Space Telescope Cosmic Origins Spectrograph Research Articles

UNCOVERING THE PUTATIVE B-STAR BINARY COMPANION OF THE SN 1993J PROGENITOR

The Type IIb supernova (SN) 1993J is one of only a few stripped-envelope supernovae with a progenitor star identified in pre-explosion images. SN IIb models typically invoke H envelope stripping by mass transfer in a binary system. For the case of SN 1993J, the models suggest that the companion grew to 22 M_solar and became a source of ultraviolet (UV) excess. Located in M81, at a distance of only 3.6 Mpc, SN 1993J offers one of the best opportunities to detect the putative companion and test the progenitor model. Previously published near-UV spectra in 2004 showed evidence for absorption lines consistent with a hot (B2 Ia) star, but the field was crowded and dominated by flux from the SN. Here we present Hubble Space Telescope (HST) Cosmic Origins Spectrograph (COS) and Wide-Field Camera 3 (WFC3) observations of SN 1993J from 2012, at which point the flux from the SN had faded sufficiently to potentially measure the UV continuum properties from the putative companion. The resulting UV spectrum is consistent with contributions from both a hot B star and the SN, although we cannot rule out line-of-sight coincidences.

FIRST ULTRAVIOLET REFLECTANCE MEASUREMENTS OF SEVERAL KUIPER BELT OBJECTS, KUIPER BELT OBJECT SATELLITES, AND NEW ULTRAVIOLET MEASUREMENTS OF A CENTAUR

ABSTRACT We observed the 2600–3200 Å (hereafter, mid-UV) reflectance of two Kuiper Belt Objects (KBOs), two KBO satellites, and a Centaur, using the Hubble Space Telescope (HST) Cosmic Origins Spectrograph (COS). Other than measurements of the Pluto system, these constitute the first UV measurements obtained of KBOs, and KBO satellites, and new HST UV measurements of the Centaur 2060 Chiron. We find significant differences among these objects, constrain the sizes and densities of Haumea's satellites, and report the detection of a possible spectral absorption band in Haumea's spectrum near 3050 Å. Comparisons of these objects to previously published UV reflectance measurements of Pluto and Charon are also made here.

Ultraviolet spectral variations of FK Comae and V 1794 Cygni

Ultraviolet spectra of FK Comae and V1794 Cygni observed with the Hubble Space Telescope Cosmic Origins Spectrograph (HST COS) and the International Ultraviolet Explorer (IUE) satellites were analyzed for the period 1981–2011. Temporal variations of line fluxes of the O I 1306Å, C II 1336Å, C IV 1550Å, He II 1640Å and Mg II k & h 2800Å, produced in the transition regions and chromospheres of these stars, imply variations in density and temperature changes in the line emitting regions as a result of the rapid rotation and magnetic fields responsible for stellar activity. Results are consistent with the models of Ramsey et al. (1981), Oliveira and Foing (1999), and Korhonen et al. (2000).

CHARACTERIZING THE CIRCUMGALACTIC MEDIUM OF NEARBY GALAXIES WITHHST/COS ANDHST/STIS ABSORPTION-LINE SPECTROSCOPY

The Circumgalactic Medium (CGM) of late-type galaxies is characterized using UV spectroscopy of 11 targeted QSO/galaxy pairs at z < 0.02 with the Hubble Space Telescope Cosmic Origins Spectrograph and ~60 serendipitous absorber/galaxy pairs at z < 0.2 with the Space Telescope Imaging Spectrograph. CGM warm cloud properties are derived, including volume filling factors of 3-5%, cloud sizes of 0.1-30 kpc, masses of 10-1e8 solar masses and metallicities of 0.1-1 times solar. Almost all warm CGM clouds within 0.5 virial radii are metal-bearing and many have velocities consistent with being bound, "galactic fountain" clouds. For galaxies with L > 0.1 L*, the total mass in these warm CGM clouds approaches 1e10 solar masses, ~10-15% of the total baryons in massive spirals and comparable to the baryons in their parent galaxy disks. This leaves >50% of massive spiral-galaxy baryons "missing". Dwarfs (<0.1 L*) have smaller area covering factors and warm CGM masses (<5% baryon fraction), suggesting that many of their warm clouds escape. Constant warm cloud internal pressures as a function of impact parameter ($P/k ~ 10 cm^{-3} K) support the inference that previous COS detections of broad, shallow O VI and Ly-alpha absorptions are of an extensive (~400-600 kpc), hot (T ~ 1e6 K) intra-cloud gas which is very massive (>1e11 solar masses). While the warm CGM clouds cannot account for all the "missing baryons" in spirals, the hot intra-group gas can, and could account for ~20% of the cosmic baryon census at z ~ 0 if this hot gas is ubiquitous among spiral groups.

Lyα DOMINANCE OF THE CLASSICAL T TAURI FAR-ULTRAVIOLET RADIATION FIELD

ABSTRACT Far-ultraviolet (FUV) radiation plays an important role in determining chemical abundances in protoplanetary disks. H i Lyman α (Lyα) is suspected to be the dominant component of the FUV emission from Classical T Tauri Stars (CTTSs), but is difficult to measure directly due to circumstellar and interstellar H i absorption. To better characterize the intrinsic Lyα radiation, we present FUV spectra of 14 CTTSs taken with the Hubble Space Telescope Cosmic Origins Spectrograph and Space Telescope Imaging Spectrograph instruments. H2 fluorescence, commonly seen in the spectra of CTTSs, is excited by Lyα photons, providing an indirect measure of the Lyα flux incident upon the warm disk surface. We use observed H2 progression fluxes to reconstruct the CTTS Lyα profiles. The Lyα flux correlates with total measured FUV flux, in agreement with an accretion-related source of FUV emission. With a geometry-independent analysis, we confirm that in accreting T Tauri systems Lyα radiation dominates the FUV flux (∼1150 Å –1700 Å). In the systems surveyed this one line comprises 70%–90% of the total FUV flux.

CHARACTERIZING CO FOURTH POSITIVE EMISSION IN YOUNG CIRCUMSTELLAR DISKS

ABSTRACT Carbon monoxide is a commonly used IR/submillimeter tracer of gas in protoplanetary disks. We present an analysis of ultraviolet CO emission in Hubble Space Telescope Cosmic Origins Spectrograph spectra for 12 Classical T Tauri stars (CTTSs). Several ro-vibrational bands of the CO A 1Π–X 1Σ+ (Fourth Positive) electronic transition system are spectrally resolved from emission of other atoms and H2. The CO A 1Π v′ = 14 state is populated by absorption of Lyα photons, created at the accretion column on the stellar surface. For targets with strong CO emission, we model the Lyα radiation field as an input for a simple fluorescence model to estimate CO rotational excitation temperatures and column densities. Typical column densities range from N CO = 1018 to 1019 cm−2. Our measured excitation temperatures are mostly below T CO = 600 K, cooler than typical M-band CO emission. These temperatures and the emission line widths imply that the UV emission originates in a different population of CO than that which is IR-emitting. We also find a significant correlation between CO emission and the disk accretion rate and age. Our analysis shows that ultraviolet CO emission can be a useful diagnostic of CTTS disk gas.

FIRST ULTRAVIOLET REFLECTANCE SPECTRA OF PLUTO AND CHARON BY THEHUBBLE SPACE TELESCOPECOSMIC ORIGINS SPECTROGRAPH: DETECTION OF ABSORPTION FEATURES AND EVIDENCE FOR TEMPORAL CHANGE

We have observed the mid-UV spectra of both Pluto and its large satellite, Charon, at two rotational epochs using the Hubble Space Telescope (HST) Cosmic Origins Spectrograph (COS) in 2010. These are the first HST/COS measurements of Pluto and Charon. Here we describe the observations and our reduction of them, and present the albedo spectra, average mid-UV albedos, and albedo slopes we derive from these data. These data reveal evidence for a strong absorption feature in the mid-UV spectrum of Pluto; evidence for temporal change in Pluto's spectrum since the 1990s is reported, and indirect evidence for a near-UV spectral absorption on Charon is also reported.

CO AND H2 ABSORPTION IN THE AA TAURI CIRCUMSTELLAR DISK*

ABSTRACT The direct study of molecular gas in inner protoplanetary disks is complicated by uncertainties in the spatial distribution of the gas, the time variability of the source, and the comparison of observations across a wide range of wavelengths. Some of these challenges can be mitigated with far-ultraviolet spectroscopy. Using new observations obtained with the Hubble Space Telescope Cosmic Origins Spectrograph, we measure column densities and rovibrational temperatures for CO and H2 observed on the line of sight through the AA Tauri circumstellar disk. CO A – X absorption bands are observed against the far-UV continuum. The CO absorption is characterized by log10(N(12CO)) = 17.5 ± 0.5 cm−2 and T rot(CO) = 500+500 −200 K, although this rotational temperature may underestimate the local kinetic temperature of the CO-bearing gas. We also detect 13CO in absorption with an isotopic ratio of ∼20. We do not observe H2 absorption against the continuum; however, hot H2 (v &gt; 0) is detected in absorption against the Lyα emission line. We measure the column densities in eight individual rovibrational states, determining a total log10(N(H2)) = 17.9+0.6 −0.3 cm−2 with a thermal temperature of T(H2) = 2500+800 −700 K. The high temperature of the molecules, the relatively small H2 column density, and the high inclination of the AA Tauri disk suggest that the absorbing gas resides in an inner disk atmosphere. If the H2 and CO are cospatial within a molecular layer ∼0.6 AU thick, this region is characterized by ∼ 105 cm−3 with an observed 〈CO/H2〉 ratio of ∼0.4. We also find evidence for a departure from a purely thermal H2 distribution, suggesting that excitation by continuum photons and H2 formation may be altering the level populations in the molecular gas.

THE FAR-ULTRAVIOLET “CONTINUUM” IN PROTOPLANETARY DISK SYSTEMS. II. CARBON MONOXIDE FOURTH POSITIVE EMISSION AND ABSORPTION*

ABSTRACT We exploit the high sensitivity and moderate spectral resolution of the Hubble Space Telescope Cosmic Origins Spectrograph to detect far-ultraviolet (UV) spectral features of carbon monoxide (CO) present in the inner regions of protoplanetary disks for the first time. We present spectra of the classical T Tauri stars HN Tau, RECX-11, and V4046 Sgr, representative of a range of CO radiative processes. HN Tau shows CO bands in absorption against the accretion continuum. The CO absorption most likely arises in warm inner disk gas. We measure a CO column density and rotational excitation temperature of N(CO) = (2 ± 1) × 1017 cm−2 and T rot(CO) 500 ± 200 K for the absorbing gas. We also detect CO A–X band emission in RECX-11 and V4046 Sgr, excited by UV line photons, predominantly H i Lyα. All three objects show emission from CO bands at λ &gt; 1560 Å, which may be excited by a combination of UV photons and collisions with non-thermal electrons. In previous observations these emission processes were not accounted for due to blending with emission from the accretion shock, collisionally excited H2, and photo-excited H2, all of which appeared as a “continuum” whose components could not be separated. The CO emission spectrum is strongly dependent upon the shape of the incident stellar Lyα emission profile. We find CO parameters in the range: N(CO) ∼ 1018–1019 cm−2, T rot(CO) ≳ 300 K for the Lyα-pumped emission. We combine these results with recent work on photo-excited and collisionally excited H2 emission, concluding that the observations of UV-emitting CO and H2 are consistent with a common spatial origin. We suggest that the CO/H2 ratio (≡ N(CO)/N(H2)) in the inner disk is ∼1, a transition between the much lower interstellar value and the higher value observed in solar system comets today, a result that will require future observational and theoretical study to confirm.

HST-COS observations of the transiting extrasolar planetary system HD 209458b

We summarize results from deep spectroscopic observations of the HD 209458 planetary system, carried out with the Hubble Space Telescope—Cosmic Origins Spectrograph. Orbitally resolved observations are used to show that hot gas emission lines, arising only in the stellar atmosphere, are not variable, while lower ionizations species found in the upper atmosphere of the hot Jupiter HD 209458b absorb stellar photons during transit. For both C II and Si III, we find mean transit attenuation of ∼8%. The firm detection of silicon is in direct conflict with previous low-resolution studies, which we attribute to long-term variability in the system. We also use these observations to search for auroral emission from the planet, detecting a statistically significant emission feature at 1582 A that is consistent with H2 photoexcited by stellar O I photons.

Multilayer resonant subwavelength gratings: effects of waveguide modes and real groove profiles

The boundary integral equation code PCGrate-S(X) is used to analyze diffraction on Hubble Space Telescope Cosmic Origins Spectrograph gratings at different boundary shapes and layer thicknesses. An effect of resonance anomalies excited in nonconformal dielectric layers overcoated on the surface of metallic grating on the efficiency is studied for the first time to our knowledge. Refractive indices (RIs) for bulk MgF2 taken from well-known references are found to be not suitable for thin optical layers at wavelengths between 115 and 170 nm. A method based on scale fitting of calculated and measured grating efficiencies is outlined for derivation of thin-film optical constants at hard to measure wavelengths. The calculated efficiency based on real boundary profiles and derived RIs of the G185M subwavelength grating is shown to fit within 9.6% or better to the measured data.

Universal method for holographic grating recording: multimode deformable mirrors generating Clebsch–Zernike polynomials

Recording methods for making aberration-corrected holographic gratings are greatly simplified by use of a plane multimode deformable mirror (MDM) upon one of the two recording beams. It is shown that MDM compensators easily provide the superposition of many interesting active optics modes, which we have named Clebsch-Zernike modes. When we apply only a uniform loading or no loading at all onto the rear side of the MDM clear aperture, the available Clebsch-Zernike modes are made to belong to a subclass of the Zernike modes that includes the three modes of the third-order aberration theory as well as a well-defined part of the Zernike higher-order modes. Such a recording method is considered to be universal, since it does not require the use of a sophisticated optical system such as a compensator. Active optics 12-arm MDM's in the vase form have been designed from the elasticity theory. The design of six-arm MDM's is currently carried out with theoretical results. As an example of the method, the recording of three holographic gratings of the Hubble Space Telescope Cosmic Origins Spectrograph has been investigated. Substantial improvements in image quality have been found by use of a six-arm MDM as recording compensator. The result is that aberrations of much higher order can simultaneously be corrected so that the residual blur images of the spectra occupy areas approximately 10 (direction of dispersion) x 3 (cross dispersion) = 30 times smaller--also in terms of pixel number--than those obtained by our American colleagues. Therefore the active optics recording method appears to provide substantial gains in resolving power and in sensitivity: (i) For all three gratings the spectral resolution would be increased by a factor of 10, and (ii), in addition, for the two higher dispersion gratings, the limiting magnitude on the sky appears to be increased by a magnitude of approximately 1-1.2.