Introduction
Star formation occurs when large clouds of gas and dust collapse due to gravity. The clouds are inhomogeneous in density and they fragment into smaller structures, where the center of each collapsing sub-cloud will become a star or system of stars. Very few of the resulting stars are massive and hot, by far the largest number will evolve into late-type stars with spectral types in the M-F range and XXX FIND FRACTION AND CITE of those are in binary or multiple systems. The infalling envelope flattens to a circumstellar disk, making the central star visible in the optical. Low-mass stars in this stage are called classical T Tauri stars (CTTS). For a few Myrs, planet formation can take place before the disk disperses. For binaries or higher-order multiple systems, the disk can belong to an individual star or surround a close binary pair depending on the mass and separation of the components.
Mass is accreted through these disks on the the stars. However, conservation of angular momentum demands that some mass is ejected and carries away a larger fraction of the angular momentum accreted through the disk, otherwise the star would spin up until it breaks apart. Mass loss occurs through wide-angle disk winds, but in some systems we can also see highly collimated jets. These jets typically have an onion-like structure with a fast component at the center surrounded by increasingly slower and less well-collimated components further out \cite{2000ApJ...537L..49B}. Forbidden optical emission lines (FELs) are a good way to find and study such jets, since the stellar photosphere and the accretion shock are too dense to contribute to the emission. If a jet is detected in several emission lines, line ratios can be used to calculate density and ionization fraction of emission components in the jet with typical densities in the range \(10^3-10^5cm^{-3}\), e.g. \cite{1999AampA...342..717B} \cite{2000AampA...356L..41L}\cite{2013AampA...550L...1S}. In turn the density and the velocity give mass loss rates. Different outflow components show different velocities. For example, in the well-studied CTTS DG Tau \citet{2013AampA...550L...1S} find [O I] in a low-velocity component (LVC, about 60 \(km\ s^{-1}\)) which can be detected as close as 15 au from the star and a medium-velocity component (MVC, about 130 \(km\ s^{-1}\)) first detected at about 50 au from the source which slows down further out. While a single spectrum is sufficient to detect the presence of an FEL, we need spatially resolved data to study how outflows accelerate and decelerate. In this work, we reanalyze archival data from the Hubble Space Telescope (HST) Program ID 9310 to search for FELs that are spatially resolved.
Observations and data reduction
Hubble Space Telescope (HST) Program ID 9310 targets binary T Tauri stars with long-slit spectroscopy using the Space Telescope Imaging Spectrograph (STIS). The long slit is always oriented such that both components of the binary are observed. \citet{2003ApJ...583..334H} analyze the spectra of both stellar components to determine stellar properties and accretion diagnostics. In this work, we aim to spatially resolve the emission in FELs along the slit, i.e. jets, around those stars.
We retrieved all data sets of Program ID 9310 from the archive, see table NUBMERHERE for a list of observations.
We start our analysis from the pipeline reduced 2-dimensional sx2
files; these files have one spectral axis and one spatial axis for the coordinate along the slit. For each column in sx2
we fit a single Gaussian, taking into account regions flagged for data quality by the pipeline. While this does not capture all features of the instrument PSF, it describes the signal close to the peak of the emission well and allows a numerically stable fit of the position of the peak.