56 research outputs found
Measurements of Mass Accretion Rates in Herbig Ae/Be Stars
Herbig Ae/Be stars(HAeBes) are young stellar objects of spectral class F2 through B0, with the central star often surrounded by a circumstellar disk of gas and dust. They are the higher mass analogs to T Tauri stars. The interaction between the star and the disk is not well understood, nor is the disk structure. The central star will often accrete mass from the disk, and the mass accretion rate is an important parameter for modeling the disk structure and evolution. The methods for measuring mass accretion rates of T Tauri stars are generally not applicable to HAeBe stars. As such, reliable measurements of mass accretion rates for HAeBes are rare. Garrison(1978) saw that the Balmer Discontinuity of HAeBes was veiled, and attributed this veiling to accretion luminosity. Building on Garrison(1978) and the work of Muzerolle et al. (2004), I determine the mass accretion rates and accretion luminosities of a large sample of HAeBe stars by measuring the veiling of the Balmer Discontinuity due to the accretion luminosity. Muzerolle et al. (1998) established a strong correlation between the accretion luminosity of T Tauri stars and the luminosity of Br gamma, and this correlation seems to extend to the evolutionary precursors to HAeBes, intermediate T Tauri stars, as well Calvet et al. (2004). I test this correlation for HAeBes and discover that it is valid for HAe stars but not for HBe stars. From examining the HAeBes of my sample from spectral range A3 to B7, there does not seem to be a particular spectral type at which the correlation fails. A few of the late HBe stars are consistent with the correlation, but most of the HBe stars have Br gamma luminosities much larger than what one would expect from the correlation. This suggests that there might be a significant stellar wind component to the Br gamma luminosity for many of the HBe stars. T Tauri stars accrete mass from their disks magnetospherically, in which the strong stellar field of the star truncates the disk at some distance from the star and the disk material than falls to the stellar surface along the magnetic field lines. HAeBe stars are not expected to have strong stellar magnetic fields, and observations have failed to find any such fields for most HAeBes (Alecian 2007). However, circumstantial evidence suggests that some HAeBe stars are accreting magnetospherically (Muzerolle et al. 2004, Brittain et al. 2009). Since the correlation between accretion luminosity and Br luminosity is valid for both T Tauri stars and HAe stars, this suggests that the same basic accretion process is occuring for both
Measuring the Stellar Accretion Rates of Herbig Ae/Be Stars
The accretion rate of young stars is a fundamental characteristic of these systems.While accretion onto T Tauri stars has been studied extensively, little work has been done on measuring the accretion rate of their intermediate-mass analogs, the Herbig Ae/Be stars. Measuring the stellar accretion rate of Herbig Ae/Bes is not straightforward both because of the dearth of metal absorption lines available for veiling measurements and the intrinsic brightness of Herbig Ae/Be stars at ultraviolet wavelengths where the brightness of the accretion shock peaks. Alternative approaches to measuring the accretion rate of young stars by measuring the luminosity of proxies such as the Br γ emission line have not been calibrated. A promising approach is the measurement of the veiling of the Balmer discontinuity. We present measurements of this veiling as well as the luminosity of Br γ . We show that the relationship between the luminosity of Br γ and the stellar accretion rate for classical T Tauri stars is consistent with Herbig Ae stars but not Herbig Be stars. We discuss the implications of this finding for understanding the interaction of the star and disk for Herbig Ae/Be stars
Chronicles of Oklahoma
Necrology section from Volume 44, Number 3, Fall 1966. It includes a document honoring the life of Marion Donehew, the first student to enroll at the University of Oklahoma and a teacher and reporter on the frontier
HD 144432: a young triple system
We present new imaging and spectroscopic data of the young Herbig star HD
144432 A, which was known to be a binary star with a separation of 1.47 arcsec.
High-resolution NIR imaging data obtained with NACO at the VLT reveal that HD
144432 B itself is a close binary pair with a separation of 0.1 arcsec.
High-resolution optical spectra, acquired with FEROS at the 2.2m MPG/ESO
telescope in La Silla, of the primary star and its co-moving companions were
used to determine their main stellar parameters such as effective temperature,
surface gravity, radial velocity, and projected rotational velocity by fitting
synthetic spectra to the observed stellar spectra. The two companions, HD
144432 B and HD 144432 C, are identified as low-mass T Tauri stars of spectral
type K7V and M1V, respectively. From the position in the HRD the triple system
appears to be co-eval with a system age of 6+/-3 Myr.Comment: Accepted for publication in Astronomy & Astrophysics, 4 pages, 4
figure
A Disk-Wind Model for the Near-Infrared Excess Emission in Protostars
Protostellar systems, ranging from low-luminosity T Tauri and Herbig Ae stars
to high-luminosity Herbig Be stars, exhibit a near-infrared (NIR) excess in
their spectra that is dominated by a bump in the monochromatic luminosity with
a peak near 3 microns. The bump can be approximated by a thermal emission
component of temperature 1500 K that is of the order of the sublimation
temperature of interstellar dust grains. In the currently popular "puffed up
rim" scenario, the bump represents stellar radiation that propagates through
the optically thin inner region of the surrounding accretion disk and is
absorbed and reemitted by the dust that resides just beyond the dust
sublimation radius, Rsub. However, this model cannot account for the strongest
bumps measured in these sources, and it predicts a large secondary bounce in
the interferometric visibility curve that is not observed. In this paper we
present an alternative interpretation, which attributes the bump to reemission
of stellar radiation by dust that is uplifted from the disk by a centrifugally
driven wind. Winds of this type are a leading candidate for the origin of the
strong outflows associated with protostars, and there is observational evidence
for disk winds originating on scales ~Rsub. Using a newly constructed Monte
Carlo radiative transfer code, we show that this model can account for the NIR
excess emission even in bright Herbig Ae stars such as AB Auriga and MWC 275,
and that it successfully reproduces the basic features of the visibilities
measured in these protostars. We argue that a robust dusty outflow in these
sources could be self-limiting to a relatively narrow launching region between
Rsub and 2Rsub. Finally, we suggest that our model could also naturally account
for the NIR and scattered-light variability exhibited by a source like MWC 275,
which may be triggered by the uplifting of dust clouds from the disk.Comment: 19 pages, 8 figures. Updated submitted version to refereed and
accepted one (accepted 8/29/2012 for publication in The Astrophysical
Journal
Measuring the mass accretion rates of Herbig Ae/Be stars with X-shooter
We present the results of our observations of eight magnetic Herbig Ae/Be
stars obtained with the X-shooter spectrograph mounted on UT2 at the VLT.
X-shooter provides a simultaneous, medium-resolution and high-sensitivity
spectrum over the entire wavelength range from 300 to 2500 nm. We estimate the
mass accretion rates M_acc of the targets from 13 different spectral
diagnostics using empiric calibrations derived previously for T Tauri-type
stars and brown dwarfs. We have estimated the mass accretion rates of our
targets, which range from 2x10^-9 to 2x10^-7 M_sun/yr. Furthermore, we have
found accretion rate variability with amplitudes of 0.10-0.40 dex taking place
on time scales from one day to tens of days. Additional future night-to-night
observations need to be carried out to investigate the character of M_acc
variability in details. Our study shows that the majority of the calibrational
relations can be applied to Herbig Ae/Be stars, but several of them need to be
re-calibrated on the basis of new spectral data for a larger number of Herbig
Ae/Be stars.Comment: 19 pages, 13 figures, nine tables, accepted for publication in
Astronomische Nachrichte
Infrared Variability of Evolved Protoplanetary Disks: Evidence for Scale Height Variations in the Inner Disk
We present the results of a multi-wavelength multi-epoch survey of five
evolved protoplanetary disks in the IC 348 cluster that show significant
infrared variability. Using 3-8micron and 24micron photometry along with
5-40micron spectroscopy from the Spitzer Space Telescope, as well as
ground-based 0.8-5micron spectroscopy, optical spectroscopy and near-infrared
photometry, covering timescales of days to years, we examine the variability in
the disk, stellar and accretion flux. We find substantial variations (10-60%)
at all infrared wavelengths on timescales of weeks to months for all of these
young stellar objects. This behavior is not unique when compared to other
cluster members and is consistent with changes in the structure of the inner
disk, most likely scale height fluctuations on a dynamical timescale. Previous
observations, along with our near-infrared photometry, indicate that the
stellar fluxes are relatively constant; stellar variability does not appear to
drive the large changes in the infrared fluxes. Based on our near-infrared
spectroscopy of the Pa-beta and Br-gamma lines we find that the accretion rates
are variable in most of the evolved disks but the overall rates are probably
too small to cause the infrared variability. We discuss other possible physical
causes for the variability, including the influence of a companion, magnetic
fields threading the disk, and X-ray flares.Comment: Accepted to ApJ. 33 pages, emulate apj forma
GASPS observations of Herbig Ae/Be stars with PACS/Herschel. The atomic and molecular content of their protoplanetary discs
We observed a sample of 20 representative Herbig Ae/Be stars and five A-type
debris discs with PACS onboard of Herschel. The observations were done in
spectroscopic mode, and cover far-IR lines of [OI], [CII], CO, CH+, H2O and OH.
We have a [OI]63 micron detection rate of 100% for the Herbig Ae/Be and 0% for
the debris discs. [OI]145 micron is only detected in 25%, CO J=18-17 in 45%
(and less for higher J transitions) of the Herbig Ae/Be stars and for [CII] 157
micron, we often found spatially variable background contamination. We show the
first detection of water in a Herbig Ae disc, HD 163296, which has a settled
disc. Hydroxyl is detected as well in this disc. CH+, first seen in HD 100546,
is now detected for the second time in a Herbig Ae star, HD 97048. We report
fluxes for each line and use the observations as line diagnostics of the gas
properties. Furthermore, we look for correlations between the strength of the
emission lines and stellar or disc parameters, such as stellar luminosity, UV
and X-ray flux, accretion rate, PAH band strength, and flaring. We find that
the stellar UV flux is the dominant excitation mechanism of [OI]63 micron, with
the highest line fluxes found in those objects with a large amount of flaring
and greatest PAH strength. Neither the amount of accretion nor the X-ray
luminosity has an influence on the line strength. We find correlations between
the line flux of [OI]63 micron and [OI]145 micron, CO J = 18-17 and [OI]6300
\AA, and between the continuum flux at 63 micron and at 1.3 mm, while we find
weak correlations between the line flux of [OI]63 micron and the PAH
luminosity, the line flux of CO J = 3-2, the continuum flux at 63 micron, the
stellar effective temperature and the Brgamma luminosity. (Abbreviated version)Comment: 20 pages, 29 figures, accepted by Astronomy and Astrophysic
A spectroscopic survey of Herbig Ae/Be stars with X-shooter – I. Stellar parameters and accretion rates
Herbig Ae/Be stars (HAeBes) span a key mass range that links low- and high-mass stars, and thus provide an ideal window from which to explore their formation. This paper presents Very Large Telescope/X-shooter spectra of 91 HAeBes, the largest spectroscopic study of HAeBe accretion to date. A homogeneous approach to determining stellar parameters is undertaken for the majority of the sample. Measurements of the ultraviolet are modelled within the context of magnetospheric accretion, allowing a direct determination of mass accretion rates. Multiple correlations are observed across the sample between accretion and stellar properties: the youngest and often most massive stars are the strongest accretors, and there is an almost 1:1 relationship between the accretion luminosity and stellar luminosity. Despite these overall trends of increased accretion rates in HAeBes when compared to classical T Tauri stars, we also find noticeable differences in correlations when considering the Herbig Ae and Herbig Be subsets. This, combined with the difficulty in applying a magnetospheric accretion model to some of the Herbig Be stars, could suggest that another form of accretion may be occurring within Herbig Be mass range
- …