Ana Inés Gómez de Castro

The origin of forbidden line emission from young stellar objects

We present a model for the origin of blueshifted, optical forbidden line emission and jets in young stellar objects based on generic properties of hydromagnetic disk winds. Magnetic stresses recollimate hydromagnetic disk winds to magnetic focal regions under very general conditions. We demonstrate that conditions in MHD shocks at these points account for the observed emission. We find that for fiducial accretion rates of 10(-7) M. yr-1 and magnetic fields at the inner edge of the disk (congruent-to 10(12) cm, gas accelerated from the innermost parts of a Keplerian accretion disk focuses into regions greater-than-or-equal-to 0.4 AU in radius that lie congruent-to 16 AU above and below the disk. The shocked gas density ranges from congruent-to 10(4) to 10(8) cm-3 where the latter occurs in the innermost part of the flow and shock. Shocked gas speeds range up to 250 km s-1 (going from the outer part of the shock at congruent-to 2 AU, to the innermost region) under these conditions. The magnetic field is moderately amplified in the shock and diverges from the flow axis in the postshock flow. It is this feature of MHD shock that produces an expanding cone of shocked gas. The opening angle of the postshock gas with respect to the flow axis is 40-degrees, and this accounts for the double-peaked character of the line profiles. Our model also predicts that the slower velocity component is associated with shocked gas of lower density than that associated with the higher velocity component. We show that the wind is largely neutral with an electron fraction of 10(-1). The wind remains largely neutral in adiabatic MHD shocks because much of the preshock kinetic energy goes into an increased postshock magnetic field. Substantial fractions of the flow energy can be liberated in these shocks.

Accretion and UV variability in BP Tauri

BP Tau is one of the few classical T Tauri stars for which the presence of a hot spot in the surface has been reported without ambiguity. The most likely source of heating is gravitational energy released by the accreting material as it shocks with the stellar surface. This energy is expected to be radiated mainly at UV wavelengths. In this work we report the variations of the UV spectrum of BP Tau for 1992 January 5-19, when the star was monitored with IUE during two rotation periods. Our data indicate that lines that can be excited by recombination processes, such as those from O I and He II, have periodic-like light curves, whereas lines that are only collisionally excited do not follow a periodic-like trend. These results agree with the expectations of the magnetically channeled accretion models. The kinetic energy released in the accretion shocks is expected to heat the gas to temperatures of ~106 K, which henceforth produces ionizing radiation. The UV (Balmer) continuum and the O I and He II lines are direct outputs of the recombination process. However, the C IV, Si II, and Mg II lines are collisionally excited not only in the shock region but also in inhomogeneous accretion events and in the active (and flaring) magnetosphere, and therefore their light curves are expected to be blurred by these irregular processes. We also report the detection of warm infalling gas from the presence of redshifted (81 km s-1) absorption components in some of the high-resolution Mg II profiles available in the IUE and Hubble Space Telescope archives.

Science with the World Space Observatory - Ultraviolet

The World Space Observatory-Ultraviolet (WSO-UV) will provide access to the UV range during the next decade. The instrumentation on board will allow to carry out high resolution imaging, high sensitivity imaging, high resolution (R~55000) spectroscopy and low resolution (R~2500) long slit spectroscopy. In this contribution, we briefly outline some of the key science issues that WSO-UV will address during its lifetime. Among them, of special interest are: the study of galaxy formation and the intergalactic medium; the astronomical engines; the Milky Way formation and evol ution, and the formation of the Solar System and the atmospheres of extrasolar p lanets.

AK SCO, FIRST DETECTION OF A HIGHLY DISTURBED ATMOSPHERE IN A PRE-MAIN-SEQUENCE CLOSE BINARY

AK Sco is a unique source: a similar to 10 Myr old pre-main-sequence (PMS) spectroscopic binary composed of two nearly equal F5 stars that at periastron are separated by barely 11 stellar radii, so the stellar magnetospheres fill the Roche lobe at periastron. The orbit is not yet circularized (e = 0.47) and very strong tides are expected. This makes AK Sco the ideal laboratory to study the effect of gravitational tides in the stellar magnetic field building up during PMS evolution. In this Letter, the detection of a highly disturbed (sigma similar or equal to 100 km s(-1)) and very dense atmosphere (n(e) = 1.6 x 10(10) cm(-3)) is reported. Significant line broadening blurs any signs of ion belts or bow shocks in the spectrum of the atmospheric plasma. The radiative losses cannot be accounted for solely by the dissipation of energy from the tidal wave propagating in the stellar atmosphere or by the accreting material. The release of internal energy from the star seems to be the most likely source of the plasma heating. This is the first clear indication of a highly disturbed atmosphere surrounding a PMS close binary.

Global aspects of dynamics and star formation in Taurus

We employ a number of data sets that include the proper motions of T-Tauri stars, molecular and H I velocities and densities, optical polarimetry, and H-alpha-luminosities of young stars in order to derive a global view of gas dynamics and star formation in the Taurus molecular cloud. We find strong correlations among cloud properties. TTS stars move in a direction that is roughly parallel to the gaseous filaments in the cloud. Assuming that stars and gas are well coupled, we infer that the cloud moves through the ISM at approximately 17 km s-1. The Taurus cloud is a member of Goulds Belt, and it is moving in the same direction as the other members. We propose that the clouds motion arises as a consequence of its formation through Parker instability in the ISM. The bulk of gas displaced by this instability moves laterally at several times the Alfven speed of the undisturbed gas as it falls back toward the galaxy. It is this motion that is consistent with that of Taurus. An odd (antisymmetric with respect to reflection through the Galactic plane) mode Parker instability produces a warped gas layer which we interpret to be the origin of Goulds Belt, of which Taurus is a small constituent. The infalling gas will shock with material in the plane of the Belt. MHD waves are generated in this process, and we suggest that this is the origin of large-scale Alfven waves in Taurus. Such waves do not damp for the life of the cloud and keep it supported against global gravitational collapse. The gravitational energy release during the Parker instability is sufficient to account for the kinematics of the cloud as well as its internal stirring. The motion of the Taurus cloud is mildly super-Alfvenic with respect to the surrounding undisturbed WNM, and this must be associated with a shock. There are indications of the presence of a shock of speed V(sh) congruent-to 6.7 km s-1 in Taurus. This shock is suggested to be responsible of the relative velocity drift of 3.8 km s-1 between the weaker H I self-absorption component and the rest of the cloud (CO, H I emission, PMS stars). Observational support for the existence of a large-scale Alfven wave in Taurus comes from both the wavelike distortion of the cloud magnetic field (wavelength of 17 pc) and the wavelike disturbance in the velocity field of this H I component which matches up with the magnetic data. Finally, the influence of the shock in the star formation process is analyzed. It is shown that there are no significant differences in the evolutionary state of groups of PMS stars located at very different positions in the cloud. It appears as if many features of the dynamics and physics of the cloud are determined by global issues such as the process of cloud formation and its interaction with the larger scale ISM.

World Space Observatory-Ultraviolet: ISSIS, the imaging instrument ✩

The Imaging and Slitless Spectroscopy Instrument (ISSIS) will be flown as part of the science instrumentation in the World Space Observatory-Ultraviolet (WSO-UV). ISSIS will be the first UV imager to operate in a high Earth orbit from a 2 m class space telescope. In this contribution, the science driving the ISSIS design and the main characteristics of this instrument are presented.

On the source of dense outflows from T Tauri stars – III. Winds driven from the star–disc shear layer

Ultraviolet observations of classical T Tauri Stars (cTTSs) have shown that there is a hot (T(e) similar or equal to 80 000 K) and dense (n(e) similar or equal to 1010 cm-3) component associated with the large-scale jet. This hot component is formed very close to the base of the jet providing fundamental information on the jet formation mechanism. In this series, we have investigated whether this component can be formed in disc winds, either cool or warm. To conclude the series, jet launching from the interface between the magnetic rotor (the star) and the disc is studied. Synthetic profiles are calculated from numerical simulations of outflow launching by star-disc interaction. Profiles are calculated for several possible configurations of the stellar field: dipolar (with surface strengths B(*) of 1, 2 and 5 kG) or dynamo fed. Also two types of discs, passive or subjected to an alpha -dynamo, are considered. These profiles have been used to define the locus of the various models in the observational diagram: dispersion versus centroid, for the profiles of the Si iii] line. Bulk motions produce an increasing broadening of the profile as the lever arm launching the jet becomes more efficient; predicted profiles are however sensitive to the disc inclination. Models are compared with observations of the Si iii] lines obtained with the Hubble Space Telescope. In addition, it is shown that the non-stationary nature of star-disc winds produce a flickering of the profile during quiescence with variations in the line flux of about 10 per cent. At outburst, accretion signatures appear in the profiles together with an enhancement of the wind, producing the correlation between accretion and outflow as reported from RU Lup, AA Tau and RW Aur observations.

The structure of the galactic magnetic field toward the high-latitude clouds

We present the results of an optical polarization survey toward the galactic anticenter, in the area 5(h) greater than or equal to alpha greater than or equal to 2(h) and 6 degrees less than or equal to delta less than or equal to 12 degrees. This region is characterized by the presence of a stream of high-velocity H I as well as high galactic latitude molecular clouds. We used our polarization data together with 100 mu m IRAS maps of the region to study the relation between the dust distribution and the geometry of the magnetic field. We find that there is a correlation between the percent polarization and the 100 mu m flux such that P(%) less than or equal to (0.16 +/- 0.05)F-100. When the IRAS flux is converted into H I column densities this becomes P(%) less than or equal to (0.13 +/- 0.03)N-20 - 0.22, which is consistent with previous interstellar medium studies on the relation between reddening and polarization. This correlation indicates that our survey is as deep as IRAS and that the magnetic field geometry does not change strongly with the optical depth in the lines of sight that we have studied. The implied lower limit to the distance of our survey is 500-700 pc at galactic latitudes b = -20 degrees and 100 pc at b = -50 degrees. Our main finding is that the magnetic field is perpendicular to the H I high-velocity stream as well as the molecular cloud MBM 16 in the high-latitude region. The field is also perpendicular to the velocity gradient in the stream. Closer to the plane, the magnetic field is parallel to the dust filaments that extend like a plume toward the halo. Our observations indicate that the galactic magnetic field toward the high-latitude clouds is toroidal. We propose a model in which flux tubes that rise out of the galactic plane become force-free and predominantly toroidal at high latitudes. The high-latitude clouds may be gas streams that are falling back toward the galactic plane within such buoyant braided ropes of magnetic flux.

Instrumentation of the WSO-UV project

Dedicated to spectroscopic and imaging observations of the ultraviolet sky, the World Space Observatory - Ultraviolet mission is a Russian-Spanish collaboration. The project consists of a 1.7m telescope with instrumentation able to perform: a) high resolution (R ≥50 000) spectroscopy by means of two echellé spectrographs covering the 115–310 nm spectral range; b) long slit (1x75 arcsec) low resolution (R ∼ 1000) spectroscopy with a near-UV channel and a far-UV channel to cover the 115–305 nm spectral range; c) near-UV and a far-UV imaging channels covering the 115-320 nm wavelength range; d) slitless spectroscopy with spectral resolution of about 500 in the full 115–320 nm spectral range. Here we present the WSO-UV focal plane instruments, their status of implementation, and the expected performances.

Finding the UV-Visible Path Forward: Proceedings of the Community Workshop to Plan the Future of UV/Visible Space Astrophysics

Proceedings from Workshop held in June 2015 at NASA GSFC on the Future of UV Astronomy from Space