Browsing by Subject "helium"

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The ACS Survey Of Galactic Globular Clusters. III. The Double Subgiant Branch Of NGC 1851
(2008-01) Milone, A. P.; Bedin, L. R.; Piotto, G.; Anderson, J.; King, I. R.; Sarajedini, A.; Dotter, A.; Chaboyer, B.; Marin-Franch, A.; Majewski, S.; Aparicio, A.; Hempel, M.; Paust, N. E. Q.; Reid, I. N.; Rosenberg, A.; Siegel, M.; Siegel, M.
Photometry with the Hubble Space Telescope Advanced Camera for Surveys (HSTACS) reveals that the subgiant branch (SGB) of the globular cluster NGC 1851 splits into two well-defined branches. If the split is due only to an age effect, the two SGBs would imply two star formation episodes separated by similar to 1 Gyr. We discuss other anomalies in NGC 1851 that could be interpreted in terms of a double stellar population. Finally, we compare the case of NGC 1851 with the other two globulars known to host multiple stellar populations, and show that all three clusters differ in several important respects.
Chandra Grating Spectroscopy Of Three Hot White Dwarfs
(2012-10) Adamczak, J.; Werner, K.; Rauch, T.; Schuh, S.; Drake, J. J.; Kruk, J. W.; Adamczak, J.
Context. High-resolution soft X-ray spectroscopic observations of single hot white dwarfs are scarce. With the Chandra Low-Energy Transmission Grating, we have observed two white dwarfs, one is of spectral type DA (LB 1919) and the other is a non-DA of spectral type PG1159 (PG1520+525). The spectra of both stars are analyzed, together with an archival Chandra spectrum of another DA white dwarf (GD246). Aims. The soft X-ray spectra of the two DA white dwarfs are investigated in order to study the effect of gravitational settling and radiative levitation of metals in their photospheres. LB1919 is of interest because it has a significantly lower metallicity than DAs with otherwise similar atmospheric parameters. GD246 is the only white dwarf known that shows identifiable individual iron lines in the soft X-ray range. For the PG1159 star, a precise effective temperature determination is performed in order to confine the position of the blue edge of the GW Vir instability region in the HRD. Methods. The Chandra spectra are analyzed with chemically homogeneous as well as stratified NLTE model atmospheres that assume equilibrium between gravitational settling and radiative acceleration of chemical elements. Archival EUV and UV spectra obtained with EUVE, FUSE, and HST are utilized to support the analysis. Results. No metals could be identified in LB1919. All observations are compatible with a pure hydrogen atmosphere. This is in stark contrast to the vast majority of hot DA white dwarfs that exhibit light and heavy metals and to the stratified models that predict significant metal abundances in the atmosphere. For GD246 we find that neither stratified nor homogeneous models can fit the Chandra spectrum. The Chandra spectrum of PG1520+525 constrains the effective temperature to T-eff = 150 000 +/- 10 000 K. Therefore, this nonpulsating star together with the pulsating prototype of the GWVir class (PG 1159-035) defines the location of the blue edge of the GWVir instability region. The result is in accordance with predictions from nonadiabatic stellar pulsation models. Such models are therefore reliable tools to investigate the interior structure of GW Vir variables. Conclusions. Our soft X-ray study reveals that the understanding of metal abundances in hot DA white dwarf atmospheres is still incomplete. On the other hand, model atmospheres of hydrogen-deficient PG 1159-type stars are reliable and reproduce well the observed spectra from soft X-ray to optical wavelengths.
Comparative Analysis of Process Stability in PBF-LB/M: (Thermal) Highspeed Imaging vs. Melt Pool Monitoring using Novel Gas Mixtures
(University of Texas at Austin, 2023) Deckers, T.; Wolf, F.; Foret, P.; Witt, G.; Kleszczynski, S.
Powder bed fusion of metals using a laser beam (PBF-LB/M) is increasingly gaining popularity in the industry. However, ensuring a consistent quality of parts processed by PBF-LB/M is crucial to compete with established manufacturing processes. In-situ process monitoring systems, such as coaxial melt pool monitoring (MPM), can contribute to this goal by minimizing post-process quality control. Three monitoring systems, a commercially available MPM system, an optical high-speed camera, and a thermal high-speed camera, were compared to identify process phenomena. Secondly, the suitability of the MPM system for in-situ quality control was tested by employing novel gas mixtures in the process. The mixtures include argon (Ar) with hydrogen (H2), helium (He), and carbon dioxide (CO2). The first results showed the capabilities of the MPM system to monitor relevant process anomalies. Also, the addition of He and H2 to the process gas resulted in an improvement in the melt pool stability and a reduction of process by-products compared to Ar.
A continuous lithium atomic beam source for cryogenic apparatus
(2010) Stoll, Brady; Daniel Heinzen
We have created an atomic beam source of lithium atoms for use in a cryogenic environment. This beam source is expected to provide a flux of 10 to the 18 atoms/s. The main features of this source are a reservoir chamber of solid lithium heated with heater wire, creating a lithium gas that travels down an ohmically heated tube and exits as an effusive beam. That beam is directed at a cold supersonic jet of helium. As lithium atoms enter the helium gas they rapidly thermalize and become entrained. The design challenges faced included directing the heat load away from the cold elements of the cryogenic chamber, obtaining the correct temperatures to provide the desired flux, and measuring the beam as it exited the reservoir. Through the use of laser induced fluorescence, we believe we have observed capture of lithium by the helium jet. Such a beam source can be used to form an intense cold atom source with the potential future application of serving as a pump source for an atom laser.
Discovery Of Pulsations, Including Possible Pressure Modes, In Two New Extremely Low Mass, He-Core White Dwarfs
(2013-03) Hermes, J. J.; Montgomery, Michael H.; Winget, D. E.; Brown, Warren R.; Gianninas, A.; Kilic, Mukremin; Kenyon, S. J.; Bell, Keaton J.; Harrold, Samuel T.; Hermes, J. J.; Montgomery, Michael H.; Winget, D. E.; Bell, Keaton J.; Harrold, Samuel T.
We report the discovery of the second and third pulsating extremely low mass (ELM) white dwarfs (WDs), SDSS J111215.82+111745.0 (hereafter J1112) and SDSS J151826.68+065813.2 (hereafter J1518). Both have masses < 0.25 M-circle dot and effective temperatures below 10,000 K, establishing these putatively He-core WDs as a cooler class of pulsating hydrogen-atmosphere WDs (DAVs, or ZZ Ceti stars). The short-period pulsations evidenced in the light curve of J1112 may also represent the first observation of acoustic (p-mode) pulsations in any WD, which provide an exciting opportunity to probe this WD in a complimentary way compared to the long-period g-modes that are also present. J1112 is a T-eff = 9590 +/- 140 K and log g = 6.36 +/- 0.06 WD. The star displays sinusoidal variability at five distinct periodicities between 1792 and 2855 s. In this star, we also see short-period variability, strongest at 134.3 s, well short of the expected g-modes for such a low-mass WD. The other new pulsating WD, J1518, is a Teff = 9900 +/- 140 K and log g = 6.80 +/- 0.05 WD. The light curve of J1518 is highly non-sinusoidal, with at least seven significant periods between 1335 and 3848 s. Consistent with the expectation that ELM WDs must be formed in binaries, these two new pulsating He-core WDs, in addition to the prototype SDSS J184037.78+642312.3, have close companions. However, the observed variability is inconsistent with tidally induced pulsations and is so far best explained by the same hydrogen partial-ionization driving mechanism at work in classic C/O-core ZZ Ceti stars.
A Gravitational Redshift Determination Of The Mean Mass Of White Dwarfs. DA Stars
(2010-03) Falcon, Ross E.; Winget, D. E.; Montgomery, Michael H.; Williams, Kurtis A.; Falcon, Ross E.; Winget, D. E.; Montgomery, Michael H.; Williams, Kurtis A.
We measure apparent velocities (v(app)) of the H alpha and H beta Balmer line cores for 449 non-binary thin disk normal DA white dwarfs (WDs) using optical spectra taken for the European Southern Observatory SN Ia progenitor survey (SPY). Assuming these WDs are nearby and comoving, we correct our velocities to the local standard of rest so that the remaining stellar motions are random. By averaging over the sample, we are left with the mean gravitational redshift, < v(g)>: we find < v(g)> = < v(app)> = 32.57 +/- 1.17 km s(-1). Using the mass-radius relation from evolutionary models, this translates to a mean mass of 0.647(-0.014)(+0.013)M(circle dot). We interpret this as the mean mass for all DAs. Our results are in agreement with previous gravitational redshift studies but are significantly higher than all previous spectroscopic determinations except the recent findings of Tremblay & Bergeron. Since the gravitational redshift method is independent of surface gravity from atmosphere models, we investigate the mean mass of DAs with spectroscopic T(eff) both above and below 12,000 K; fits to line profiles give a rapid increase in the mean mass with decreasing T(eff). Our results are consistent with no significant change in mean mass: < M >(hot) = 0.640 +/- 0.014 M(circle dot) and < M >(cool) = 0.686(-0.039)(+0.035)M(circle dot).
Helium charge exchange recombination spectroscopy on Alcator C-Mod Tokamak
(2014-05) Liao, Kenneth Teh-Yong; Gentle, Kenneth W.; Rowan, William L.
The Wide-View Charge Exchange Recombination Spectroscopy (CXRS) diagnostic at Alcator C-Mod, originally designed for measurement of boron, has been modified to fit several different roles. By measuring the He¹⁺ (n = 4 [rightwards arrow] 3) emission line at 4686Å and surrounding spectra, we can measure ⁴He and ³He density, temperature, and velocity profiles and use this information to study turbulent impurity transport. The transport is characterized using a standard ansatz for the radial particle flux: [mathematical equation]. This effort is designated He CXRS. Also, direct measurement of ³He are used to test models of Ion Cyclotron Resonance Heating (ICRH). We look for evidence of fast ion production and the effect of the minority ion profile on fast wave heating. Several modifications were made to the hardware. Light is collected via two optical arrays: poloidal and toroidal. The toroidal array has been upgraded to increase throughput and spatial resolution, increasing the number of toroidal channels from 10 to 22. A new protective shroud was installed on the poloidal array. Additional diagnostics (a 11 channel beam duct view, neutralizer view, duct pressure monitor) were added to the Diagnostic Neutral Beam to improve DNB modeling for CXRS. This work includes investigation of plasmas where helium is at low concentration (<1%), acting passively, as well as scenarios with a large fraction (>~20%). Using the STRAHL code, time-dependent helium density profiles are used to obtain anomalous transport parameters. Thermodiffusion and curvature pinch terms are also estimated from experimental scaling studies. Results are compared with neoclassical results from the NCLASS code and calculations by the GENE gyrokinetic code. Another focus is verification of power deposition models which are crucially dependent on minority ion density, for which ³He is used. At low ³He fraction, direct absorption by ³He generates fast ions with anisotropic velocity-space distribution functions. At high ³He fraction, mode conversion heating of electrons is dominant. The minority distribution function and predicted wave deposition are simulated using AORSA and CQL3D. This work provides the first measurements of helium transport on C-Mod and expands our understanding of helium transport and fast wave heating.
Kepler Observations of Transiting Hot Compact Objects
(2010-04) Rowe, Jason F.; Borucki, William J.; Koch, David; Howell, Steve B.; Basri, Gibor; Batalha, Natalie; Brown, Timothy M.; Caldwell, Douglas; Cochran, William D.; Dunham, Edward; Dupree, Andrea K.; Fortney, Jonathan J.; Gautier, Thomas N., III; Gilliland, Ronald L.; Jenkins, Jon; Latham, David W.; Lissauer, Jack J.; Marcy, Geoff; Monet, David G.; Sasselov, Dimitar; Welsh, William F.; Cochran, William D.
Kepler photometry has revealed two unusual transiting companions: one orbiting an early A-star and the other orbiting a late B-star. In both cases, the occultation of the companion is deeper than the transit. The occultation and transit with follow-up optical spectroscopy reveal a 9400 K early A-star, KOI-74 (KIC 6889235), with a companion in a 5.2 day orbit with a radius of 0.08R(circle dot) and a 10,000 K late B-star KOI-81 (KIC 8823868) that has a companion in a 24 day orbit with a radius of 0.2R(circle dot). We infer a temperature of 12,250 K for KOI-74b and 13,500 K for KOI-81b. We present 43 days of high duty cycle, 30 minute cadence photometry, with models demonstrating the intriguing properties of these objects, and speculate on their nature.
Presentation: Origins of the Universe
(Environmental Science Institute, 2001-03-30) Bash, Frank; Environmental Science Institute; PDF and PPT updated 10/12/17.
Radius Constraints From High-Speed Photometry Of 20 Low-Mass White Dwarf Binaries
(2014-09) Hermes, J. J.; Brown, Warren R.; Kilic, Mukremin; Gianninas, A.; Chote, Paul; Sullivan, Denis J.; Winget, D. E.; Bell, Keaton J.; Falcon, Ross E.; Winget, K. I.; Mason, Paul A.; Harrold, Samuel T.; Montgomery, Michael H.; Hermes, J. J.; Winget, K. I.; Harrold, Samuel T.; Montgomery, Michael H.
We carry out high-speed photometry on 20 of the shortest-period, detached white dwarf binaries known and discover systems with eclipses, ellipsoidal variations (due to tidal deformations of the visible white dwarf), and Doppler beaming. All of the binaries contain low-mass white dwarfs with orbital periods of less than four hr. Our observations identify the first eight tidally distorted white dwarfs, four of which are reported for the first time here. We use these observations to place empirical constraints on the mass-radius relationship for extremely low-mass (<= 0.30 M-circle dot) white dwarfs. We also detect Doppler beaming in several of these binaries, which confirms their high-amplitude radial-velocity variability. All of these systems are strong sources of gravitational radiation, and long-term monitoring of those that display ellipsoidal variations can be used to detect spin-up of the tidal bulge due to orbital decay.
Rapid Orbital Decay in the 12.75-Minute Binary White Dwarf J0651+2844
(2012-10) Hermes, J. J.; Kilic, Mukremin; Brown, Warren R.; Winget, D. E.; Prieto, Carlos Allende; Gianninas, A.; Mukadam, Anjum S.; Cabrera-Lavers, Antonio; Kenyon, Scott J.; Hermes, J. J.; Winget, D. E.
We report the detection of orbital decay in the 12.75-minute, detached binary white dwarf (WD) SDSS J065133.338+284423.37 (hereafter J0651). Our photometric observations over a 13 month baseline constrain the orbital period to 765.206543(55) s and indicate that the orbit is decreasing at a rate of (-9.8 +/- 2.8) x 10(-12) s s(-1) (or -0.31 +/- 0.09 ms yr(-1)). We revise the system parameters based on our new photometric and spectroscopic observations: J0651 contains two WDs with M-1 = 0.26 +/- 0.04 M-circle dot and M-2 = 0.50 +/- 0.04 M-circle dot. General relativity predicts orbital decay due to gravitational wave radiation of (-8.2 +/- 1.7) x 10(-12) s s(-1) (or -0.26 +/- 0.05 ms yr(-1)). Our observed rate of orbital decay is consistent with this expectation. J0651 is currently the second-loudest gravitational wave source known in the milli-Hertz range and the loudest non-interacting binary, which makes it an excellent verification source for future missions aimed at directly detecting gravitational waves. Our work establishes the feasibility of monitoring this system's orbital period decay at optical wavelengths.
Relativistic Ab initio Study On PtF and HePtF
(2010-01) Zou, Wenli L.; Liu, Yang; Boggs, James E.; Zou, Wenli L.; Liu, Yang; Boggs, James E.
The electronic structures and spectroscopic constants of the first three low-lying electronic states (Omega = 1/2, 3/2, and 5/2) of the linear HePtF complex were investigated by highly accurate relativistic ab initio methods, in which the spin-orbit coupling was taken into account, and compared with the results of PtF. It shows that the complex is significantly different from the typical van der Waals systems because of short He-Pt bond distances (1.80 similar to 1.87 angstrom), large He-Pt stretching frequencies (500 similar to 600 cm(-1)), considerable binding energies (1400 similar to 2500 cm(-1) with corrections), and a small electron transfer from helium (about 0.06). However, the topological analysis of the electron density distribution indicates that there is strong van der Waals interaction in the He-Pt bond instead of weak covalent one.
SDSS J184037.78+642312.3: the First Pulsating Extremely Low Mass White Dwarf
(2012-05) Hermes, J. J.; Montgomery, M. H.; Winget, D. E.; Brown, Warren R.; Kilic, Mukremin; Kenyon, Scott J.; Hermes, J. J.; Montgomery, M. H.; Winget, D. E.
We report the discovery of the first pulsating extremely low mass (ELM) white dwarf (WD), SDSS J184037.78+642312.3 (hereafter J1840). This DA (hydrogen-atmosphere) WD is by far the coolest and the lowest-mass pulsating WD, with T-eff = 9100 +/- 170 K and log g = 6.22 +/- 0.06, which corresponds to a mass of similar to 0.17 M-circle dot. This low-mass pulsating WD greatly extends the DAV (or ZZ Ceti) instability strip, effectively bridging the log g gap between WDs and main-sequence stars. We detect high-amplitude variability in J1840 on timescales exceeding 4000 s, with a non-sinusoidal pulse shape. Our observations also suggest that the variability is multi-periodic. The star is in a 4.6 hr binary with another compact object, most likely another WD. Future, more extensive time-series photometry of this ELM WD offers the first opportunity to probe the interior of a low-mass, presumably He-core WD using the tools of asteroseismology.
The Seismic Properties Of Low-Mass He-Core White Dwarf Stars
(2012-11) Corsico, A. H.; Romero, A. D.; Althaus, L. G.; Hermes, J. J.; Hermes, J. J.
Context. In recent years, many low-mass (less than or similar to 0.45 M-circle dot) white dwarf stars expected to harbor He cores have been detected in the field of the Milky Way and in several galactic globular and open clusters. Until recently, no objects of this kind showed pulsations. This situation has changed recently with the exciting discovery of SDSS J184037.78+642312.3, the first pulsating low-mass white dwarf star. Aims. Motivated by this extremely important finding, and in view of the very valuable asteroseismological potential of these objects, we present here a detailed pulsational study applied to low-mass He-core white dwarfs with masses ranging from 0.17 to 0.46 M-circle dot, based on full evolutionary models representative of these objects. This study is aimed to provide a theoretical basis from which to interpret future observations of variable low-mass white dwarfs. Methods. The background stellar models on which our pulsational analysis was carried out were derived by taking into account the complete evolutionary history of the progenitor stars, with special emphasis on the diffusion processes acting during the white dwarf cooling phase. We computed nonradial g-modes to assess the dependence of the pulsational properties of these objects with stellar parameters such as the stellar mass and the effective temperature, and also with element diffusion processes. We also performed a g- and p-mode pulsational stability analysis on our models and found well-defined blue edges of the instability domain, where these stars should start to exhibit pulsations. Results. We found substantial differences in the seismic properties of white dwarfs with M-* less than or similar to 0.20 M-circle dot and the extremely low-mass (ELM) white dwarfs (M-* less than or similar to 0.20 M-circle dot). Specifically, g-mode pulsation modes in ELM white dwarfs mainly probe the core regions and are not dramatically affected by mode-trapping effects by the He/H interface, whereas the opposite is true for more massive He-core white dwarfs. We found that element diffusion processes substantially affects the shape of the He/ H chemical transition region, leading to non-negligible changes in the period spectrum of low-mass white dwarfs, in particular in the range of stellar masses characteristic of ELM objects. Finally, our stability analysis successfully predicts the pulsations of the only known variable low-mass white dwarf (SDSS J184037.78+642312.3) at the right effective temperature, stellar mass and range of periods. Conclusions. Our computations predict both g-and p-mode pulsational instabilities in a significant number of known low-mass and ELM white dwarfs. It is worth observing these stars in order to discover if they pulsate.
The Spectral Evolution of Convective Mixing White Dwarfs, the Non-DA Gap, and White Dwarf Cosmochronology
(2012-07) Chen, E. Y.; Hansen, B. M. S.; Chen, E. Y.
The spectral distribution of field white dwarfs shows a feature called the "non-DA gap." As defined by Bergeron et al., this is a temperature range (5100-6100 K) where relatively few non-DA stars are found, even though such stars are abundant on either side of the gap. It is usually viewed as an indication that a significant fraction of white dwarfs switch their atmospheric compositions back and forth between hydrogen-rich and helium-rich as they cool. In this Letter, we present a Monte Carlo model of the Galactic disk white dwarf population, based on the spectral evolution model of Chen and Hansen. We find that the non-DA gap emerges naturally, even though our model only allows white dwarf atmospheres to evolve monotonically from hydrogen-rich to helium-rich through convective mixing. We conclude by discussing the effects of convective mixing on the white dwarf luminosity function and the use thereof for Cosmochronology.
Two New Tidally Distorted White Dwarfs
(2012-04) Hermes, J. J.; Kilic, Mukremin; Brown, Warren R.; Montgomery, Michael H.; Winget, D. E.; Hermes, J. J.; Montgomery, Michael H.; Winget, D. E.
We identify two new tidally distorted white dwarfs (WDs), SDSS J174140.49+652638.7 and J211921.96-001825.8 (hereafter J1741 and J2119). Both stars are extremely low mass (ELM, <= 0.2 M-circle dot)WDs in short-period, detached binary systems. High-speed photometric observations obtained at the McDonald Observatory reveal ellipsoidal variations and Doppler beaming in both systems; J1741, with a minimum companion mass of 1.1 M-circle dot, has one of the strongest Doppler beaming signals ever observed in a binary system (0.59% +/- 0.06% amplitude). We use the observed ellipsoidal variations to constrain the radius of each WD. For J1741, the star's radius must exceed 0.074R(circle dot). For J2119, the radius exceeds 0.10R(circle dot). These indirect radius measurements are comparable to the radius measurements for the bloated WD companions to A-stars found by the Kepler spacecraft, and they constitute some of the largest radii inferred for any WD. Surprisingly, J1741 also appears to show a 0.23% +/- 0.06% reflection effect, and we discuss possible sources for this excess heating. Both J1741 and J2119 are strong gravitational wave sources, and the time-of-minimum of the ellipsoidal variations can be used to detect the orbital period decay. This may be possible on a timescale of a decade or less.