Data Release 6

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: 9 non-ULLYSES targets 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:
  1. PGMW3120: multiple stars in aperture
  2. AV22: multiple stars in aperture
  3. AV287: Target not in aperture
  4. 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:
  1. Begin by examining the NVO file, which was initially created by splicing together pieces of the extracted spectra from the eight FUSE detector segments.
  2. 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.
  3. 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.
  4. 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.
  1. Archival spectra were added for 25 massive stars in the low-metallicity galaxies WLM, Leo P, Sextans A, and IC 1613.
  2. 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".
  3. The ULLYSES name for the target previously known as LH9-34 has been changed to PGMW-1363. This target was included in previous releases.*
  4. 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.*
  5. 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: 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: New HST exposure- and sub-exposure-level products are available for:

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

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.


A description of the ULLYSES observations and data products is given in:

For more information on how to cite ULLYSES data, see ULLYSES References.

Charting young stars’ ultraviolet light with Hubble.

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