Nonadiabatic Analysis Research Articles

High-Resolution Diatomic Spectroscopy near the Dissociation Threshold

Current progress in and prospects for high-resolution molecular laser spectroscopy used for quantum-mechanical modeling of the energy and radiation properties of the rovibronic states of diatomic molecules near the dissociation threshold are discussed at the experimental (spectroscopic) level of accuracy, which is impossible without taking into account all types of intramolecular interactions. The weakly bound, quasibound, and continuum rovibronic states localized near the dissociation threshold actively participate in the formation of stable molecules during spontaneous or laser-stimulated association of colliding atoms, which leads to cooling of the initial reaction medium. Laser-induced fluorescence (LIF) combined with high-resolution Fourier transform spectroscopy is a unique experimental technique, which allows the study of all the three (bound, quasibound, and continuum) parts of the molecular spectrum simultaneously. LIF experiments combined with precision ab initio electronic structure calculations and global nonadiabatic analysis of quasidegenerate rovibronic states converging to the same dissociation limit make it possible to study the structural and dynamic properties of isolated molecules over a very wide range of their electronic-vibrational-rotational excitation.

The interplay of covalency, cooperativity, and coupling strength in governing C-H bond activation in Ni2E2 (E = O, S, Se, Te) complexes.

Dinickel dichalcogenide complexes hold vital multifaceted significance across catalysis, electron transfer, magnetism, materials science, and energy conversion. Understanding their structure, bonding, and reactivity is crucial for all aforementioned applications. These complexes are classified as dichalcogenide, subchalcogenide, or chalcogenide based on metal oxidation and coordinated chalcogen, and due to the associated complex electronic structure, ambiguity often lingers about their classification. In this work, using DFT, CASSCF/NEVPT2, and DLPNO-CCSD(T) methods, we have studied in detail [(NiL)2(E2)] (L = 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane; E = O, S, Se and Te) complexes and explored their reactivity towards C-H bond activation for the first time. Through a comprehensive analysis of the structure, bonding, and reactivity of a series of [(NiL)2(E2)] complexes with E = O, S, Se, and Te, our computational findings suggest that {Ni2O2} and {Ni2S2} are best categorised as dichalcogenide-type complexes. In contrast, {Ni2Se2} and {Ni2Te2} display tendencies consistent with the subchalcogenide classification, and this aligns with the earlier structural correlation proposed (Berry and co-workers, J. Am. Chem. Soc. 2015, 137, 4993) reports on the importance of the E-E bond strength. Our study suggests the reactivity order of {Ni2O2} > {Ni2S2} > {Ni2Se2} > {Ni2Te2} for C-H bond activation, and the origin of the difference in reactivity was attributed to the difference in the Ni-E bond covalency, and electronic cooperativity between two Ni centres that switch among the classification during the reaction. Further non-adiabatic analysis at the C-H bond activation step demonstrates a decrease in coupling strength as we progress down the group, indicating a correlation with metal-ligand covalency. Notably, the reactivity trend is found to be correlated to the strength of the antiferromagnetic exchange coupling constant J via developing a magneto-structural-barrier map - offering a hitherto unknown route to fine-tune the reactivity of this important class of compound.

Pulsation in faint blue stars

ABSTRACT Following the discovery of blue large-amplitude pulsators (BLAPs) by the OGLE survey, additional hot, high-amplitude pulsating stars have been discovered by the Zwicky Transient Facility. It has been proposed that all of these objects are low-mass pre-white dwarfs and that their pulsations are driven by the opacity of iron-group elements. With this expanded population of pulsating objects, it was decided to compute a sequence of post-common-envelope stellar models using the mesa stellar evolution code and to examine the pulsation properties of low-mass pre-white dwarfs using non-adiabatic analysis with the gyre stellar oscillation code. By including the effects of atomic diffusion and radiative levitation, it is shown that a large region of instability exists from effective temperatures of 30 000 K up to temperatures of at least 50 000 K and at a wide range of surface gravities. This encompasses both groups of pulsator observed so far, and confirms that the driving mechanism is through iron group element opacity. We make some conservative estimates about the range of periods, masses, temperatures, and gravities in which further such pulsators might be observed.

Early hydration calorimetric study of the sewage sludge incinerated waste streams Portland cement-based binders: technological implications

The research study forms a part of the “Management of municipal water waste treatment plants potential by-products of sewage sludge ash type, as active or non-active additions to Portland cement-based binders” project’s outcomes (H2020-MSCA-IF-2016-746830). Based on the applied techniques characteristics (non-isothermal and non-adiabatic differential micro-calorimeter, SEM/EDS and XRF analysis), this study comprises the very first stage for a sustainable conversion of each of the considered 190114 and 190107* waste streams into a value-added by-product for specific constructive applications. Provided technical requirements regarding the designed cementitious products handling and suggested industrial application are focused on the early performance of the tested for quality and consistency, pastes, mortars and concretes. In detail, the obtained physical–chemical parameters show substantial differences of the original waste streams regarding their technological origin, seasonability and geographical localisation. Consequently, the calorimetric curves exhibit diverse early chemistry of the produced cements, showing the considerable variations in mineral nature (siliceous, aluminous) or hydraulic factor contents (reactive silica, SiO2r−, and reactive alumina, Al2O3r−) of the observed pozzolanic activities. Setting retarding phenomenon and shrinkage reducing effects are also carefully discussed. All the calorimetric and pozzolanic activity related data is contrasted with two other reference materials, fly ash and granulated blast furnace slag, standard behaviour.

Period–luminosity relations of fast-rotating B-type stars in the young open cluster NGC 3766

We study the pulsational properties of rapidly rotating main-sequence B-type stars using linear non-adiabatic analysis of non-radial low-frequency modes taking into account the effect of rotation. We compare the properties of prograde sectoral $g$ and retrograde $r$ modes excited by the $\kappa$ mechanism at the Fe opacity peak with the newly discovered period-luminosity relation that is obeyed by a group of fast-rotating B-type stars in the young open cluster NGC 3766. The observed relation consists of two sequences in the period versus magnitude diagram, at periods shorter than 0.5 days. We find that this property is consistent with similar period-luminosity relations predicted for excited sectoral prograde $g$-modes of azimuthal orders $m=-1$ and $m=-2$ in fast-rotating stars along an isochrone. We further show that some of the rapidly rotating stars that have photometric variability with periods longer than a day may be caused by $r$-mode pulsation predicted to be excited in these stars. One fast-rotating star, in particular, shows both short and long periods that can be explained by the simultaneous excitation of $g$- and $r$-mode pulsations in models of fast-rotating stars.

The fundamental parameters of the Ap star 78 Virginis

Context. Determining the effective temperature of Ap stars, including the roAp stellar pulsators, is a difficult task owing to their strong magnetic field and their related spotted surfaces. It is, however, an important step towards constraining models of their complex atmosphere and testing proposed pulsation excitation mechanisms. Aims. Using the unique angular resolution provided by long-baseline visible interferometry, we aim at deriving accurate angular diameters of a number of Ap targets, so as to determine their unbiased effective temperature (Teff) and their accurate position in the Hertzsprung-Russell diagram, to estimate their mass and age, and to test non-adiabatic pulsation models. Interferometric results on four Ap stars have been published in earlier works. Here we report the results on a fifth, significantly hotter star. Methods. We observed 78 Vir with the visible spectrograph VEGA installed at the combined focus of the CHARA long-baseline optical array. We derived the limb-darkened diameter of this Ap star from our interferometric measurements. Based on photometric and spectroscopic data available in the literature, we estimated the star’s bolometric flux and used it, in combination with its parallax and angular diameter, to determine the star’s luminosity and effective temperature. We then used the derived fundamental parameters to perform a non-adiabatic pulsation analysis. Results. We determined a limb-darkened angular diameter of 0.346 ± 0.006 mas and deduced a linear radius of R = 2.11 ± 0.04 R� . Considering a bolometric flux of 2.73 ± 0.20 10 −7 erg/cm 2 /s we obtained a luminosity of L/L� = 27 ± 2a nd an effective temperature of Teff = 9100 ± 190 K. The non-adiabatic pulsation modeling allows us to predict that high overtone pulsations could be excited in 78 Vir at frequencies ranging from 1.2 to 1.9 mHz, provided that the magnetic field is capable of suppressing envelope convection in the polar regions. Conclusions. Visible long-baseline interferometry is a unique means of deriving accurate fundamental parameters of Ap stars. The Ap star 78 Vir is found to be a promising roAp-star candidate and one that would allow us to extend recent tests on the roAp stars’ excitation mechanism towards the blue edge of the instability strip. Asteroseismic data of this star would, therefore, be of strong interest.

Nonadiabatic analysis of rosette modes of oscillations in rotating stars and their contribution to angular momentum transport

Rosette modes compose a peculiar group of oscillation modes in rotating stars. Although they have been investigated in some theoretical works, it has not been clear yet whether these modes are excited in real stars or not. In order to study this problem, a stability analysis of the modes is performed. It turns out that some rosette modes are unstable due to the kappa mechanism in a model of slowly pulsating B stars. It is also examined how unstable rosette modes can transport angular momentum in stars.

Nonadiabatic analysis of strange-modes in hot massive stars with time-dependent convection

We carry out nonadiabatic analysis of strange-modes in hot massive stars with time-dependent convection (TDC). We find that the instability of the modes excited at the Fe bump is weaker with TDC than with frozen-in convection (FC). But the instability still remains with TDC, and could be a possible candidate for the trigger of luminous blue variable (LBV) phenomena.

Pulsations of pre-white dwarfs with hydrogen-dominated atmospheres

Abstract We carried out a fully non-adiabatic analysis for nonradial oscillations of pre-white dwarfs evolved from the post-Asymptotic Giant Branch (AGB) with hydrogen-dominated envelopes. It is shown that nuclear reactions in the hydrogen burning shell excite low-degree g-modes in the period range of about 40–200 s for the pre-white dwarf models with Teff = 40000 K–300000 K. It is also shown that the amount of hydrogen has a significant influence on the instability domain of such pre-white dwarfs in the Hertzsprung-Russel (H-R) diagram. Thus, the thickness of hydrogen-dominated envelopes may be well constrained by observing the presence of the g-mode oscillations.

Stability analysis of strange-modes in hot massive stars with time-dependent convection

Abstract We carry out a nonadiabatic analysis of strange-modes in hot massive stars with time-dependent convection (TDC). In the envelopes of such stars, convective luminosity is not so dominant as it is in the envelopes of stars on the redder side of the classical instability strip. Around the Fe opacity bump, however, convection contributes nonnegligibly to energy transfer. Indeed, we find that the instability of modes excited at the Fe bump is likely to be weaker with TDC as compared with the case of adopting the frozen-in convection (FC) approximation. But we confirm that unstable strange-modes certainly remain in hot massive stars even by taking into account TDC. We also examine properties of the strange-mode instability, which is related to destabilization of strange-modes without adiabatic counterparts. In this type of instability, a phase lag between density and pressure varies from 0° to 180° in an excitation zone, unlike the case of the κ-mechanism. In addition, we confirm by comparing two models with Z = 0 and Z = 0.02 that the dominance of radiation pressure is important for this type of instability.

Analysis of strange-mode instability with time-dependent convection in hot massive stars

AbstractWe carry out nonadiabatic analysis of strange-modes in hot massive stars with time-dependent convection (TDC) for the first time. Although convective luminosity in envelopes of hot massive stars is not as dominative as in stars near the red edge of the classical Cepheid instability strip in the Hertzsprung-Russell (H-R) diagram, we have found that the strange-mode instability can be affected by the treatment of convection. However, existence of the instability around and over the Humphreys-Davidson (H-D) limit is independent of the treatment. This implies that the strange-mode instability could be responsible for the lack of observed stars over the H-D limit regardless of uncertainties on convection theories.

Testing the effects of opacity and the chemical mixture on the excitation of pulsations in B stars of the Magellanic Clouds

The B-type pulsators known as \beta Cephei and Slowly Pulsating B (SPB) stars present pulsations driven by the \kappa mechanism, which operates thanks to an opacity bump due to the iron group elements. In low-metallicity environments such as the Magellanic Clouds, \beta Cep and SPB pulsations are not expected. Nevertheless, recent observations show evidence for the presence of B-type pulsator candidates in both galaxies. We seek an explanation for the excitation of \beta Cep and SPB modes in those galaxies by examining basic input physics in stellar modelling: i) the specific metal mixture of B-type stars in the Magellanic Clouds; ii) the role of a potential underestimation of stellar opacities. We first derive the present-day chemical mixtures of B-type stars in the Magellanic Clouds. Then, we compute stellar models for that metal mixture and perform a non-adiabatic analysis of these models. In a second approach, we simulate parametric enhancements of stellar opacities due to different iron group elements. We then study their effects in models of B stars and their stability. We find that adopting a representative chemical mixture of B stars in the Small Magellanic Cloud cannot explain the presence of B-type pulsators there. An increase of the opacity in the region of the iron-group bump could drive B-type pulsations, but only if this increase occurs at the temperature corresponding to the maximum contribution of Ni to this opacity bump. We recommend an accurate computation of Ni opacity to understand B-type pulsators in the Small Magellanic Cloud, as well as the frequency domain observed in some Galactic hybrid \beta Cep-SPB stars.

Vibrational instability of Population III very massive main-sequence stars due to the -mechanism

ABSTRACT Very massive stars are thought to be formed in the early Universe because of a lack of cooling process by heavy elements, and might have been responsible for the later evolution of the Universe. We had an interest in the vibrational stability of their evolution and have carried out a linear non-adiabatic analysis of radial and non-radial oscillations for Population III very massive main-sequence stars with . We found that only the radial fundamental mode becomes unstable due to the -mechanism for these stars. The instability appears just after the CNO cycle is activated and the nuclear energy generation rate becomes large enough to stop the pre-main-sequence contraction, and continues during the early stage of core hydrogen burning. Also, we have roughly estimated the amount of mass loss due to the instability to evaluate its significance.

Fully Nonadiabatic Analysis of Vibrational Instability of Population III Stars due to the ɛ-Mechanism

Abstract A linear nonadiabatic analysis of the vibrational stability of population III main-sequence stars was carried out. It was demonstrated that, in the case of massive stars with $M$$\gtrsim$ 5 $M_\odot$, helium burning (triple alpha reaction) starts during the main-sequence stage and produces $^{12}$C, leading to the activation of a part of the CNO-cycle. It was found that, despite of that, those stars with $M$$\lesssim$ 13 $M_\odot$ become unstable against the dipole g$_1$- and g$_2$-modes due to the $\varepsilon $-mechanism, during the early evolutionary phase at which the pp-chain is still the dominant nuclear energy source. The instability due to the $\varepsilon $-mechanism occurs against g-modes having a large amplitude in the off-centered $^3$He accumulation shell in the deep interior, and the growth time is much shorter than the evolutionary timescale. This instability is therefore likely to induce mixing in the stellar interior, and have a significant influence on the evolution of these stars.

Pulsations in the atmosphere of the roAp star HD 24712 - II. Theoretical models

We discuss pulsations of the rapidly oscillating Ap (roAp) star HD 24712 (HR 1217) based on nonadiabatic analyses taking into account the effect of dipole magnetic fields. We have found that all the pulsation modes appropriate for HD 24712 are damped; i.e., the kappa-mechanism excitation in the hydrogen ionization layers is not strong enough to excite high-order p-modes with periods consistent with observed ones, all of which are found to be above the acoustic cut-off frequencies of our models. The main (2.721 mHz) and the highest (2.806 mHz) frequencies are matched with modified $l=2$ and $l=3$ modes, respectively. The large frequency separation ($\approx 68 \mu$Hz) is reproduced by models which lay within the error box of HD 24712 on the HR diagram. The nearly equally spaced frequencies of HD 24712 indicate the small frequency separation to be as small as $\approx 0.5\mu$Hz. However, the small separation derived from theoretical $l=1$ and 2 modes are found to be larger than $\sim 3\mu$Hz. The problem of equal spacings could be resolved by assuming that the spacings correspond to pairs of $l=2$ and $l=0$ modes. The amplitude distribution on the stellar surface is strongly affected by the magnetic field resulting in the predominant concentration at the polar regions. Amplitudes and phases of radial-velocity variations for various spectral lines are converted to relations of amplitude/phase versus optical depth in the atmosphere. Oscillation phase delays gradually outward in the outermost layers indicating the presence of waves propagating outward. The phase changes steeply around $\log\tau\sim-3.5$, which supports a $T-\tau$ relation having a small temperature inversion there.

Hot DQ white dwarfs: a pulsational test of the mixing scenario for their formation

Hot DQ white dwarfs constitute a new class of white dwarf stars, uncovered recently within the framework of SDSS project. There exist nine of them, out of a total of several thousands white dwarfs spectroscopically identified. Recently, three hot DQ white dwarfs have been reported to exhibit photometric variability with periods compatible with pulsation g-modes. In this contribution, we presented the results of a non-adiabatic pulsation analysis of the recently discovered carbon-rich hot DQ white dwarf stars. Our study relies on the full evolutionary models of hot DQ white dwarfs recently developed by Althaus et al. (2009), that consistently cover the whole evolution from the born-again stage to the white dwarf cooling track. Specifically, we performed a stability analysis on white dwarf models from stages before the blue edge of the DBV instability strip (Teff ≈ 30000 K) until the domain of the hot DQ white dwarfs (18000-24000 K), including the transition DB→hot DQ white dwarf. We explore evolutionary models with M*= 0.585M⊙ and M* = 0.87M⊙, and two values of thickness of the He-rich envelope (MHe = 2 × 10−7M* and MHe = 10−8M*).

Nonradial Oscillations in Cepheids

We analyse the presence of nonradial oscillations in Cepheids, a problem that has not been theoretically revised since the work of Dziembowsky (1977, Acta Astron., 27, 95) and Osaki (1977, Publ. Astron. Soc. Japan, 29, 235). Our analysis is motivated by a work of Moskalik et al. (2004, ASPC, 310, 498), which reports the detection of low-amplitude periodicities in a few Cepheids of the large Magellanic cloud. These newly discovered periodicities were interpreted as nonradial modes. Based on linear nonadiabatic stability analysis, our goal is to reanalyse the presence and stability of nonradial modes, taking into account improvement in the main input physics required for the modelling of Cepheids.

Multisite Observations of δ Scuti Stars 7 Aql and 8 Aql (a New δ Scuti Variable): The Twelfth STEPHI Campaign in 2003

We present an analysis of the pulsation behavior of the δ Scuti stars 7 Aql (HD 174532) and 8 Aql (HD 174589), a new variable star, observed in the framework of the STEPHI XII campaign during 2003 June and July; 183 hr of high-precision photometry were acquired by using four-channel photometers at three sites on three continents during 21 days. The light curves and amplitude spectra were obtained following a classical scheme of multichannel photometry. Observations in different filters were also obtained and analyzed. Six and three frequencies have been unambiguously detected above a 99% confidence level in the range 190-300 μHz and 100-145 μHz in 7 Aql and 8 Aql, respectively. A comparison of observed and theoretical frequencies shows that 7 Aql and 8 Aql may oscillate with p-modes of low radial orders, typical among δ Scuti stars. In terms of radial oscillations the range of 8 Aql goes from n = 1 to 3, while for 7 Aql the range spans from n = 4 to 7. Nonradial oscillations have to be present in both stars as well. The expected range of excited modes according to a nonadiabatic analysis goes from n = 1 to 6 in both stars.

Stability analysis of sdO equilbrium models

Inhalt (u.a.): M. BREGER: Delta Scuti stars: Observational aspects; M.S. Cunha: Theory of rapidly oscillating Ap stars; M. SACHKOV et al.: Vertical structure of pulsations in roAp stars; G. HOUDEK: Theoretical asteroseismology of solar-like oscillations; B.L. POPIELSKI: Core modes as a seismic probe of mixing beyond the convective core; M. GODART: Temperature gradients in the core overshooting region; W.A. DZIEMBOWSKI – J. DASZYŃSKA-DASZKIEWICZ – A.A. PAMYATNYKH: Interpretation of the Be star HD 163868 oscillation spectrum based on the MOST observations; D. KILKENNY: Pulsating Hot Subdwarfs – An Observational Review; M.H. MONTGOMERY: Mapping Convection using Pulsating White Dwarf Stars; M. ROTH: The Network Activities in HELAS; I. KUDZEJ et al.: A New Slovak Observatory 500 km from Vienna; W.W. WEISS: Microsatellites; G. KOPACKI – S. FRANDSEN: RR Lyrae stars in M4

Effect of convection modeling on the non-adiabatic seismic observables of δ Scuti stars

AbstractPulsation frequencies in δ Scuti stars do not fall in the asymptotic domain, therefore, multi-band photometric methods are currently used to identify the pulsation modes. Theoretical photometric amplitude ratios and phase differences between photometric bands depend, however, on the treatment of convection in surface stellar layers. In this poster we present the results of applying the non-adiabatic analysis by Dupret et al. (2003) to δ Scuti stellar models computed by using the FST (Canuto et al. 1996, CGM) treatment of convection in the interior and in the atmosphere. We determine the amplitude ratios and phase difference for several bands in the Strömgren photometric system, and we compare the results obtained with the FST treatment and with the classical Mixing-length theory. We show that the differences in the external thermal structure are clearly reflected in the photometric phase differences.