity. Blanco 1 members
that are photometric binaries show few or no detected 24 um excesses whereas a
quarter of the apparently single Blanco 1 members do have excesses. We have
examined the MIPS data for two other clusters of similar age to Blanco 1 -- NGC
2547 and the Pleiades. The AFGK photometric binary star members of both of
these clusters also show a much lower frequency of 24 um excesses compared to
stars that lie near the single-star main sequence. We provide a new
determination of the relation between V-Ks color and Ks-[24] color for main
sequence photospheres based on Hyades members observed with MIPS. As a result
of our analysis of the Hyades data, we identify three low mass Hyades members
as candidates for having debris disks near the MIPS detection limit.Comment: Accepted to Ap
CCN measurements at the Princess Elisabeth Antarctica research station during three austral summers
For three austral summer seasons (2013â2016, each from December to February)
aerosol particles arriving at the Belgian Antarctic research station Princess
Elisabeth (PE) in Dronning Maud Land in East Antarctica were characterized.
This included number concentrations of total aerosol
particles (NCN) and cloud condensation nuclei (NCCN),
the particle number size distribution (PNSD), the aerosol particle
hygroscopicity, and the influence of the air mass origin on NCN
and NCCN. In general NCN was found to range from
40Â to 6700 cmâ3, with a median of 333 cmâ3, while
NCCNÂ was found to cover a range between less than 10Â and
1300 cmâ3 for supersaturations (SSs) between 0.1 % and 0.7 %. It is
shown that the aerosol is dominated by the Aitken mode, being characterized by a
significant amount of small, and therefore likely secondarily formed, aerosol
particles, with 94 % and 36 % of the aerosol particles smaller than
90Â and â35 nm, respectively. Measurements of the basic
meteorological parameters as well as the history of the air masses arriving
at the measurement station indicate that the station is influenced by both
marine air masses originating from the Southern Ocean and coastal areas
around Antarctica (marine events â MEs) and continental air masses
(continental events â CEs). CEs, which were defined as instances when the air
masses spent at least 90 % of the time over the
Antarctic continent during the last 10Â days prior to arrival at the measurements station, occurred during 61 % of the time during which
measurements were done. CEs came along with rather constant NCN
and NCCNÂ values, which we denote as Antarctic continental
background concentrations. MEs, however, cause large fluctuations
in NCN and NCCN, with low concentrations likely caused
by scavenging due to precipitation and high concentrations likely originating
from new particle formation (NPF) based on marine precursors. The application
of HYSPLIT back trajectories in form of the potential source contribution
function (PSCF) analysis indicate that the region of the Southern Ocean is a
potential source of Aitken mode particles. On the basis of PNSDs, together
with NCCNÂ measured at an SS of 0.1 %, median values for the
critical diameter for cloud droplet activation and the aerosol particle
hygroscopicity parameter Îș were determined to be 110 nm and 1,
respectively. For particles larger than â110 nm the Southern Ocean
together with parts of the Antarctic ice shelf regions were found to be
potential source regions. While the former may contribute sea spray particles
directly, the contribution of the latter may be due to the emission of sea salt
aerosol particles, released from snow particles from surface snow layers,
e.g., during periods of high wind speed, leading to drifting or blowing snow.
The region of the Antarctic inland plateau, however, was not found to feature a
significant source region for aerosol particles in general or for cloud condensation nuclei measured at the PEÂ station in
the austral summer.</p
GJ 900: A new hierarchical system with low-mass components
Speckle interferometric observations made with the 6 m telescope of the
Special Astrophysical Observatory of the Russian Academy of Sciences in 2000
revealed the triple nature of the nearby ( mas)
low-mass young ( Myr) star GJ 900. The configuration of the triple
system allowed it to be dynamically unstable. Differential photometry performed
from 2000 through 2004 yielded - and -band absolute magnitudes and
spectral types for the components to be =6.660.08,
=9.150.11, =10.080.26, =4.840.08,
=6.760.20, =7.390.31, K5--K7,
M3--M4, M5--M6. The ``mass--luminosity''
relation is used to estimate the individual masses of the components:
,
,
. From the observations of the
components relative motion in the period 2000--2006, we conclude that GJ 900 is
a hierarchical triple star with the possible orbital periods
P80 yrs and P20 yrs. An analysis of the 2MASS
images of the region around GJ 900 leads us to suggest that the system can
include other very-low-mass components.Comment: 7 pages, 5 figure
Measured Mass Loss Rates of Solar-like Stars as a Function of Age and Activity
Collisions between the winds of solar-like stars and the local ISM result in
a population of hot hydrogen gas surrounding these stars. Absorption from this
hot H I can be detected in high resolution Lyman-alpha spectra of these stars
from the Hubble Space Telescope. The amount of absorption can be used as a
diagnostic for the stellar mass loss rate. We present new mass loss rate
measurements derived in this fashion for four stars (Epsilon Eri, 61 Cyg A, 36
Oph AB, and 40 Eri A). Combining these measurements with others, we study how
mass loss varies with stellar activity. We find that for the solar-like GK
dwarfs, the mass loss per unit surface area is correlated with X-ray surface
flux. Fitting a power law to this relation yields Mdot ~ Fx^(1.15+/-0.20). The
active M dwarf Proxima Cen and the very active RS CVn system Lambda And appear
to be inconsistent with this relation. Since activity is known to decrease with
age, the above power law relation for solar-like stars suggests that mass loss
decreases with time. We infer a power law relation of Mdot ~ t^(-2.00+/-0.52).
This suggests that the solar wind may have been as much as 1000 times more
massive in the distant past, which may have had important ramifications for the
history of planetary atmospheres in our solar system, that of Mars in
particular.Comment: 34 pages, 9 figures; AASTEX v5.0 plus EPSF extensions in mkfig.sty;
accepted by Ap
Time scales of Li evolution: a homogeneous analysis of open clusters from ZAMS to late-MS
We have performed a new and homogeneous analysis of all the Li data available
in the literature for main sequence stars (spectral-types from late F to K) in
open clusters. In the present paper we focus on a detailed investigation of MS
Li depletion and its time scales for stars in the 6350-5500 K effective
temperature range. For the first time, we were able to constrain the age at
which non-standard mixing processes, driving MS Li depletion, appear. We have
also shown that MS Li depletion is not a continuous process and cannot be
simply described by a t^(-alpha) law. We confirm that depletion becomes
ineffective beyond an age of 1-2 Gyr for the majority of the stars, leading to
a Li plateau at old ages. We compared the empirical scenario of Li as a
function of age with the predictions of three non-standard models. We found
that models including only gravity waves as main mixing process are not able to
fit the Li vs. age pattern and thus this kind of mixing can be excluded as the
predominant mechanism responsible for Li depletion. On the other hand, models
including slow mixing induced by rotation and angular momentum loss, and in
particular those including also diffusive processes not related to rotation,
can explain to some extent the empirical evidence. However, none of the
currently proposed models can fit the plateau at old ages.Comment: 20 pages, 10 figures A&A accepte
The detection of M-dwarf UV flare events in the GALEX data archives
We present the preliminary results from implementing a new software tool that
enables inspection of time-tagged photon data for the astronomical sources
contained within individual GALEX ultraviolet images of the sky. We have
inspected the photon data contained within 1802 GALEX images to reveal rapid,
short-term (<500 sec) UV source variability in the form of stellar flares. The
mean associated change in NUV magnitude due to this flaring activity is
2.7+/-0.3 mag. A list of 49 new UV variable-star candidates is presented,
together with their associated Sloan Digital Sky Survey (SDSS) photometric
magnitudes. From these data we can associate the main source of these UV flare
events with magnetic activity on M-dwarf stars. Photometric parallaxes have
been determined for 32 of these sources, placing them at distances ranging from
approximately 25 to 1000pc. The average UV flare energy for these flare events
is 2.5E30 ergs, which is of a similar energy to that of U-band, X-ray and EUV
flares observed on many local M-dwarf stars. We have found that stars of
classes M0 to M5 flare with energies spanning a far larger range and with an
energy approximately 5 times greater than those of later (M6 to M8) spectral
type.Comment: Accepted for the Astrophysical Journal Supplement, GALEX Special
Issu