Periodic radial velocity variations in RU Lupi

Context. RU Lup is a Classical T Tauri star with unusually strong emission lines, which has been interpreted as manifestations of accretion. Recently, evidence has accumulated that this star might have a variable radial velocity. Aims. We intended to investigate in more detail the possible variability in radial velocity using a set of 68 high-resolution spectra taken at the VLT (UVES), the AAT (UCLES) and the CTIO (echelle). Methods. Using standard cross-correlation techniques, we determined the radial velocity of RU Lup. We analysed these results with Phasedispersion minimization and the Lomb-Scargle periodogram and searched for possible periodicities in the obtained radial velocities. We also analysed changes in the absorption line shapes and the photometric variability of RU Lup. Results. Our analysis indicated that RU Lup exhibits variations in radial velocity with a periodicity of 3.71 days and an amplitude of 2.17 km/s. These variations can be explained by the presence of large spots, or groups of spots, on the surface of RU Lup. We also considered a low-mass companion and stellar pulsations as alternative sources for these variations but found these to be unlikely.Comment: 8 pages, 4 figures, Accepted by A&

Streaming Video over HTTP with Consistent Quality

In conventional HTTP-based adaptive streaming (HAS), a video source is encoded at multiple levels of constant bitrate representations, and a client makes its representation selections according to the measured network bandwidth. While greatly simplifying adaptation to the varying network conditions, this strategy is not the best for optimizing the video quality experienced by end users. Quality fluctuation can be reduced if the natural variability of video content is taken into consideration. In this work, we study the design of a client rate adaptation algorithm to yield consistent video quality. We assume that clients have visibility into incoming video within a finite horizon. We also take advantage of the client-side video buffer, by using it as a breathing room for not only network bandwidth variability, but also video bitrate variability. The challenge, however, lies in how to balance these two variabilities to yield consistent video quality without risking a buffer underrun. We propose an optimization solution that uses an online algorithm to adapt the video bitrate step-by-step, while applying dynamic programming at each step. We incorporate our solution into PANDA -- a practical rate adaptation algorithm designed for HAS deployment at scale.Comment: Refined version submitted to ACM Multimedia Systems Conference (MMSys), 201

Doppler Probe of Accretion onto a T Tauri star

The YY Ori stars are T Tauri stars with prominent time-variable redshifted absorption components that flank certain emission lines. One of the brightest in this class is S CrA, a visual double star. We have obtained a series of high-resolution spectra of the two components during four nights with the UVES spectrograph at the Very Large Telescope. We followed the spectral changes occurring in S CrA to derive the physical structure of the accreting gas. We found that both stars are very similar with regard to surface temperature, radius, and mass. Variable redshifted absorption components are particularly prominent in the SE component. During one night, this star developed a spectrum unique among the T Tauri stars: extremely strong and broad redshifted absorption components appeared in many lines of neutral and ionized metals, in addition to those of hydrogen and helium. The absorption depths of cooler, low ionization lines peak at low velocities - while more highly ionized lines have peak absorption depths at high velocities. The different line profiles indicate that the temperature and density of the accretion stream increase as material approaches the star. We derive the physical conditions of the flow at several points along the accretion funnel directly from the spectrum of the infalling gas. We estimated mass accretion rates of about 10^(-7) solar masses per year, which is similar to that derived from the relation based on the strength of H alpha emission line. This is the first time the density and temperature distributions in accretion flows around a T Tauri star have been inferred from observations. Compared with predictions from standard models of accretion in T Tauri stars, which assume a dipole stellar magnetic field, we obtained higher densities and a steeper temperature rise toward the star.Comment: Replaces 1408.1846 4 pages, 4 figures. Appears in Astronomy and Astrophysics, 201

Unveiling extremely veiled T Tauri stars

Photospheric absorption lines in classical T Tauri stars (CTTS) are weak compared to normal stars. This so-called veiling is normally identified with an excess continuous emission formed in shock-heated gas at the stellar surface below the accretion streams. We have selected four stars (RW Aur A, RU Lup, S CrA NW and S CrA SE) with unusually strong veiling to make a detailed investigation of veiling versus stellar brightness and emission line strengths for comparisons to standard accretion models. We have monitored the stars photometrically and spectroscopically at several epochs. In standard accretion models a variable accretion rate will lead to a variable excess emission. Consequently, the stellar brightness should vary accordingly. We find that the veiling of absorption lines in these stars is strongly variable and usually so large that it would require the release of several stellar luminosities of potential energy. At states of very large line dilution, the correspondingly large veiling factors derived correlate only weakly with brightness. Moreover, the emission line strengths violate the expected trend of veiling versus line strength. The veiling can change dramatically in one night, and is not correlated with the phase of the rotation periods found for two stars. We show that in at least three of the stars, when the veiling becomes high, the photospheric lines become filled-in by line emission, which produces large veiling factors unrelated to changes in any continuous emission from shocked regions. We also consider to what extent extinction by dust and electron scattering in the accretion stream may affect veiling measures in CTTS. We conclude that the degree of veiling cannot be used as a measure of accretion rates in CTTS with rich emission line spectra.Comment: Accepted for publication in A&A Letters. New language-edited version. (4 pages, 3 figures

Modelling Circumbinary Gas Flows in Close T Tauri Binaries

Young close binaries open central gaps in the surrounding circumbinary accretion disc, but the stellar components may still gain mass from gas crossing through the gap. It is not well understood how this process operates and how the stellar components are affected by such inflows. Our main goal is to investigate how gas accretion takes place and evolves in close T Tauri binary systems. In particular, we model the accretion flows around two close T Tauri binaries, V4046 Sgr and DQ Tau, both showing periodic changes in emission lines, although their orbital characteristics are very different. In order to derive the density and velocity maps of the circumbinary material, we employ two-dimensional hydrodynamic simulations with a locally isothermal equation of state. The flow patterns become quasi-stable after a few orbits in the frame co-rotating with the system. Gas flows across the circumbinary gap through the co-rotating Lagrangian points, and local circumstellar discs develop around both components. Spiral density patterns develop in the circumbinary disc that transport angular momentum efficiently. Mass is preferentially channelled towards the primary and its circumstellar disc is more massive than the disc around the secondary. We also compare the derived density distribution to observed line profile variability. The line profile variability tracing the gas flows in the central cavity shows clear similarities with the corresponding observed line profile variability in V4046 Sgr, but only when the local circumstellar disc emission was excluded. Closer to the stars normal magnetospheric accretion may dominate while further out the dynamic accretion process outlined here dominates. Periodic changes in the accretion rates onto the stars can explain the outbursts of line emission observed in eccentric systems such as DQ Tau.Comment: Accepted for publication in MNRA

Radio observations of globulettes in the Carina nebula

Context. The Carina nebula hosts a large number of globulettes. An optical study of these tiny molecular clouds shows that the majority are of planetary mass, but there are also those with masses of several tens up to a few hundred Jupiter masses. Aims. We seek to search for, and hopefully detect, molecular line emission from some of the more massive objects; in case of successful detection we aim to map their motion in the Carina nebula complex and derive certain physical properties. Methods. We carried out radio observations of molecular line emission in (CO)-C-12 and (CO)-C-13 (2-1) and (3-2) of 12 globulettes in addition to positions in adjacent shell structures using APEX. Results. All selected objects were detected with radial velocities shifted relative to the emission from related shell structures and background molecular clouds. Globulettes along the western part of an extended dust shell show a small spread in velocity with small velocity shifts relative to the shell. This system of globulettes and shell structures in the foreground of the bright nebulosity surrounding the cluster Trumpler 14 is expanding with a few km s(-1) relative to the cluster. A couple of isolated globulettes in the area move at similar speed. Compared to similar studies of the molecular line emission from globulettes in the Rosette nebula, we find that the integrated line intensity ratios and line widths are very different. The results show that the Carina objects have a different density/temperature structure than those in the Rosette nebula. In comparison the apparent size of the Carina globulettes is smaller, owing to the larger distance, and the corresponding beam filling factors are small. For this reason we were unable to carry out a more detailed modelling of the structure of the Carina objects in the way as performed for the Rosette objects. Conclusions. The Carina globulettes observed are compact and denser than objects of similar mass in the Rosette nebula. The distribution and velocities of these globulettes suggest that they have originated from eroding shells and elephant trunks. Some globulettes in the Trumpler 14 region are quite isolated and located far from any shell structures. These objects move at a similar speed as the globulettes along the shell, suggesting that they once formed from cloud fragments related to the same foreground shell.Peer reviewe

The Instability Strip for Pre--Main-Sequence Stars

We investigate the pulsational properties of Pre--Main-Sequence (PMS) stars by means of linear and nonlinear calculations. The equilibrium models were taken from models evolved from the protostellar birthline to the ZAMS for masses in the range 1 to 4 solar masses. The nonlinear analysis allows us to define the instability strip of PMS stars in the HR diagram. These models are used to constrain the internal structure of young stars and to test evolutionary models. We compare our results with observations of the best case of a pulsating young star, HR~5999, and we also identify possible candidates for pulsational variability among known Herbig Ae/Be stars which are located within or close to the instability strip boundaries.Comment: 14 pages, three postscript figures, accepted for publication on the Astrophysical Journal Letter

Hubble Space Telescope STIS spectrum of RW Aurigae A: Evidence for an ionized beltlike structure and mass ejection in timescales of a few hours

Profiles of the UV semiforbidden lines of C III](1908) and Si III](1892) of RW Aur have been obtained with the Hubble Space Telescope (HST) STIS. The C III](1908) profile shows two high-velocity components at upsilon = +/-170 km s(-1) and a central one. The Si III](1892) profile is very broad (FWHM = 293 km s(-1)), and the high-velocity components are unresolved. The high-velocity components are most probably produced in a rotating belt similar to that detected in other sources of bipolar outflows. A radius between 2.7 R-* and the corotation radius (6.1 R-*) is derived, and a log T-e(K) similar or equal to 4.7 and log n(e)(cm(-3)) = 11.6 are estimated. The belt is clumpy, and the most likely source of heating is local X-ray radiation, probably associated with the release of magnetic energy. In addition, profiles of the optical lines of He I, Fe II, and Halpha retrieved from the HST archive have been analyzed. The spectra were obtained by shifting the STIS slit between exposures in the transverse direction to the flow. Two features vary from one exposure to another: a blueshifted emission feature (detected in the Fe II and He I lines) and a redshifted absorption feature (detected in Halpha), which are observed at the velocity of the blueshifted and redshifted components of the jet, respectively. There is a clear-cut correlation between the equivalent widths of these two components. These components are not associated with the flaring activity of the source, since they remain stable timescales of similar or equal to40 minutes. They are most likely associated with variations in the stellar/circumstellar environment in timescales of a few (similar or equal to5) hours, consistent with reports by other authors for variations of the veiling and the appearance of shell signatures in the optical spectrum

Modelling Circumbinary Gas Flows in Close T Tauri Binaries

Young close binaries open central gaps in the surrounding circumbinary accretion disc, but the stellar components may still gain mass from gas crossing through the gap. It is not well understood how this process operates and how the stellar components are affected by such inflows. Our main goal is to investigate how gas accretion takes place and evolves in close T Tauri binary systems. In particular, we model the accretion flows around two close T Tauri binaries, V4046 Sgr and DQ Tau, both showing periodic changes in emission lines, although their orbital characteristics are very different. In order to derive the density and velocity maps of the circumbinary material, we employ two-dimensional hydrodynamic simulations with a locally isothermal equation of state. The flow patterns become quasi-stable after a few orbits in the frame co-rotating with the system. Gas flows across the circumbinary gap through the co-rotating Lagrangian points, and local circumstellar discs develop around both components. Spiral density patterns develop in the circumbinary disc that transport angular momentum efficiently. Mass is preferentially channelled towards the primary and its circumstellar disc is more massive than the disc around the secondary. We also compare the derived density distribution to observed line profile variability. The line profile variability tracing the gas flows in the central cavity shows clear similarities with the corresponding observed line profile variability in V4046 Sgr, but only when the local circumstellar disc emission was excluded. Closer to the stars normal magnetospheric accretion may dominate while further out the dynamic accretion process outlined here dominates. Periodic changes in the accretion rates onto the stars can explain the outbursts of line emission observed in eccentric systems such as DQ Tau.Comment: Accepted for publication in MNRA