December 14, 2021
ULLYSES Data Release 4 (DR4) consists of high-level science products (HLSPs)
for 25 new ULLYSES targets and updated HLSPs for 226 targets previously
released in DRs 1-3, for a total of 251 ULLYSES targets to date.
Products are also provided for 8 companion targets present in STIS long-slit observations.
Updated HLSPs from previous DRs now include recent COS and STIS pipeline
and/or calibration improvements. HLSPs from previous DRs should be
replaced with newly downloaded products that incorporate these upgrades.
DR4 Target Breakdown
251 ULLYSES targets:
- 196 massive stars from the LMC and SMC
- 103 massive stars also have FUSE spectra
- 51 Survey T Tauri stars
- 31 stars have LCOGT (Las Cumbres Observatory Global Telescope) imaging observations
- 31 stars have HST COS/FUV, STIS/NUV, and STIS/Optical spectra
- 4 Monitoring T Tauri stars
- 1 monitoring star (TW Hya) has LCOGT imaging observations
8 non-ULLYSES targets
- These targets are present in STIS long-slit observations of T Tauri stars
- Products for these companions may be used to disentangle their
contributions to the spectra of primary ULLYSES T Tauri stars
Finally, imaging HLSPs exist for 1 low-metallicity galaxy, NGC3109.
Changes to the _sed File Extension
Beginning in DR4 and all subsequent DRs, the filename extension used for level
4 HLSPs has been changed. Previously this extension was “_sed.fits”, it has
now been renamed “_preview-spec.fits”. Level 4 products are
created by abutting all coadded input spectra
for a given target. These
products are not spectral energy distributions (SEDs), as the units are in
flux density (erg/cm2/s/Angstrom). Additionally, the selection of the
transition wavelength from one abutted spectrum to the next is currently
defined to be the middle of the overlap region. As such, it is possible that
these products may use lower-quality data when higher-quality data exist.
While these preview spectra may be suitable for many forms of scientific analysis,
it is recommended that they be carefully evaluated (e.g. by comparing with
the HLSPs for each individual spectrographic configuration) before being
used for detailed measurements.
Spectroscopic time-series products
are available for both survey and
monitoring T Tauri stars. For survey stars, only exposure level time-series
with LCOGT data (with extension “_tss.fits”) are available. It should be
noted that LCOGT time-series “spectra” consist of only two wavelength samples
corresponding to the central wavelengths of each imaging filter used. HST
COS and STIS UV data are time-tagged, which allows for sub-exposure time-series
spectra. Sub-exposure products (with extension “_split-tss.fits”) are only
generated from HST data of monitoring stars. HST exposure-level time-series
spectra are also created for monitoring stars. The data formats of both
exposure- and sub-exposure-level products are identical regardless of input
data type (LCOGT or HST) or target type (survey or monitoring star), but the
input data calibration and exact methods of creation differ. See the
Data Description webpage
for complete details.
In DR4, new LCOGT exposure-level products are available for:
- 18 survey T Tauri stars
- 1 monitoring star: TW Hydra
New HST exposure- and sub-exposure-level products are available for:
- 3 monitoring stars: BP Tau, RU Lup, and GM Aur
Additionally, updated DR3 time-series products are available for download.
These products benefit from updated calibration and processing.
Time and Wavelength Sampling
The time and wavelength sampling for sub-exposure time-series spectra are
optimized to probe the smallest time interval
possible while maintaining
a S/N ≥ 5 per resolution element at the peak of the most important emission
lines: C IV (1548 Å), and Mg II (2800 Å). The final sampling is also chosen
to ensure that products are ready for use out-of-the-box and require no
additional binning for most scientific goals. However, if a higher S/N is
required, data can be binned further in time or wavelength by the user.
For all NUV observations, the optimal time sampling and wavelength sampling
were determined to be, respectively, 10s and the native wavelength grid
(i.e. no wavelength binning, or ~0.415 Å). For FUV spectra, the following
time and wavelength bins have been adopted for the four monitoring T Tauri stars:
- TW Hya: time = 30s, wavelength = 3 pixels
- RU Lup: time = 30s, wavelength = 6 pixels
- BP Tau: time = 30s, wavelength = 6 pixels
- GM Aur: time = 90s, wavelength = 6 pixels
For reference, the COS/FUV resolution element is 6 pixels, or ~0.073 Å, in the dispersion direction.
COS Calibration Updates
In DR3, routine data quality checks revealed a region of transient elevated
counts, known as a “hotspot”, on the COS/FUVB segment. The hotspot is located
in one of the background regions used by CalCOS, and therefore its flux is
subtracted from the science spectrum resulting in a corresponding flux
depression. Analysis performed by the COS team indicated the hotspot has
been present since February 2018. Hotspots can be identified and flagged
for removal using a COS reference file called
On November 9, 2021, the COS team delivered a new SPOTTAB that now flags and
ignores all pixels affected by this hotspot. All affected COS HLSPs in DR4
have been created using this SPOTTAB.
Caveats and Known Issues
Observations of the monitoring T Tauri star, GM Aur, were interrupted by
the October 2021 safing of HST. As a result, only 7 of the 12 expected
visits were executed. Of these 7, only 6 were immediately usable with 1
requiring special calibration. In an effort to deliver data for this target
as soon as possible, this 7th visit was omitted from DR4 but will be
included in DR5.
For regions where all contributing datasets have data quality issues, flux is set to 0.
The selection of the transition wavelength from one abutted spectrum to
the next is currently defined to be the middle of the overlap region.
In future releases, the transition wavelengths will be selected to ensure
the signal-to-noise ratio is similar at the transition wavelength.
Twenty of the T Tauri stars introduced in DR3 include archival data only.
While low-resolution STIS spectra (G230L, G430L, and/or G750L) exist for
these targets, these modes require careful manual calibration. As such,
these modes will be included in a future DR.
Data Description & Download
A full description of the ULLYSES data products and how they are created can
be found here
. Data may be downloaded
from this website
(HLSPs and their contributing data),
the MAST Data Discovery Portal
(HLSPs and their contributing data), or directly as a High-Level Science Product
collection using the
A description of the ULLYSES observations and data products is given in:
For more information on how to cite ULLYSES data, see