88 research outputs found
Detection of DCO+ in a circumstellar disk
We report the first detection of DCO+ in a circumstellar disk. The DCO+ J=5-4 line at 360.169 GHz is observed with the 15m James Clerk Maxwell Telescope in the disk around the pre-main sequence star TW Hya. Together with measurements of the HCO+ and H13CO+ J=4-3 lines, this allows an accurate determination of the DCO+/HCO+ ratio in this disk. The inferred value of 0.035+-0.015 is close to that found in cold pre-stellar cores and is somewhat higher than that measured in the envelope around the low-mass protostar IRAS 16293 -2422. It is also close to the DCN/HCN ratio obtained for pristine cometary material in the jet of comet Hale-Bopp. The observed DCO+/HCO+ ratio for TW Hya is consistent with theoretical models of disks which consider gas-phase fractionation processes within a realistic 2-D temperature distribution and which include the effects of freeze-out onto grains
H_2 and CO Emission from Disks around T Tauri and Herbig Ae Pre-Main-Sequence Stars and from Debris Disks around Young Stars: Warm and Cold Circumstellar Gas
We present ISO Short-Wavelength Spectrometer observations of H_2 pure-rotational line emission from the disks around low- and intermediate-mass pre-main-sequence stars as well as from young stars thought to be surrounded by debris disks. The pre-main-sequence sources have been selected to be isolated from molecular clouds and to have circumstellar disks revealed by millimeter interferometry. We detect "warm" (T ≈100-200 K) H_2 gas around many sources, including tentatively the debris-disk objects. The mass of this warm gas ranges from ~ 10^(-4) M_☉ up to 8 x 10^(-3) and can constitute a nonnegligible fraction of the total disk mass. Complementary single-dish ^(12)CO 3-2/^(13)CO 3-2, and ^(12)CO 6-5 observations have been obtained as well. These transitions probe cooler gas at T ≈ 20-80 K. Most objects show a double-peaked CO emission profile characteristic of a disk in Keplerian rotation, consistent with interferometer data on the lower J lines. The ratios of the ^(12)CO 3-2/^(13)CO 3-2 integrated fluxes indicate that ^(12)CO 3-2 is optically thick but that ^(13)CO 3-2 is optically thin or at most moderately thick. The ^(13)CO 3-2 lines have been used to estimate the cold gas mass. If a H_2/CO conversion factor of 1 x 10^(-4) is adopted, the derived cold gas masses are factors of 10-200 lower than those deduced from 1.3 millimeter dust emission assuming a gas/dust ratio of 100, in accordance with previous studies. These findings confirm that CO is not a good tracer of the total gas content in disks since it can be photodissociated in the outer layers and frozen onto grains in the cold dense part of disks, but that it is a robust tracer of the disk velocity field. In contrast, H_2 can shield itself from photodissociation even in low-mass "optically thin" debris disks and can therefore survive longer. The warm gas is typically 1%-10% of the total mass deduced from millimeter continuum emission, but it can increase up to 100% or more for the debris-disk objects. Thus, residual molecular gas may persist into the debris-disk phase. No significant evolution in the H_2 CO, or dust masses is found for stars with ages in the range of 10^6-10^7 yr, although a decrease is found for the older debris-disk star β Pictoris. The large amount of warm gas derived from H_2 raises the question of the heating mechanism(s). Radiation from the central star as well as the general interstellar radiation field heat an extended surface layer of the disk, but existing models fail to explain the amount of warm gas quantitatively. The existence of a gap in the disk can increase the area of material influenced by radiation. Prospects for future observations with ground- and space-borne observations are discussed
The gas temperature in flaring disks around pre-main sequence stars
A model is presented which calculates the gas temperature and chemistry in
the surface layers of flaring circumstellar disks using a code developed for
photon-dominated regions. Special attention is given to the influence of dust
settling. It is found that the gas temperature exceeds the dust temperature by
up to several hundreds of Kelvins in the part of the disk that is optically
thin to ultraviolet radiation, indicating that the common assumption that
Tgas=Tdust is not valid throughout the disk. In the optically thick part, gas
and dust are strongly coupled and the gas temperature equals the dust
temperature. Dust settling has little effect on the chemistry in the disk, but
increases the amount of hot gas deeper in the disk. The effects of the higher
gas temperature on several emission lines arising in the surface layer are
examined. The higher gas temperatures increase the intensities of molecular and
fine-structure lines by up to an order of magnitude, and can also have an
important effect on the line shapes.Comment: 14 pages, 10 figures, accepted for publication in A&
Warm Molecular Layers in Protoplanetary Disks
We have investigated molecular distributions in protoplanetary disks,
adopting a disk model with a temperature gradient in the vertical direction.
The model produces sufficiently high abundances of gaseous CO and HCO+ to
account for line observations of T Tauri stars using a sticking probability of
unity and without assuming any non-thermal desorption. In regions of radius R >
10 AU, with which we are concerned, the temperature increases with increasing
height from the midplane. In a warm intermediate layer, there are significant
amounts of gaseous molecules owing to thermal desorption and efficient
shielding of ultraviolet radiation by the flared disk. The column densities of
HCN, CN, CS, H2CO, HNC and HCO+ obtained from our model are in good agreement
with the observations of DM Tau, but are smaller than those of LkCa15.
Molecular line profiles from our disk models are calculated using a
2-dimensional non-local-thermal-equilibrium (NLTE) molecular-line radiative
transfer code for a direct comparison with observations. Deuterated species are
included in our chemical model. The molecular D/H ratios in the model are in
reasonable agreement with those observed in protoplanetary disks.Comment: 11 pages, Latex (aa.cls), to be published in Astronomy and
Astrophysic
Warm molecular layers in protoplanetary disks
Wetensch. publicatieFaculteit der Wiskunde en Natuurwetenschappe
H_2 Emission From Disks Around Herbig Ae and T Tauri Stars
We present the initial results of a deep ISO-SWS survey
for the low J pure rotational emission lines of H2
toward a number of Herbig Ae and T Tauri stars.
The objects are selected to be as isolated as possible
from molecular clouds, with a spectral energy distribution
characteristic of a circumstellar disk. For most
of them the presence of a disk has been established
directly by millimeter interferometry. The S (1) line is
detected in most sources with a peak flux of 0.3-1 Jy.
The S(0) line is definitely seen in 2 objects: GG Tau
and HD 163296. The observations suggest the presence
of "warm" gas at T_(kin) ≈ 100 K with a mass of a
few % of the total gas+ dust mass, derived assuming
a gas-to-dust ratio of 100:1. The S(1) peak flux does
not show a strong correlation with spectral type of
the central star or continuum flux at 1.3 millimeter.
Possible origins for the warm gas seen in H_2 are discussed,
and comparisons with model calculations are
made
Warm molecular gas and kinematics in the disc around HD 100546
The disc around the Herbig Ae/Be star HD 100546 is one of the most
extensively studied discs in the southern sky. Although there is a wealth of
information about its dust content and composition, not much is known about its
gas and large scale kinematics. We detect and study the molecular gas in the
disc at spatial resolution from 7.7" to 18.9" using the APEX telescope. The
lines 12CO J=7-6, J=6-5, J=3-2, 13CO J=3-2 and [C I] 3P2-3P1 are observed,
diagnostic of disc temperature, size, chemistry, and kinematics. We use
parametric disc models that reproduce the low-J 12CO emission from Herbig~Ae
stars and vary the basic disc parameters - temperature, mass and size. Using
the molecular excitation and radiative transfer code RATRAN we fit the observed
spectral line profiles. Our observations are consistent with more than 0.001
Msun of molecular gas in a disc of approximately 400 AU radius in Keplerian
rotation around a 2.5 Msun star, seen at an inclination of 50 degrees. The
detected 12CO lines are dominated by gas at 30-70~K. The non-detection of the
[C I] line indicates excess ultraviolet emission above that of a B9 type model
stellar atmosphere. Asymmetry in the 12CO line emission suggests that one side
of the outer disc is colder by 10-20~K than the other, possibly due to a shadow
by a warped geometry of the inner disc. Pointing offsets, foreground cloud
absorption and asymmetry in the disc extent are excluded scenarios. Efficient
heating of the outer disc ensures that low- and high-J 12CO lines are dominated
by the outermost disc regions, indicating a 400 AU radius. The 12CO J=6--5 line
arises from a disc layer higher above disc midplane, and warmer by 15-20~K than
the layer emitting the J=3--2 line. The existing models of discs around Herbig
Ae stars, assuming a B9.5 type model stellar atmosphere overproduce the [CI]
3P2--3P1 line intensity from HD 100546 by an order of magnitude.Comment: 9pages, 3figures, Accepted for publication in Astronomy &
Astrophysic
Genicular artery embolization as a novel treatment for mild to moderate knee osteoarthritis:protocol design of a randomized sham-controlled clinical trial
INTRODUCTION: Knee osteoarthritis is a common disease with pain as the most prevalent symptom. Previous cohort studies have shown genicular artery embolization to reduce pain symptoms in patients with mild to moderate knee osteoarthritis. Patients resistant to conservative therapy but not eligible yet for surgical treatment due to young age or comorbidities may profit from an effective and sustained pain reduction treatment. This study is a randomized sham-controlled trial to evaluate the efficacy of genicular artery embolization in patients with knee osteoarthritis. METHODS AND ANALYSIS: Fifty-eight patients with mild-to-moderate knee osteoarthritis will be recruited and randomly allocated to the treatment or control group in a 1:1 ratio. Participants in the treatment group will undergo genicular artery embolization. Patients in the control group will undergo sham treatment. Outcome measurements will be assessed at baseline and after 1, 4, 8, and 12 months with questionnaires, pressure pain threshold testing, and MR imaging. The MR imaging protocol is designed to (semi)quantitatively assess osteoarthritis in the knee joint. The primary outcome is the change from baseline of the Knee injury and Osteoarthritis Outcome Score (KOOS) pain subscale after 4 months. Secondary outcomes include change in osteoarthritis-related questionnaires, pressure pain threshold, and OA-related MRI features, particularly synovitis and bone marrow lesions. ETHICS AND DISSEMINATION: This trial will determine the efficacy of genicular artery embolization compared to a sham treatment. This is of importance to assess before proceeding to larger-scale efficiency studies and, ultimately, implementing this treatment into day to day clinical practice. TRIAL REGISTRATION: ClinicalTrials.gov NCT03884049. Registered on 21 March 201
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