Engineering topics

Hillenbrand, Lynne A.

Publications and source records attributed to Hillenbrand, Lynne A..

10 records

DOE OSTI2021

Cataclysmic Variables in the Second Year of the Zwicky Transient Facility

Using a filter in the GROWTH Marshal based on color and the amplitude and timescale of variability, we have identified 372 objects as known or candidate cataclysmic variables (CVs) during the second year of the operation of the Zwicky Transient Facility. From the available difference imaging data, we found that 93 are previously confirmed CVs and 279 are strong candidates. Spectra of four of the candidates confirm them as CVs by the presence of Balmer emission lines, while one of the four has prominent He ii lines indicative of containing a magnetic white dwarf. Gaia EDR3 parallaxes are available for 154 of these systems, resulting in distances from 108–2096 pc and absolute magnitudes in the range of 7.5–15.0, with the largest number of candidates between 10.5 and 12.5. The total numbers are 21% higher than from the previous year of the survey with a greater number of distances available but a smaller percentage of systems close to the Galactic plane. Comparison of these findings with a machine-learning method of searching all the light curves reveals large differences in each data set related to the parameters involved in the search process.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2021

Outbursting Young Stellar Object PGIR 20dci in the Perseus Arm

We report the discovery of a likely outbursting Class I young stellar object, associated with the star-forming region NGC 281-W (distance ~2.8 kpc). The source is currently seen only at infrared wavelengths, appearing in both the Palomar Gattini InfraRed (1.2 μm) and the Near-Earth Object Wide-field Infrared Survey Explorer (3.4 and 4.6 μm) photometric time-domain surveys. Recent near-infrared imaging reveals a new, extended scattered light nebula. Recent near-infrared spectroscopy confirms the similarity of PGIR 20dci to FU Ori–type sources, based on strong molecular absorption in CO, H{sub 2}O, and OH; weak absorption in several atomic lines; and a warm wind/outflow as indicated by a P Cygni profile in the He i λ10830 line. This is a rare case of an FU Ori star with a well-measured long-term photometric rise before a sharper outburst, and the second instance of an FU Ori star with a documented two-step brightening in the mid-infrared.

74 ATOMIC AND MOLECULAR PHYSICS↗

DOE OSTI2020

An Asymmetric Eclipse Seen toward the Pre-main-sequence Binary System V928 Tau

K2 observations of the weak-lined T Tauri binary V928 Tau A and B show the detection of a single, asymmetric eclipse, which may be due to a previously unknown substellar companion eclipsing one component of the binary with an orbital period >66 days. Over an interval of about 9 hr, one component of the binary dims by around 60%, returning to its normal brightness about 5 hr later. From modeling of the eclipse shape, we find evidence that the eclipsing companion may be surrounded by a disk or a vast ring system. The modeled disk has a radius of 0.9923 ± 0.0005 R&lowast;, with an inclination of 56.°78 ± 0.°03, a tilt of 41.°22 ± 0.°05, an impact parameter of -0.2506 ± 0.0002 R&lowast;, and an opacity of 1.00. The occulting disk must also move at a transverse velocity of 6.637 ± 0.002 R&lowast; day -1 , which, depending on whether it orbits V928 Tau A or B, corresponds to approximately 73.53 or 69.26 km s -1 . A search in ground-based archival data reveals additional dimming events, some of which suggest periodicity, but no unambiguous period associated with the eclipse observed by K2. We present a new epoch of astrometry that is used to further refine the orbit of the binary, presenting a new lower bound of 67 yr, and constraints on the possible orbital periods of the eclipsing companion. The binary is also separated by 18' (~2250 au) from the lower-mass CFHT-BD-Tau 7, which is likely associated with V928 Tau A and B. We also present new high-dispersion optical spectroscopy that we use to characterize the unresolved stellar binary.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2020

Emission-line Data Cubes of the HH 32 Stellar Jet

We analyze data cubes of over 60 emission lines in the HH 32 stellar jet acquired with the Keck Cosmic Web Imager (KCWI). The data cover the less explored blue portion of the spectrum between 3586 and 6351 Å and have both high spectral (R ∼ 10,000) and spatial (≲1″) resolution. The study includes all three major ionization states of oxygen, three Balmer lines, multiple lines of Fe ii and Fe iii, and the first data cubes ever acquired for important unblended diagnostic lines such as He ii λ4686, Ca i λ3933, and Mg i] λ4571. The data cubes generally sort according to excitation and have a relatively continuous progression from the highest-excitation ions (He ii, O iii) through the intermediate-excitation ions (O i and H i) to the lowest-excitation ions (Ca ii and Mg i). Merging the KCWI cubes with Hubble Space Telescope images leads to several new insights about the flow, including evidence for bow shocks, partial bow shocks, spur shocks, Mach disks, jet deflection shocks, a wiggling jet, and potential shock precursors. The most surprising result is that one of the velocity components of Fe ii in the Mach disk suddenly increases in flux relative to other lines by a factor of two, implying that the Mach disk vaporizes dust in the jet. Hence, jets must accelerate or entrain dust to speeds of over 300 km s{sup −1} without destroying the grains.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2020

Pleiades or Not? Resolving the Status of the Lithium-rich M Dwarfs HHJ 339 and HHJ 430

Oppenheimer et al. discovered two M5 dwarfs in the Pleiades with nearly primordial lithium. These stars are not low enough in mass to represent the leading edge of the lithium depletion boundary at Pleiades age (∼125 Myr). A possible explanation for the enhanced lithium in these stars is that they are actually not members of the Pleiades but instead are members of a younger moving group seen in projection toward the Pleiades. We have used data from Gaia DR2 to confirm that these two stars, HHJ 339 and HHJ 430, are indeed not members of the Pleiades. Based on their space motions, parallaxes, and positions in a Gaia-based color–magnitude diagram, it is probable that these two stars are about 40 parsecs foreground to the Pleiades and have ages of ∼25 Myr. Kinematically they are best matched to the 32 Ori moving group.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2020

Cataclysmic Variables in the First Year of the Zwicky Transient Facility

Using selection criteria based on amplitude, time, and color, we have identified 329 objects as known or candidate cataclysmic variables (CVs) during the first year of testing and operation of the Zwicky Transient Facility. Of these, 90 are previously confirmed CVs, 218 are strong candidates based on the shape and color of their light curves obtained during 3–562 days of observation, and the remaining 21 are possible CVs but with too few data points to be listed as good candidates. Almost half of the strong candidates are within 10 deg of the galactic plane, in contrast to most other large surveys that have avoided crowded fields. The available Gaia parallaxes are consistent with sampling the low mass transfer CVs, as predicted by population models. Our follow-up spectra have confirmed Balmer/helium emission lines in 27 objects, with four showing high-excitation He ii emission, including candidates for an AM CVn, a polar, and an intermediate polar. Our results demonstrate that a complete survey of the Galactic plane is needed to accomplish an accurate determination of the number of CVs existing in the Milky Way.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS2015

CSI 2264: Characterizing Young Stars in NGC 2264 With Short-Duration Periodic Flux Dips in Their Light Curves

We identify nine young stellar objects (YSOs) in the NGC 2264 star-forming region with optical CoRoT light curves exhibiting short-duration, shallow, periodic flux dips. All of these stars have infrared (IR) excesses that are consistent with their having inner disk walls near the Keplerian corotation radius. The repeating photometric dips have FWHM generally less than one day, depths almost always less than 15%, and periods (3 < P < 11 days) consistent with dust near the Keplerian co-rotation period. The flux dips vary considerably in their depth from epoch to epoch, but usually persist for several weeks and, in two cases, were present in data collected on successive years. For several of these stars, we also measure the photospheric rotation period and find that the rotation and dip periods are the same, as predicted by standard \disk-locking" models. We attribute these flux dips to clumps of material in or near the inner disk wall, passing through our line of sight to the stellar photosphere. In some cases, these dips are also present in simultaneous Spitzer IRAC light curves at 3.6 and 4.5 microns. We characterize the properties of these dips, and compare the stars with light curves exhibiting this behavior to other classes of YSO in NGC 2264. A number of physical mechanisms could locally increase the dust scale height near the inner disk wall, and we discuss several of those mechanisms; the most plausible mechanisms are either a disk warp due to interaction with the stellar magnetic field or dust entrained in funnel- ow accretion columns arising near the inner disk wall.

CSI 2264↗

NASA NTRS1993

Angular momentum regulation in low-mass young stars surrounded by accretion disks

From study of a sample of 34 T Tauri stars with photometrically derived rotation periods and spectral types later than KS, we find that the observed periods appear to be related to the presence or absence of an accretion disk. Those stars which we infer to be surrounded by accretion disks have rotation periods P(rot) over 4 days with a most probable P(rot) of about 8.5 days, while those stars which lack accretion disk signatures cover a wide range of P(rot) from 1.5 to 16 days, including a significant number of objects with P(rot) less than 4 days. This suggests the possibility that the 'initial' angular momentum of a star is not established until it dissipates its circumstellar accretion disk. During the disk accretion phase, the stellar angular velocity appears to be regulated at a low value, countering the tendency of the star to spin up both from contraction toward the main sequence and from the accretion of inner disk material of high specific angular momentum. When the accretion disk is dissipated, this regulation mechanism will cease to function. At this point, the star is no longer maintained at a low angular velocity, but is 'free' to conserve its angular momentum, and thus to increase its angular velocity in response to contraction and changes in moment of inertia. This hypothesis, combined with a spread in disk dispersal time scales, provides a context for explaining the observed distribution of stellar rotational velocities for stars on the ZAMS in young clusters.

Edwards, Suzan↗

NASA NTRS1992

Herbig Ae/Be stars - Intermediate-mass stars surrounded by massive circumstellar accretion disks

The proposition that Herbig Ae/Be stars are young intermediate mass stars surrounded by optically thick accretion disks is explored. From a study of 47 such objects, a subset of 30 stars is identified whose spectral energy distributions can be interpreted convincingly in terms of pre-main sequence stars surrounded by massive optically thick circumstellar accretion disks. Constraints on the physical properties of the disks, such as size, mass, accretion rate, lifetime, and radial structure are derived from the photometric data.

Hillenbrand, Lynne A.↗

NASA NTRS1988

Spectroscopic evidence supporting the gravitational lens hypothesis for 1635+267 A,B

The gravitational lens hypothesis is tested for 1613+267 A,B by comparing the detailed line widths and shapes of the 2799-A Mg II and semiforbidden 1909-A C-III lines in each component. Following subtraction of an interpolating polynomial fit to the continua and the determination of a single optimum scaling factor (an amplification ratio of 2.83), reasonable agreement between the profiles of both lines in the two composites is obtained. Comparison of these lines to those in an unrelated quasar with a similar redshift and apparent magnitude does not produce a good match. It is suggested that the observed match in the 1635+267 A,B spectra arises from gravitational lensing.

Turner, Edwin L.↗