ULLYSES Observing Strategies and Technical Specifications

The ULLYSES program is divided into two primary observational campaigns of high- and low-mass stars, with about 500 orbits allocated to each component.

High Mass Stars

The focus on high-mass stars includes observations of OB stars in each of the Large (50% solar metallicity) and Small (20% solar metallicity) Magellanic Clouds, as well as six additional stars, which are accessible in the even lower metallicity Local Group galaxies NGC 3109 (10 to 20% solar metallicity), and Sextans A (10% solar metallicity). Observations of OB stars in the Magellanic Clouds will utilize a subset of the COS G130M (cenwaves 1096 and 1291), COS G160M (cenwave 1611), STIS E140M, COS G185M (cenwaves 1953 and 1986), and STIS E230M (cenwaves 1978 and 2707) gratings (see example in Figure 1). Massive stars in Sextans A and NGC 3109 will be observed with G140L/800. Observations of high-mass stars will consist of ~225 orbits in each of the LMC and SMC, and 50 orbits for Sextans A and NGC 3109. The goal signal-to-noise ratio values for each mode to be used in massive star observations are listed in Table 1.

Table 1: Signal to Noise Goals for Planned Instrument Configurations
Mode Wavelength S/N (on the continuum)

COS/G130M/1096

1080 Å

20

COS/G130M/1291

1150 Å

30

COS/G160M/1611

1590 Å

30

COS/G185M/1953

1860 Å

20

COS/G185M/1986

1980 Å

20

STIS/E140M/1425

1200 Å

20

STIS/E230M/1978

1800 Å

20

STIS/E230M/2707

2800 Å

20

COS/G140L/800 (Sextans A and NGC 3109 only)

1600 Å

15

 

UV spectrum of Sk-68 26
Figure 1: Example of UV spectrum (FUV+NUV) of LMC massive star Sk-68 26, a BC2 Ia supergiant. For this star, archival coverage exists with FUSE, but this spectral range could be observed as part of ULLYSES with the COS/G130M/1096 setting. The STIS E140M or COS G130M+G160M gratings will cover the longest wavelengths in the FUV, while massive stars in the LMC and SMC will be observed with STIS E230M/1978 (and E230M/2707 for the the brightest late B supergiant).

 

Low-Mass Stars

For low-mass stars, observations will focus on about 60 K- and M-type T Tauri stars and brown dwarfs within the Milky Way, including time-domain monitoring of four prototypical T Tauri stars with well-known rotation periods and magnetic configurations. The targets will sample mass, age, and accretion rate over ~400 orbits. 100 additional orbits will be allocated to the time-domain variability aspect of ULLYSES. All survey T Tauri stars will be observed with COS/G130M/1291, COS/G160M/1611, STIS/G230L, STIS/G430L, and STIS/G750L (see example in Figure 2). The goal signal-to-noise for each mode to be used in the T Tauri star observations is listed in Table 2.

Table 2: Signal to Noise Goals for Planned Instrument Configurations
Mode Wavelength S/N
COS/G130M/1291 Peak of N V (~1239 Å ) 15 (line)
COS/G160M/1611 Peak of C IV (~1548-1550 Å) 30 (line)
STIS/G230L Peak of Mg II (~2800 Å) 20 (line)
STIS/G430L 4000 Å 20 (continuum)
STIS/G750L 5700 Å 20 (continuum)

 

Full spectral coverage of TW Hya
Figure 2: Example of full spectral coverage of T Tauri star TW Hya. The FUV part of the spectrum has been observed with STIS E140M. Similar objects selected in the ULLYSES samples will be observed in the FUV with either STIS E140M or COS G130M+G160M. The NUV, optical, and NIR parts of the spectrum of T Tauri stars in ULLYSES will be observed with STIS G230L, G430L, and G750L, respectively.

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