arXiv subjects

Henrik Hartman

Publications and source records attributed to Henrik Hartman.

13 recordsLinked to original sources

arXiv2025

Rubidium Abundances in Cool Giants from High-Resolution H-band Spectra: A New Diagnostic for Galactic Chemical Evolution

The Galactic Center and inner disk of the Milky Way contain complex stellar populations obscured by heavy dust extinction. To study their chemical composition, high-resolution near-infrared (near-IR) spectroscopy is necessary. Expanding the set of elements measurable in the near-IR, especially neutron-capture elements, improves our ability to trace nucleosynthesis and Galactic chemical evolution. This work aims to identify and characterize a spectral line suitable for determining rubidium (Rb) abundances. Rb is produced in roughly equal parts by the r- and s-processes. We analyze high-resolution (R = 45,000) IGRINS near-IR spectra of 40 M giants in the solar neighborhood, most observed with Gemini South. We perform spectral synthesis of the Rb I line at 15289.48 A, using new log(gf) values and including an astrophysical calibration of the blending Fe I lines. The resulting [Rb/Fe] ratios are compared to other neutron-capture elements and interpreted with chemical evolution models. We demonstrate that the used Rb line is a reliable abundance indicator in M giants and the coolest K giants, but becomes too weak at higher temperatures. [Rb/Fe] shows a decreasing trend with metallicity, mirroring that of ytterbium (Yb), another mixed r-/s-process element. Our results agree with optical studies, validating the use of this near-IR line. Comparisons with chemical evolution models confirm that both s- and r-process sources are needed to explain the Rb trend. This work adds Rb to the list of elements measurable in high-resolution H- and K-band spectra, enabling studies of one more neutron-capture element in dust-obscured regions like the Galactic Center and inner disk.

astro-ph.GA↗

arXiv2024

Eta Carinae left a curious ladder to climb

Eta Carinae underwent the Great Eruption in the 1840s and a Lesser Eruption in the 1890s. Its apparent spectrum, modified by intervening ejecta, the Homunculus and Little Homunculus, continues to evolve but contains information pertaining to events in the 19th century. The LOS spectrum contains narrow absorption velocities, from -122 to -1665 km/s: rungs of a broken ladder caused by shells formed by the interacting winds. Estimated shell origin dates correlate with origin dates of expanding emission structures preceding the Great Eruption. The LOS absorption velocities extend the record post Great Eruption to the Lesser Eruption. We suggest that these shells originated from a binary merger within a triple system. Shells formed not only from periastron passages of the current secondary, but also from ear-like extensions preceding and following the periastron event. Additional models need to be considered.

astro-ph.SR↗

arXiv2023

Experimental oscillator strengths of Al I lines for near-infrared astrophysical spectroscopy

Elemental abundances can be determined from stellar spectra, making it possible to study galactic formation and evolution. Accurate atomic data is essential for the reliable interpretation and modeling of astrophysical spectra. In this work, we perform laboratory studies on neutral aluminium. This element is found, for example, in young, massive stars and it is a key element for tracing ongoing nucleosynthesis throughout the Galaxy. The near-infrared (NIR) wavelength region is of particular importance, since extinction in this region is lower than for optical wavelengths. This makes the NIR wavelength region a better probe for highly obscured regions, such as those located close to the Galactic center. We investigate the spectrum of neutral aluminium with the aim to provide oscillator strengths (f-values) of improved accuracy for lines in the NIR and optical regions (670 - 4200 nm). Measurements of high-resolution spectra were performed using a Fourier transform spectrometer and a hollow cathode discharge lamp. The f-values were derived from experimental line intensities combined with published radiative lifetimes. We report oscillator strengths for 12 lines in the NIR and optical spectral regions, with an accuracy between 2 and 11%, as well as branching fractions for an additional 16 lines.

astro-ph.SR↗

arXiv2023

Eta Carinae: the dissipating occulter is an extended structure

Previous STIS long-slit observations of Eta Carinae identified numerous absorption features in both the stellar spectrum, and in the adjacent nebular spectra, along our line-of-sight. The absorption features became temporarily stronger when the ionizing FUV radiation field was reduced by the periastron passage of the secondary star. Subsequently, dissipation of a dusty structure in our LOS has led to a long-term increase in the apparent magnitude of \ec, an increase in the ionizing UV radiation, and the disappearance of absorptions from multiple velocity-separated shells extending across the foreground Homunculus lobe. We use HST/STIS spectro-images, coupled with published infrared and radio observations, to locate this intervening dusty structure. Velocity and spatial information indicate the occulter is ~1000 au in front of Eta Carinae. The Homunculus is a transient structure composed of dusty, partially-ionized ejecta that eventually will disappear due to the relentless rain of ionizing radiation and wind from the current binary system along with dissipation and mixing with the ISM. This evolving complex continues to provide an astrophysical laboratory that changes on human timescales.

astro-ph.SR↗

arXiv2022

Changes in the Na D$_1$ Absorption Components of $η$ Carinae Provide Clues on the Location of the Dissipating Central Occulter

The Na D absorption doublet in the spectrum of $η$ Carinae is complex, with multiple absorption features associated with the Great Eruption (1840s), the Lesser Eruption (1890s), and interstellar clouds. The velocity profile is further complicated by the P Cygni profile originating in the system's stellar winds and blending with the He I $λ$5876 profile. The Na D profile contains a multitude of absorption components, including those at velocities of $-$145 km s$^{-1}$, $-$168 km s$^{-1}$, and $+$87 km s$^{-1}$ that we concentrate on in this analysis. Ground-based spectra recorded from 2008 to 2021 show significant variability of the $-$145 km s$^{-1}$ absorption throughout long-term observations. In the high ionization phases of $η$ Carinae prior to the 2020 periastron passage, this feature disappeared completely but briefly reappeared across the 2020 periastron, along with a second absorption at $-$168 km s$^{-1}$. Over the past few decades, $η$ Car has been gradually brightening demonstrated to be caused by a dissipating occulter. The decreasing absorption of the $-$145 km s$^{-1}$ component, coupled with similar trends seen in absorptions of ultraviolet resonant lines, indicate that this central occulter was possibly a large clump associated with the Little Homunculus or another clump between the Little Homunculus and the star. We also report on a foreground absorption component at $+$87 km s$^{-1}$. Comparison of Na D absorption in the spectra of nearby systems demonstrates that this red-shifted component likely originates in an extended foreground structure consistent with a previous ultraviolet spectral survey in the Carina Nebula.

astro-ph.SR↗

arXiv2022

Eta Carinae: an evolving view of the central binary, its interacting winds and its foreground ejecta

FUV spectra of Eta Car, recorded across two decades with HST/STIS, document multiple changes in resonant lines caused by dissipating extinction in our line of sight. The FUV flux has increased nearly ten-fold which has led to increased ionization of the multiple shells within the Homunculus and photo-destruction of molecular hydrogen. Comparison of observed resonant line profiles with CMFGEN model profiles allows separation of wind-wind collision and shell absorptions from the primary wind, P Cygni profiles.The dissipating occulter preferentially obscured the central binary and interacting winds relative to the very extended primary wind. We are now able to monitor changes in the colliding winds with orbital phase. High velocity transient absorptions occurred across the most recent periastron passage, indicating acceleration of the primary wind by the secondary wind which leads to a downstream, high velocity bowshock that is newly generated every orbital period. There is no evidence of changes in the properties of the binary winds.

astro-ph.SR↗

arXiv2019

MCDHF and RCI calculations of energy levels, lifetimes, and transition rates in Si III and Si IV

We present extensive multiconfiguration Dirac-Hartree-Fock and relativistic configuration interaction calculations including 106 states in doubly ionized silicon (Si III) and 45 states in triply ionized silicon (Si iv), which are important for astrophysical determination of plasma properties in different objects. These calculations represents an important extension and improvement of earlier calculations especially for Si IV. The calculations are in good agreement with available experiments for excitation energies, transition properties, and lifetimes. Important deviations from the NIST-database for a selection of perturbed Rydberg series are discussed in detail.

physics.atom-ph↗

arXiv2017

Benchmarking Atomic Data for Astrophysics: Be-like Ions between B II and Ne VII

Large-scale self-consistent multiconfiguration Dirac--Hartree--Fock and relativistic configuration interaction calculations are reported for the $n \leq 6$ levels in Be-like ions from B II to Ne VII. Effects from electron correlation are taken into account by means of large expansions in terms of a basis of configuration state functions, and a complete and accurate data set of excitation energies, lifetimes, wavelengths, and electric dipole, magnetic dipole, electric quadrupole, and magnetic quadrupole line strengths, transition rates, and oscillator strengths for these levels is provided for each ion. Comparisons are made with available experimental and theoretical results. The uncertainty of excitation energies is assessed to be 0.01\% on average, which makes it possible to find and rule out misidentifications and aid new line identifications involving high-lying levels in astrophysical spectra. The complete data set is also useful for modeling and diagnosing astrophysical plasmas.

physics.atom-ph↗

arXiv2012

Silver and palladium help unveil the nature of a second r-process

The rapid neutron-capture process, creating about half of the heaviest elements in the Solar System was believed to be unique. Many recent studies have shown that this does not include the formation of lighter elements (in particular 38 < Z < 48). Among those, palladium (Pd) and especially silver (Ag) are expected to be key indicators of a possible second r-process, but until recently they have been studied only in a few stars. Therefore we target Pd and Ag in a large sample of stars and compare these abundances to those of Sr, Y, Zr, Ba and Eu produced by the slow (s-) and rapid (r-) neutron-capture processes. Hereby we investigate the nature of the formation process of Ag and Pd. Through a homogeneous 1D LTE analysis of 71 stars we derive stellar abundances using the spectrum synthesis code MOOG, and MARCS model atmospheres. We calculate abundance ratio trends and compare the derived abundances to site-dependent yield predictions (low mass O-Ne-Mg cc SN, and parametrised high entropy winds), to extract characteristics of the second r-process. The abundance ratios of the heavy elements yield correlations and anti-correlations. These trends lead to clear indications of the existence of a second/weak r-process, responsible for the formation of Pd and Ag. By comparing to the model predictions, we find that the conditions under which this process takes place differ from the main r-process in needing lower neutron number densities, neutron-to-seed ratios, entropies and/or favour higher electron abundances. Our analysis confirms that Pd and Ag form via a r-process that differs from the main r-process, the main and weak s-processes, and charged particle freeze-outs. This process is efficiently working down to [Fe/H] = -3.3 (where our sample ends). Our results may indicate that a combination of these explosive sites is needed to explain the observationally-derived abundance patterns.

astro-ph.SR↗

arXiv2010

Experimental and theoretical lifetimes and transition probabilities in Sb I

We present experimental atomic lifetimes for 12 levels in Sb I, out of which seven are reported for the first time. The levels belong to the 5p$^2$($^3$P)6s $^{2}$P, $^{4}$P and 5p$^2$($^3$P)5d $^{4}$P, $^{4}$F and $^{2}$F terms. The lifetimes were measured using time-resolved laser-induced fluorescence. In addition, we report new calculations of transition probabilities in Sb I using a Multiconfigurational Dirac-Hartree-Fock method. The physical model being tested through comparisons between theoretical and experimental lifetimes for 5d and 6s levels. The lifetimes of the 5d $^4$F$_{3/2, 5/2, 7/2}$ levels (19.5, 7.8 and 54 ns, respectively) depend strongly on the $J$-value. This is explained by different degrees of level mixing for the different levels in the $^4$F term.

physics.atom-ph↗

arXiv2010

Detection of high-velocity material from the wind-wind collision zone of Eta Carinae across the 2009.0 periastron passage

We report near-IR spectroscopic observations of the Eta Carinae massive binary system during 2008-2009 using VLT/CRIRES. We detect a strong, broad absorption wing in He I 10833 extending up to -1900 km/s across the 2009.0 spectroscopic event. Archival HST/STIS ultraviolet and optical data shows a similar high-velocity absorption (up to -2100 km/s) in the UV resonance lines of Si IV 1394, 1403 across the 2003.5 event. UV lines from low-ionization species, such as Si II 1527, 1533 and C II 1334, 1335, show absorption up to -1200 km/s, indicating that the absorption with v from -1200 to -2100 km/s originates in a region markedly faster and more ionized than the nominal wind of the primary star. Observations obtained at the OPD/LNA during the last 4 spectroscopic cycles (1989-2009) also display high-velocity absorption in He I 10833 during periastron. Based on the OPD/LNA dataset, we determine that material with v < -900 km/s is present in the phase range 0.976 < phi < 1.023 of the spectroscopic cycle, but absent in spectra taken at phi < 0.947 and phi > 1.049. Therefore, we constrain the duration of the high-velocity absorption to be 95 to 206 days (or 0.047 to 0.102 in phase). We suggest that the high-velocity absorption originates from shocked gas in the wind-wind collision zone, at distances of 15 to 45 AU in the line-of-sight to the primary star. Using 3-D hydrodynamical simulations of the wind-wind collision zone, we find that the dense high-velocity gas is in the line-of-sight to the primary star only if the binary system is oriented in the sky so that the companion is behind the primary star during periastron, corresponding to a longitude of periastron of omega ~ 240 to 270 degrees. We study a possible tilt of the orbital plane relative to the Homunculus equatorial plane and conclude that our data are broadly consistent with orbital inclinations in the range i=40 to 60 degrees.

astro-ph.SR↗

arXiv2010

The CRIRES Search for Planets Around the Lowest-Mass Stars. I. High-Precision Near-Infrared Radial Velocities with an Ammonia Gas Cell

Radial velocities measured from near-infrared spectra are a potentially powerful tool to search for planets around cool stars and sub-stellar objects. However, no technique currently exists that yields near-infrared radial velocity precision comparable to that routinely obtained in the visible. We describe a method for measuring high-precision relative radial velocities of these stars from K-band spectra. The method makes use of a glass cell filled with ammonia gas to calibrate the spectrograph response similar to the "iodine cell" technique that has been used very successfully in the visible. Stellar spectra are obtained through the ammonia cell and modeled as the product of a Doppler-shifted template spectrum of the object and a spectrum of the cell, convolved with a variable instrumental profile model. A complicating factor is that a significant number of telluric absorption lines are present in the spectral regions containing useful stellar and ammonia lines. The telluric lines are modeled simultaneously as well using spectrum synthesis with a time-resolved model of the atmosphere over the observatory. The free parameters in the complete model are the wavelength scale of the spectrum, the instrumental profile, adjustments to the water and methane abundances in the atmospheric model, telluric spectrum Doppler shift, and stellar Doppler shift. Tests of the method based on the analysis of hundreds of spectra obtained for late M dwarfs over six months demonstrate that precisions of ~5 m/s are obtainable over long timescales, and precisions of better than 3 m/s can be obtained over timescales up to a week. The obtained precision is comparable to the predicted photon-limited errors, but primarily limited over long timescales by the imperfect modeling of the telluric lines.

astro-ph.EP↗

arXiv2010

The CRIRES Search for Planets Around the Lowest-Mass Stars. II. The Proposed Giant Planet Orbiting VB10 Does Not Exist

We present high-precision relative radial velocities of the very low-mass star VB10 that were obtained over a time span of 0.61 yr as part of an ongoing search for planets around stars at the end of the main sequence. The radial velocities were measured from high-resolution near-infrared spectra obtained using the CRIRES instrument on the VLT with an ammonia gas cell. The typical internal precision of the measurements is 10 m/s. These data do not exhibit significant variability and are essentially constant at a level consistent with the measurement uncertainties. Therefore, we do not detect the radial velocity variations of VB10 expected due to the presence of an orbiting giant planet similar to that recently proposed by Pravdo and Shaklan based on apparent astrometric perturbations. In addition, we do not confirm the ~1 km/s radial velocity variability of the star tentatively detected by Zapatero Osorio and colleagues with lower precision measurements. Our measurements rule out planets with M_p > 3 M_Jup and the orbital period and inclination suggested by Pravdo and Shaklan at better than 5 sigma confidence. We conclude that the planet detection claimed by Pravdo and Shaklan is spurious on the basis of this result. Although the outcome of this work is a non-detection, it illustrates the potential of using ammonia cell radial velocities to detect planets around very low-mass stars.

astro-ph.EP↗