X-ray Diagnostics of Grain Depletion in Matter Accreting onto T Tauri Stars

Recent analysis of high resolution Chandra X-ray spectra has shown that the Ne/O abundance ratio is remarkably constant in stellar coronae. Based on this result, we point out the utility of the Ne/O ratio as a discriminant for accretion-related X-rays from T Tauri stars, and for probing the measure of grain-depletion of the accreting material in the inner disk. We apply the Ne/O diagnostic to the classical T Tauri stars BP Tau and TW Hya--the two stars found to date whose X-ray emission appears to originate, at least in part, from accretion activity. We show that TW Hya appears to be accreting material which is significantly depleted in O relative to Ne. In constrast, BP Tau has an Ne/O abundance ratio consistent with that observed for post-T Tauri stars. We interpret this result in terms of the different ages and evolutionary states of the circumstellar disks of these stars. In the young BP Tau disk (age 0.6 Myr) dust is still present near the disk corotation radius and can be ionized and accreted, re-releasing elements depleted onto grains. In the more evolved TW Hya disk (age 10 Myr), evidence points to ongoing coagulation of grains into much larger bodies, and possibly planets, that can resist the drag of inward-migrating gas, and accreting gas is consequently depleted of grain-forming elements.Comment: 13 pages, 1 Figure, ApJ Letters, in pres

X-rays from accretion shocks in T Tauri stars: The case of BP Tau

We present an XMM-Newton observation of the classical T Tauri star BP Tau. In the XMM-Newton RGS spectrum the O {\sc vii} triplet is clearly detected with a very weak forbidden line indicating high plasma densities and/or a high UV flux environment. At the same time concurrent UV data point to a small hot spot filling factor suggesting an accretion funnel shock as the site of the X-ray and UV emission. Together with the X-ray data on TW Hya these new observations suggest such funnels to be a general feature in classical T Tauri stars.Comment: 4 pages, 4 figures, accepted by A&

Emission Line Variability of the Accreting Young Brown Dwarf 2MASSW J1207334-393254: From Hours to Years

We have obtained a series of high-resolution optical spectra for the brown dwarf 2MASSW J1207334-393254 (2M1207) using the ESO Very Large Telescope with the UVES spectrograph during two consecutive observing nights (time resolution of ~12 min) and the Magellan Clay telescope with the MIKE spectrograph. Combined with previously published results, these data allow us to investigate changes in the emission line spectrum of 2M1207 on timescales of hours to years. Most of the emission line profiles of 2M1207 are broad, in particular that of Halpha, indicating that the dominant fraction of the emission must be attributed to disk accretion rather than to magnetic activity. From the Halpha 10% width we deduce a relatively stable accretion rate between 10^(-10.1...-9.8) Msun/yr for two nights of consecutive observations. Therefore, either the accretion stream is nearly homogeneous over (sub-)stellar longitude or the system is seen face-on. Small but significant variations are evident throughout our near-continuous observation, and they reach a maximum after ~8 h, roughly the timescale on which maximum variability is expected across the rotation cycle. Together with past measurements, we confirm that the accretion rate of 2M1207 varies by more than one order of magnitude on timescales of months to years. Such variable mass accretion yields a plausible explanation for the observed spread in the accretion rate vs. mass diagram. The magnetic field required to drive the funnel flow is on the order of a few hundred G. Despite the obvious presence of a magnetic field, no radio nor X-ray emission has been reported for 2M1207. Possibly strong accretion suppresses magnetic activity in brown dwarfs, similar to the findings for higher mass T Tauri stars.Comment: accepted for publication in Ap

Electrification of short sea shipping in Scandinavia - The influence on electrifying ferries within short sea shipping in the Scandinavian countries

For many years in history, the maritime sector has propelled ferries using fossil fuels which have led to great carbon emissions, among others. This bachelor thesis is focused on the fossil-free fuel of the future, electricity. In addition, the thesis will investigate the international and national conventions which affects the development of electric ferries. The thesis has been limited to short sea shipping in the Scandinavian countries and investigates at how these countries could differ when it comes to the development of electric ferries. The work has been carried out as a comparative systematic literature study including a semi-structured interview method. The research questions cover the extent of electric ferries, different incitements to manage the development of electrifying ferries as well as the national and international conventions. Based on the interviews that were held with companies from the Swedish shipping industry, a depper research could be conducted. There are several challenges when it comes to converting to electric propulsion such as the distance between the operating ports, the possibility of charging the batteries and finding an electricity network with enough capacity. Conventions from the International Maritime Organization and the European Union have been considered as the most important influence, which also has a big influence to create national laws and regulations. The International Maritime Organization is a part of the United Nations that is focusing One conclusion that can be taken from the research is that the development of electric propulsion in short sea shipping are an important and necessary development. The development includes multiple beneficial incitements but requires a lot as well. A consideration every company and country must make. During our research, we discovered that although Scandinavia is seen to be at the forefront when it comes to the development of electric solutions for ferries the number of projects is relatively small. Companies in Norway and Denmark who are currently working on developing electric solutions for ferries did unfortunately not have time or the possibility to participate in any interviews and therefore the report was limited to interview Swedish companies

On the origin of ionising photons emitted by T Tauri stars

We address the issue of the production of Lyman continuum photons by T Tauri stars, in an attempt to provide constraints on theoretical models of disc photoionisation. By treating the accretion shock as a hotspot on the stellar surface we show that Lyman continuum photons are produced at a rate approximately three orders of magnitude lower than that produced by a corresponding black body, and that a strong Lyman continuum is only emitted for high mass accretion rates. When our models are extended to include a column of material accreting on to the hotspot we find that the accretion column is extremely optically thick to Lyman continuum photons. Further, we find that radiative recombination of hydrogen atoms within the column is not an efficient means of producing photons with energies greater than 13.6eV, and find that an accretion column of any conceivable height suppresses the emission of Lyman continuum photons to a level below or comparable to that expected from the stellar photosphere. The photospheric Lyman continuum is itself much too weak to affect disc evolution significantly, and we find that the Lyman continuum emitted by an accretion shock is similarly unable to influence disc evolution significantly. This result has important consequences for models which use photoionisation as a mechanism to drive the dispersal of circumstellar discs, essentially proving that an additional source of Lyman continuum photons must exist if disc photoionisation is to be significant.Comment: 6 pages, 4 figures. Accepted for publication in MNRA

The 2008 extreme outburst of the young eruptive variable star Ex Lupi

In early 2008, the young low-mass star EX Lupi, the prototype of the EXor class of eruptive variables, optically brightened by over five magnitudes for a period of 7 months. The previous time a change of such amplitude had been observed in EX Lup was over 50 years ago. In this Letter we present new optical and near-IR high resolution spectroscopy of EX~Lup during the 2008 outburst. We investigate the physical characteristics of the outburst both soon after it began and some four months later, and consider the energetics and kinematics observed. Emission line strengths, widths, and profiles changed significantly between the two observations. Also, modeling of the 2.2935 um CO overtone bandhead emission suggests that an inner gap in the circumstellar gas disk around the star may be present and it is from the inner edge of the gas disk that the CO overtone emission probably arises. We derive a mass accretion luminosity and rate during the extreme outburst of ~2+-0.5~Lsun and ~2+-0.5x10^-7 Msun yr^-1, respectively, which suggests that this outburst was indeed one of the strongest witnessed in EX Lup, yet not as strong as those observed in FU Orionis stars.Comment: 5 pages, 4 figure

Profiles of Strong Permitted Lines in Classical T Tauri Stars

We present a spectral analysis of 30 T Tauri stars observed with the Hamilton echelle spectrograph over more than a decade. One goal is to test magnetospheric accretion model predictions. Observational evidence previously published supporting the model, such as emission line asymmetry and a high frequency of redshifted absorption components, are considered. We also discuss the relation between different line forming regions and search for good accretion rate indicators. In this work we confirm several important points of the models, such as the correlation between accretion and outflow, broad emission components that are mostly central or slightly blueshifted and only the occasional presence of redshifted absorption. We also show, however, that the broad emission components supposedly formed in the magnetospheric accretion flow only partially support the models. Unlike the predictions, they are sometimes redshifted, and are mostly found to be symmetric. The published theoretical profiles do not have a strong resemblance to our observed ones. We emphasize the need for accretion models to include a strong turbulent component before their profiles will match the observations. The effects of rotation, and the outflow components, will also be needed to complete the picture.Comment: 25 pages including 9 figures, 3 tables, accepted for publication in the Astronomical Journa

Characterizing the IYJ Excess Continuum Emission in T Tauri Stars

We present the first characterization of the excess continuum emission of accreting T Tauri stars between optical and near-infrared wavelengths. With nearly simultaneous spectra from 0.48 to 2.4 microns acquired with HIRES and NIRSPEC on Keck and SpeX on the IRTF, we find significant excess continuum emission throughout this region, including the I, Y, and J bands, which are usually thought to diagnose primarily photospheric emission. The IYJ excess correlates with the excess in the V band, attributed to accretion shocks in the photosphere, and the excess in the K band, attributed to dust in the inner disk near the dust sublimation radius, but it is too large to be an extension of the excess from these sources. The spectrum of the excess emission is broad and featureless, suggestive of blackbody radiation with a temperature between 2200 and 5000 K. The luminosity of the IYJ excess is comparable to the accretion luminosity inferred from modeling the blue and ultraviolet excess emission and may require reassessment of disk accretion rates. The source of the IYJ excess is unclear. In stars of low accretion rate, the size of the emitting region is consistent with cooler material surrounding small hot accretion spots in the photosphere. However, for stars with high accretion rates, the projected area is comparable to or exceeds that of the stellar surface. We suggest that at least some of the IYJ excess emission arises in the dust-free gas inside the dust sublimation radius in the disk.Comment: Accepted to ApJ, 31 pages, 21 figure

Protoplanetary Disk Resonances and Type I Migration

Waves reflected by the inner edge of a protoplanetary disk are shown to significantly modify Type I migration, even allowing the trapping of planets near the inner disk edge for small planets in a range of disk parameters. This may inform the distribution planets close to their central stars, as observed by the Kepler mission.Comment: 6 pages, 3 figures, accepted by Ap