Star HD Research Articles

Cover Picture: Astron. Nachr. 4/2024

Variability of the Hα emission line profile of the magnetic O‐type star HD 54897 over one decade, from 2014 to 2024. In the left panels, each profile is plotted with a different color as is indicated in the legend, showing the corresponding observing date and instrument used: H: HARPS, E: ESPaDOnS, U: UVES, and P: PEPSI. The upper panel shows the profiles based on the spectra observed recently, while the bottom panel shows all available profiles published previously. The right panel shows the same data in a color scale plot. The red color corresponds to the strongest emission, while the blue color shows the Hα line wings appearing in absorption. The observed Hα emission is thought to originatein a disk of increased gas density in the equatorial plane of the magnetic field. For details see the article by Küker et al., this issue, e230169. image

Radial and Vertical Constraints on the Icy Origin of H2CO in the HD 163296 Protoplanetary Disk

H2CO is a small organic molecule widely detected in protoplanetary disks. As a precursor to grain-surface formation of CH3OH, H2CO is considered an important precursor of O-bearing organic molecules that are locked in ices. Still, since gas-phase reactions can also form H2CO, there remains an open question on the channels by which organics form in disks, and how much the grain versus the gas pathways impact the overall organic reservoir. We present spectrally and spatially resolved Atacama Large Millimeter/submillimeter Array observations of several ortho- and para-H2CO transitions toward the bright protoplanetary disk around the Herbig Ae star HD 163296. We derive column density, excitation temperature, and ortho-to-para ratio (OPR) radial profiles for H2CO, as well as disk-averaged values of N T ∼ 4 × 1012 cm−2, T ex ∼ 20 K, and OPR ∼ 2.7, respectively. We empirically determine the vertical structure of the emission, finding vertical heights of z/r ∼ 0.1. From the profiles, we find a relatively constant OPR ∼ 2.7 with radius, but still consistent with 3.0 among the uncertainties, a secondary increase of N T in the outer disk, and low T ex values that decrease with disk radius. Our resulting radial, vertical, and OPR constraints suggest an increased UV penetration beyond the dust millimeter edge, consistent with an icy origin but also with cold gas-phase chemistry. This Herbig disk contrasts previous results for the T Tauri disk, TW Hya, which had a larger contribution from cold gas-phase chemistry. More observations of other sources are needed to disentangle the dominant formation pathway of H2CO in protoplanetary disks.

Forty Hypervelocity Stars from Gaia DR3

Forty hypervelocity stars (HVS) are detected from a search for HVS in Gaia DR3 data whose G magnitudes are less than 17.0 and radial velocities are more +400 km s−1. The Gaia DR3 astrophysical parameters and spectra indicate that most of the HVS reported in this study are F, G, and K type metal-poor stars. Eleven stars are at high galactic latitudes. Included in this study are the B components of six nearby visual double stars BD-6 885B, HD 47503B, HD 53229B, HD 95123B, HD 180683B, and HD 202276B which are found to be HVS. The A components of these six visual doubles are less massive late-type stars with normal radial velocities. The source of hypervelocity of the B components is not clear. They may have less luminous and relatively massive degenerate component stars which are not yet detected. Further study of all these HVS is needed.

The FUV Fluxes of A-type Star Members of the Hyades

The International Ultraviolet Explorer database has been searched for the FUV spectra of the A stars which are members of the Hyades. Eight out of 16 confirmed members of the Hyades have FUV absolute fluxes obtained with the SWP camera through the large aperture. The observed sharp decrease of the FUV fluxes from spectral type A2 to A9 is essentially a temperature effect. The FUV flux of the A7 IV star HD 28310, a δ Scuti star, is clearly variable.

A comprehensive X-ray analysis of the massive O-type binary HD 93250 over two decades

Massive star winds are known to be responsible for X-ray emission arising from wind plasma heated by the strong shocks up to temperatures of 10$^6$--10$^7$ K in the case of colliding wind binaries. The investigation of X-ray emission from massive stars thus constitutes a valuable tool for identifying binaries, which is otherwise a difficult task using classical techniques. We investigated thermal and nonthermal X-ray emission from the massive O-type star HD\,93250 to unveil its binary orbital parameters independently. To meet our goal, we analyzed X-ray data obtained with European Photon Imaging Camera on board XMM-Newton spanning over sim 19 years. Additionally, we analyzed NuSTAR observations of HD\,93250 taken at various epochs. We determine the variability timescale of the X-ray emission to be 193.8pm 1.3\,d, in full agreement with the 194.3pm 0.4\,d period derived from the astrometric orbit. The X-ray spectrum of HD\,93250 is well explained by a three-temperature thermal plasma emission model with temperatures of 0.26, 1.0, and 3.3 keV. The resulting X-ray flux varies in compliance with the typical colliding wind emission from eccentric massive binaries where it enhances near periastron passage and decreases gradually close to apastron in proportion to the inverse of the binary separation. The periastron-to-apastron X-ray emission ratio points to an eccentricity range of 0.20-0.25, once again in agreement with the previously determined astrometric orbit. Finally, we do not detect any hard X-ray emission attributable to nonthermal emission above 10 keV. Given the derived plasma temperature, the strong phase-locked variability, and the significant over-luminosity in X-rays, we establish that the X-ray emission from HD\,93250 is dominated by the colliding-wind region. Our results lend support to the idea that X-ray time analysis of massive stars constitutes a relevant tool for investigating their multiplicity and for extracting relevant information on their basic orbital parameters ---such as period and eccentricity--- independently of any orbital solution derived from classical techniques.

First ALMA observations of the HD 105211 debris disc: A warm dust component close to a gigayear-old star

Most debris discs consist of a gas-poor, cold dust belt located tens to hundreds of astronomical units away from the host star. Many cold dust belts exhibit distinct structures attributed to the dynamic interaction of planetary systems. Moreover, in a few systems, additional warm components can be found closer to the central star, resembling the asteroid belt or zodiacal dust in our Solar System. In this work, we investigate the structure of the disc surrounding the nearby F2V star HD 105211 (eta Cru, HIP 59072), which has a warm excess (seen with Spitzer /MIPS at 24 mu m) and a potential asymmetry in the cold belt (seen in the Herschel /PACS images). We applied the CASA pipeline to obtain the ALMA 1.3 mm continuum images of HD105211. Then we constructed the spectral energy distribution (SED) of the system and performed Markov Chain Monte Carlo (MCMC) simulations to fit a model to the ALMA visibility data. To characterise the disc asymmetry, we analysed the ALMA images of two individual observation blocks (OB1, conducted on 2017 March 28, and OB2, conducted on 2018 May 9) and compared them to the previous Herschel images. Our modelling reveals that the disc around HD105211 is a narrow ring (23.6pm 4.6 au) with low eccentricity ($e positioned at a distance of 133.7pm 1.6 au from the central star, which differs from the broad disc ($100 au) starting at an inner edge of $87 au, inferred from the Herschel images. We found that both observation blocks show excess emission at the stellar position ($>3 while OB1 shows an offset between the star and the phase centre ($ and OB2 shows brightness clumps ($ We used a two-temperature model to fit the infrared SED and used the ALMA detection to constrain the warm component to a nearly pure blackbody model. The relatively low ratio of actual radius to blackbody radius of the HD105211 debris disc indicates that this system is depleted in small grains, which could indicate that it is dynamically cold. The excess emission from the stellar position suggests that there should be a warm millimetre-sized dust component close to the star, for which we suggest two possible origins: in situ asteroid belt or comet delivery.

The Hybrid Debris Disk Host Star HD 21997 is a High-frequency Delta Scuti Pulsator

HD 21997 is host to a prototypical “hybrid” debris disk characterized by debris disk-like dust properties and a CO gas mass comparable to a protoplanetary disk. We use Transiting Exoplanet Survey Satellite time series photometry to demonstrate that HD 21997 is a high-frequency delta Scuti pulsator. If the mode identification can be unambiguously determined in future works, an asteroseismic age of HD 21997 may become feasible.

Signs of magnetic star-planet interactions in HD 118203

Context. Planetary systems with close-in giant planets can experience magnetic star-planet interactions that modify the activity levels of their host stars. The induced activity is known to strongly depend on the magnetic moment of the interacting planet. Therefore, such planet-induced activity should be more readily observable in systems with close-in planets in eccentric orbits, since those planets are expected to rotate faster than in circular orbits. However, no evidence of magnetic interactions has been reported in eccentric planetary systems to date. Aims. We intend to unveil a possible planet-induced activity in the bright (V = 8.05 ± 0.03 mag) and slightly evolved star HD 118203, which is known to host an eccentric (e = 0.32 ± 0.02) and close-in (a = 0.0864 ± 0.0006 au) Jupiter-sized planet. Methods. We characterized the planetary system by jointly modelling 56 ELODIE radial velocities and four sectors of TESS photometry. We computed the generalized Lomb-Scargle periodogram of the TESS, ELODIE, and complementary ASAS-SN data to search for planet-induced and rotation-related activity signals. We studied the possible origins of the stellar variability found, analysed its persistence and evolution, and searched for possible links with the eccentric orbital motion of HD 118203 b. Results. We found evidence of an activity signal within the TESS photometry that matches the 6.1-day orbital period of its hosted planet HD 118203 b, which suggests the existence of magnetic star-planet interactions. We did not find, however, any additional activity signal that could be unambiguously interpreted as the rotation of the star, so we cannot discard stellar rotation as the actual source of the signal found. Nevertheless, both the evolved nature of the star and the significant orbital eccentricity make the synchronous stellar rotation with the planetary orbit very unlikely. Conclusions. The planetary system HD 118203 represents the best evidence that magnetic star–planet interactions can be found in eccentric planetary systems, and it opens the door to future dedicated searches in such systems that will allow us to better understand the interplay between close-in giant planets and their host stars.

CO outflows from young stars in the NGC2023 cluster

Young early-type HAeBe stars are still embedded in the molecular clouds in which they formed. They illuminate reflection nebulae, which shape the surrounding molecular cloud and may trigger star formation. They are therefore ideal places to search for ongoing star formation activity. NGC\,2023 is illuminated by the Herbig Be star HD\,37903. It is the most massive member of a small young cluster with about 30 PMS stars, several of which are Class I objects that still heavily accrete. It might therefore be expected that they might drive molecular outflows. We examined the whole region for outflows. We analyzed previously published APEX data to search for and characterize the outflows in the NGC\,2023 region. This is the first systematic search for molecular outflows in this region. Since the outflows were mapped in several CO transitions, we can determine their properties quite well. We have discovered four molecular outflows in the vicinity of NGC\,2023, three of which are associated with Class I objects. MIR-63, a bright mid-infrared and submillimeter Class I source, is a binary with a separation of 2 and drives two bipolar outflows orthogonal to each other. The large southeast--northwest outflow excites the Herbig-Haro flow HH\,247. MIR-73, a Class I object, which is also a far-infrared source, drives a pole-on outflow. MIR-62 is a Class II object with strong infrared excess and a luminosity of 7 It is not detected in the far-infrared. The Class I sources have bolometric luminosities of about 20 or lower, that is, they are all low-mass stars. One other far-infrared source, MIR-75, may have powered an outflow in the past because it now illuminates an egg-shaped cavity. The four outflows are all powered by young stars and are located in the immediate vicinity of NGC\,2023. They are at a similar evolutionary stage, which suggests that their formation may have been triggered by the expanding region.

Characterizing the dynamical magnetosphere of the extremely slowly rotating magnetic O9.7 V star HD 54879 using rotational modulation of the Hα profile

AbstractThe magnetic field in the O9.7 V star HD 54879 has been monitored for almost a decade. Spectropolarimetric observations reveal a rather strong mean longitudinal magnetic field that varies with a period of about 7.41 years. Observations in the H line show a variation with the same period, while the H line shows only little variation. Assuming the periodic variation to be caused by a slow rotation and a dipolar magnetic field, we find a magnetic field strength of 2 kG at the magnetic poles. With the relatively low mass loss rate of year−1, this star is a case of extremely strong magnetic confinement. Both theoretical arguments and numerical simulations indicate the presence of an extended disk of increased gas density in the equatorial plane of the magnetic field, where gas from the line‐driven stellar wind is trapped. This disk is likely to be the origin of the observed H emission, which peaks together with the strongest line‐of‐sight magnetic field. The profile of the H line is resolved in several components and shows a remarkable variability with the rotation period.

Modeling the Light Curve of HD 222925's Progenitor Kilonova

The leading theory for heavy element nucleosynthesis is that a majority of isotopes heavier than iron are formed in neutron star mergers (NSMs), visible through the kilonovae that proceed them. In this work, we derive the potential kilonova light curve associated with the elemental abundances of the “gold standard” metal-poor star HD 222925, which is thought to be enriched by a single NSM. Previous work explores which initial kilonova conditions best match the elemental abundances in HD 222925 and finds two main solutions: either the ejecta is highly constrained, or the underlying nuclear microphysics model is incomplete. We construct the light curves for these two cases in hopes of using kilonova to break this degeneracy. We find that our merger models produce light curves that are observationally too alike to robustly distinguish between the two valid models, offering little hope for constraining kilonova ejecta or the behavior of neutron-rich nuclei.

HD\,7977 and its possible influence on Solar System bodies

In the latest Gaia third data release, one can find extremely small proper motion components for the star HD\,7977. This, together with the radial velocity measurement lead to the conclusion that this star passed very close to the Sun in the recent past. Such a very close approach of a one solar mass star must have resulted in noticeable changes in the motion of all Solar System bodies, especially those on less tight orbits, namely long-period comets (LPCs) and transneptunian objects (TNOs). We estimate and discuss these effects. Our current knowledge on the solar neighbourhood found in the latest Gaia catalogues allowed us to perform numerical integrations and prepare a list of potential stellar perturbers of LPCs. We used this list, made available in the StePPeD database. To study the past motion of LPCs under the simultaneous action of the Galactic potential and passing stars, we used precise original cometary orbits taken from the current CODE catalogue. We examined the reliability of the extremely small proper motion of HD\,7977 and conclude that this star can be an unresolved binary; however, according to the astrometry covering more than a century, the current Gaia data cannot be ruled out. We present the parameters of a very close passage of this star near the Sun. We also show examples of the strong influence of this passage on the past motion of some LPCs. We also discuss the possible influence of this perturber on other Solar System bodies. It is possible that 2.47 Myr ago the one solar mass star HD\,7977 passed as close as 1000 au from the Sun. Such an event constitutes a kind of dynamical horizon for all studies of the past Solar System bodies' dynamics.

On the chemical composition of the evolved very bright metal‐poor star HD 1936

AbstractWe present chemical abundances of the very bright metal‐poor star HD 1936 based on high‐resolution and high SNR spectra from AUKR. We obtain the abundances of 29 atomic species with atomic numbers between 3 and 63. In this context, the derived lithium abundance of 1.01 is consistent with the thin Li plateau observed in lower red giant branch stars. The star is a carbon‐normal with a ratio of −0.31, just like other low‐luminosity red giants on the thin Li plateau. We find the ratios of [Eu/Fe] = 0.43 and [Ba/Eu] = −0.64, indicating very little s‐process contamination. These ratios allow us to classify the star as a moderately r‐process‐enhanced (r‐I) metal‐poor star for the first time. It is worth mentioning that the star has a metallicity of −1.74, a [Cu/Fe] of −0.74, a [Zn/Fe] of 0.04, and a [Mg/C] of 0.69. The results suggest that it may be a second‐generation star formed in a multi‐enriched environment, rather than being a descendant of very massive first‐generation stars. A last point worth mentioning is the possibility that HD 1936 may host a sub‐stellar component with a mass of . Although our study does not confirm or deny this, we briefly discuss the possibility of the star hosting a planet.

Iron Complexes as Potential Carriers of Diffuse Interstellar Bands: The Photodissociation Spectrum of Fe+(H2O) at Optical Wavelengths.

The fullerene ion C60+ is the only carrier of diffuse interstellar bands (DIBs) identified so far. Transition-metal compounds feature electronic transitions in the visible and near-infrared regions, making them potential DIB carriers. Since iron is the most abundant transition metal in the cosmos, we here test this idea with Fe+(H2O). Laboratory spectra were obtained by photodissociation spectroscopy at 80 K. Spectra were modeled with the reflection principle. A high-resolution spectrum of the DIB standard star HD 183143 served as an observational reference. Two broad bands were observed from 4120 to 6800 Å. The 4120-4800 Å band has sharp features emerging from the background, which have the width of DIBs but do not match the band positions of the reference spectrum. Calculations show that the spectrum arises from a d-d transition at the iron center. While no match was found for Fe+(H2O) with known DIBs, the observation of structured bands with line widths typical for DIBs shows that small molecules or molecular ions containing iron are promising candidates for DIB carriers.

Passing Stars as an Important Driver of Paleoclimate and the Solar System’s Orbital Evolution

Reconstructions of the paleoclimate indicate that ancient climatic fluctuations on Earth are often correlated with variations in its orbital elements. However, the chaos inherent in the solar system’s orbital evolution prevents numerical simulations from confidently predicting Earth’s past orbital evolution beyond 50–100 Myr. Gravitational interactions among the Sun’s planets and asteroids are believed to set this limiting time horizon, but most prior works approximate the solar system as an isolated system and neglect our surrounding Galaxy. Here we present simulations that include the Sun’s nearby stellar population, and we find that close-passing field stars alter our entire planetary system’s orbital evolution via their gravitational perturbations on the giant planets. This shortens the timespan over which Earth’s orbital evolution can be definitively known by a further ∼10%. In particular, in simulations that include an exceptionally close passage of the Sun-like star HD 7977 2.8 Myr ago, new sequences of Earth’s orbital evolution become possible in epochs before ∼50 Myr ago, which includes the Paleocene–Eocene Thermal Maximum. Thus, simulations predicting Earth’s past orbital evolution before ∼50 Myr ago must consider the additional uncertainty from passing stars, which can open new regimes of past orbital evolution not seen in previous modeling efforts.

Stellar activity and differential rotation of HD 111395

Aims. Stellar activity cycles and rotation periods are important parameters for characterising the stellar dynamo, which operates in late-type main-sequence stars. However, the number of stars with well-known cycle and rotation periods is rather low, so new detections are still important. Methods. To find activity cycles and rotation periods, we utilised the TIGRE telescope to monitor stars for periodic variations in chromospheric activity indicators. We employed the widely used CaII H&K lines and the CaII infrared triplet lines as stellar activity indicators. To verify a periodic variation and to determine the corresponding period, we performed a frequency analysis via the generalised Lomb-Scargle method of the taken time series. Results. We studied CaII data of the G5V star HD 111395 and derive an activity cycle period of 949 ± 5 d (≈2.6 yr). This cycle is coincident with coronal measurements from the X-ray telescope eROSITA on board SRG. Furthermore, the TIGRE CaII time series show a long-term trend that indicates an additional long-term cycle. Using the few available literature S-index data points, we estimate a probable cycle length of 12–15 yr for this potential long-term cycle. Finally, we determined rotation periods from each observation season. We computed a mean rotation period of 16.76 ± 0.36 d averaged over all observation seasons and chromospheric indicators. However, we also find a strong variation in the mean seasonal rotation periods, which follows the derived cycle period; therefore, we interpret this behaviour as a sign of surface differential rotation.

Vertical Structure of Gas and Dust in Four Debris Disks

We present high-spectral-resolution M-band spectra from iSHELL on NASA’s Infrared Telescope Facility along the line of sight to the debris disk host star HD 32297. We also present a Gemini Planet Imager H-band polarimetric image of the HD 131488 debris disk. We search for fundamental CO absorption lines in the iSHELL spectra of HD 32297, but do not detect any. We place an upper limit on the CO column density of ∼6 × 1015 cm−2. By combining the column density upper limit, the CO mass measured with the Atacama Large Millimeter/submillimeter Array (ALMA), and the geometrical properties of the disk, we estimate the scale height of the CO to be ≲2 au across the radial extent of the disk (∼80–120 au). We use the same method to estimate the CO scale height of three other edge-on, CO-rich debris disks that all have CO observed in absorption with the Hubble Space Telescope as well as in emission with ALMA: β Pictoris, HD 110058, and HD 131488. We compare our estimated CO scale heights of these four systems to the millimeter dust scale heights and find that, under the assumption of hydrostatic equilibrium, there is a potential correlation between the CO and millimeter dust scale heights. There are multiple factors that affect the gas vertical structure such as turbulence, photodissociation with weak vertical mixing, as well as where the gas originates. One possible explanation for the potential correlation could be that the gas and dust are of a similar secondary origin in these four systems.

Two Long-period Giant Planets around Two Giant Stars: HD 112570 and HD 154391

We present the discoveries of two giant plants orbiting the red-giant-branch star HD 112570 and the red-clump star HD 154391, based on the radial-velocity (RV) measurements from the Xinglong station and Okayama Astrophysical Observatory. Spectroscopic and asteroseismic analyses suggest that HD 112570 has a mass of 1.15 ± 0.12 M ☉, a radius of 9.85 ± 0.23 R ☉, a metallicity [Fe/H] of −0.46 ± 0.1, and logg of 2.47 ± 0.1. With the joint analysis of RV and Hipparcos-Gaia astrometry, we obtain a dynamical mass of Mp=3.42−0.84+1.4MJup , a period of P=2615−77+85 days, and a moderate eccentricity of e=0.20−0.14+0.16 for the Jovian planet HD 112570 b. For HD 154391, it has a mass of 2.07 ± 0.03 M ☉, a radius of 8.56 ± 0.05 R ☉, a metallicity [Fe/H] of 0.07 ± 0.1, and logg of 2.86 ± 0.1. The super-Jupiter HD 154391 b has a mass of Mp=9.1−1.9+2.8MJup , a period of P=5163−57+60 days, and an eccentricity of e=0.20−0.04+0.04 . We found that HD 154391 b has one of the longest orbital periods among those ever discovered orbiting evolved stars, which may provide a valuable case in our understanding of planetary formation at wider orbits. Moreover, while a mass gap at 4 M Jup seems to be present in the population of giant stars, there appear to be no significant differences in the distribution of metallicity among giant planets with masses above or below this threshold. Finally, the origin of the abnormal accumulation near 2 au for planets around large evolved stars (R ⋆ > 21 R ☉), remains unclear.

Detection of Tm ii Lines in the Spectra of Two s-process Rich Stars

Abstract Thulium abundance is unknown in the atmospheres of low and medium-mass evolved stars. The reason is the weakness of lines and the crowded spectra. We examined the spectra of two late-type stars in a broad wavelength region to search for thulium lines. Thulium abundance was estimated in the atmospheres of barium star HD 204075 and post-asymptotic giant branch star HD 235858 for the first time, log ϵ ( Tm ) ≃ 0.1 and ≲1.3, respectively.

Amplitude Modulation in a δ Sct star HD 118660

In this paper, we report the detection of amplitude modulation in a δ Scuti star HD 118660. We found that the p-mode frequency at 24.3837 d−1 varies periodically in amplitude with frequency 0.0558±0.00147 d−1. However, all other modes are stable in both amplitude and phase which is clear evidence of non-conservation of visible pulsation mode energy. We constructed a two-frequency model by superimposing two sinusoïds with frequencies ν1 = 24.3837 d−1 and ν2 = 24.4420 d−1 and corresponding phases ϕ1 = 0.5211 rad and ϕ2 = 0.9481 rad to mimic the observed variations of amplitude and phase with time. The plausible explanation of the amplitude modulation in HD 118660 is due to beating of two unresolved closed frequencies ν1 and ν2.