March 21, 2023
ULLYSES Data Release 6 (DR6) consists of new high-level science products (HLSPs) for
59 ULLYSES targets observed since DR5 (June 2022) and updated HLSPs for 339 targets
included in all previous releases, for a total of 398 ULLYSES targets to date.
Updated HLSPs from previous DRs 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.
DR6 Target Breakdown
398 ULLYSES targets:
- 286 massive stars from the LMC, SMC, and low-metallicity galaxies NGC 3109, Sextans A,
IC 1613, WLM, and Leo-P have HST/UV spectra
- 146 massive stars also have FUSE spectra
- 108 survey T Tauri stars have HST/UV spectra
- 57 stars have LCOGT (Las Cumbres Observatory Global Telescope) imaging observations in the V and i’ bands
- 1 star (SZ82) has LCOGT imaging observations in the u’, V and i’ bands
- 60 stars have STIS/Optical spectra
- 4 Monitoring T Tauri stars
- All monitoring stars have LCOGT imaging observations in the u’, V, and i’ bands
- All monitoring stars have additional archival COS and/or STIS spectra
9 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, HST/WFC3 imaging HLSPs exist for both low-metallicity galaxies,
NGC 3109 and Sextans A.
Improving FUSE Data
All archival FUSE data used in the ULLYSES sample are examined and vetted by the ULLYSES team.
Some targets exhibit various issues in their spectra, such as spectral channel drifting.
In DR6, the ULLYSES team has begun to deliver improved spectra for such targets.
Using the strategy outlined below, FUSE data for 23 targets previously excluded from the sample
were able to be rectified and included in products. Even with extra processing, 4 targets were still unable to
be rectified and will not be included in the ULLYSES sample:
- PGMW3120: multiple stars in aperture
- AV22: multiple stars in aperture
- AV287: Target not in aperture
- SK-69D220: multiple stars in aperture
Flux Differences Due to Guiding
Some ULLYSES FUSE data suffer from drifts among the spectral channels;
FUSE was essentially four independent spectrographs, and thermal instabilities on orbit could cause each
one to drift out of alignment. One of the four channels was used for guiding, and the flux in this channel
was generally the most accurate. Usually, thermal drifts in the other channels led the target to drift out
of the aperture, resulting in lower count rates and thus spectral fluxes. In crowded fields, these drifts
could allow a neighboring star to drift into the aperture, resulting in higher fluxes in the final spectra.
The following strategy was used to repair these data:
- Begin by examining the NVO file, which was initially created by splicing together pieces of the extracted
spectra from the eight FUSE detector segments.
- If the NVO file does not meet data quality needs (e.g., depressed flux or mismatching flux at channel
transition points), a new NVO file is created by using the eight individual extracted spectra in the “ALL” files.
These eight spectra are shifted to a common wavelength zero point and rescaled to create a new NVO file.
- The guide channel is identified (LiF1A for the first half of the mission, and LiF2A for the second)
and its spectrum adopted as a reference.
- If the spectra from other channels are less than 50% brighter than the reference, then they are
rescaled to match the reference in the region of overlap.
- If they are more than 50% brighter than the reference, they are assumed to be contaminated by
nearby stars and not included in the final spectrum.
Even with these corrections, some flux mismatches still remain at the transition point between FUSE and
HST data- these are not corrected by the ULLYSES team. If smooth transitions are required, one of the
contributing spectra may be manually scaled to the other.
Background Subtraction Corrections
Parts of the FUSE calibration pipeline (CalFUSE) were run only in cases where the background subtraction failed.
FUSE did not have a shutter, so the detector received light from all three apertures (LWRS, MDRS, and HIRS)
at all times. CalFUSE assumes that only the target aperture contains a star and fits a background model to
the rest of the detector. In crowded fields, nearby stars occasionally fell in a non-target aperture, leading
to an over-subtraction of the background. In these cases, the region of the detector used to model the background
(stored as header keywords in the intermediate data file) was modified, and spectra were re-calibrated.
Changes to the ULLYSES Sample
Notable updates to the ULLYSES sample are included below.
-
Archival spectra were added for 25 massive stars in the low-metallicity galaxies
WLM, Leo P, Sextans A, and IC 1613.
-
Following MAST best practice naming standards, HLSP filenames always include
the target name. Due to recent updates in the HLSP delivery process, filenames
with the "+" symbol are no longer allowed. Therefore, all filenames with RA,
Dec coordinates in their name were renamed to change "+" to "p" (for plus),
and "-" to "m" (for minus). E.g. the target "echa-j0843.3-7915" was changed to
"echa-j0843m7915" in the filename only (periods and trailing digits are also
stripped, as periods are not acceptable filename characters). The 0th header
keyword "TARGNAME" will remain as the formally adopted ULLYSES name of the
target, in this case: "echa-j0843.3-7915".
-
The ULLYSES name for the target previously known as LH9-34 has been changed
to PGMW-1363. This target was included in previous releases.*
-
The ULLYSES target previously known as V-RW-AUR-A has been changed to
V-RW-AUR. Initially, the ULLYSES team classified these two names as
separate objects, however in archival data the names are used
interchangeably. This target was included in previous releases.*
-
The ULLYSES target previously known as HD-104237A has been changed to
HD-104237. Initially, the ULLYSES team classified these two names as
separate objects, however in archival data the names are used interchangeably.
This target is new to DR6. This target name change does not affect the
star HD-104237E, which is a distinct target.
* This affects the target name used in both the ULLYSES catalog and in product
filenames, however, the change does not affect the underlying HST observations
downloaded directly from MAST.
HLSP Creation Code
The ULLYSES HLSP creation code was previous released publicly in DR5, but with DR6 the
code used to
coadd spectra
has now been made generic for use with any COS and STIS data.
Example Jupyter notebooks are forthcoming. For a description of the
coaddition algorithm, see the
Data Description
webpage.
Contributions and feedback
on this project are welcome! Contribution guidelines
are included in the repository. If you have questions or concerns regarding the
software, please
open an issue
or contact the
HST Help Desk.
Time-series Spectra
In previous DRs, HST spectroscopic time-series products were only available
for the four monitoring T Tauri stars.
The ULLYSES team has now begun delivering
serendipitous HST spectroscopic time-series products for any targets that
would benefit from it. Such targets are typically T Tauri stars that have
been observed over multiple epochs (either by the ULLYSES team, archival
programs, or a combination of both) and whose flux varies as a function of
time and cannot be coadded. For these targets, only exposure-level products
are delivered (see the
Data Description
webpage). Bundling all observations of variable targets in one file
allows for convenient time-domain spectroscopic analysis.
In DR6, time-series spectra are created for three such T Tauri stars:
- RECX-6 (due to special circumstances, this target was previously delivered in DR5)
- SZ45
- V505-ORI
All three targets above have also undergone
manual calibration
to improve the science spectra. In addition to these serendipitous time-series targets,
typical ULLYSES time-series HLSPs are also included in DR6, as detailed below.
The ULLYSES team obtained COS G160M and G230L observations over two epochs for
4 monitoring T Tauri stars: TW Hya, BP Tau, RU Lup, GM Aur (for more details,
see the
T Tauri target tables).
Additional archival COS and/or STIS spectra have been now added to the
exposure-level time-series products for these stars, greatly increasing not
only the wavelength range of data, but also the time domain.
New
LCOGT exposure-level products
are available for:
- 28 survey T Tauri stars in V and i’ bands, including 6 stars that previously had
LCOGT observations, but were serendipitously observed again when targeting new stars
- 1 survey T Tauri star in the u’, V, and i’ bands
- 2 monitoring T Tauri stars (RU Lup, TW Hya) in u’, V, and i’ bands
New HST exposure- and sub-exposure-level products are available for:
- The second (and final) epoch of 3 monitoring stars: BP Tau, RU Lup, and GM Aur
Targets Requiring Special Calibration
In addition to the FUSE recalibration detailed in
Improving FUSE Data,
various other corrections were made to ULLYSES data.
When possible, COS/NUV vignetting is corrected by scaling vignetted flux to
overlapping clean spectra, however not all NUV observations have overlapping spectra.
For archival TW Hya data (data obtained not as part of the ULLYSES program),
the vignetted regions- defined as the first 200 pixels of each stripe- of
such observations are now flagged and discarded. In DR7, this correction will
be extended to all targets with COS/NUV data that cannot be rescaled.
As in previous DRs, all STIS/G230L, G430L, and G750L data of T Tauri stars continue
to require tailored calibrations. Special calibration steps taken for
these observations can include: custom hot pixel identification and flagging,
defringing for G750L observations, and customized spectral extraction
parameters for T Tauri stars and any companions. Level0 HLSPs (manually
calibrated 1-D spectra, and calibration configuration files) are provided for these stars.
DR6 Caveats and Known Issues
-
For regions where all contributing datasets have data quality issues, the flux has a value of 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.
- The FUV flux in many T Tauri stars can be quite low- in these cases,
the default CalCOS and CalSTIS background subtractions can cause the
continuum flux to dip below zero, particularly longward of 1600Å.
-
The STScI STIS team is in the process of re-deriving
flux calibrations.
As of February 2023, the following modes have been completed: G140L, E140M, E230M,
G230L, G230LB, and G430L. Until the remaining modes are completed, there may be
slight mismatches between the flux of adjacent modes (e.g. between G430L and G750L).
- The abutting algorithm does not scale fluxes that are mismatched between
different gratings or instruments.
- A limited number of archival STIS E140M and E230M spectra of LMC/SMC massive stars contain
multiple SCI extensions due to the particular design of the observations.
Currently, only the first SCI extension is included in ULLYSES HLSPs. This
limitation will be addressed for DR7.
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 the
ULLYSES search form
(HLSPs only), the
MAST Data Discovery Portal
(HLSPs and their contributing data), or directly as a High-Level Science Product
collection using the
DOI.
Publications
A description of the ULLYSES observations and data products is given in:
For more information on how to cite ULLYSES data, see
ULLYSES References.
