arXiv subjects

D. Jack

Publications and source records attributed to D. Jack.

15 recordsLinked to original sources

arXiv2022

Chromospheric activity and photospheric variation of $\alpha$ Ori during the great dimming event in 2020

The so-called great dimming event of alpha Ori in late 2019 and early 2020 sparked our interest in the behaviour of chromospheric activity during this period. To study the timeline of chromospheric activity, we derive a S_MWO time series of TIGRE and Mount Wilson values, and we compare this long time series with photometric data from the AAVSO database. In addition, we determine the absolute and normalised excess flux of the Ca II H&K lines. To do so, we estimate the changing effective temperature from TIGRE spectra and find a clear drop of about 80 K between November 2019 and February 2020, which coincides with the minimum of visual brightness. During the same period, the S-index increased significantly, yet this is a mere contrast effect, because the normalised excess flux of the Ca II H&K lines did not change significantly. However, the latter dropped immediately after this episode. Comparing the combined S_MWO values and visual magnitude time series, we find a similar increase in the S-index during another noticeable decrease in the visual magnitude of alpha Ori, which took place in 1984 and 1985. To also probe the dynamics of the upper photosphere, we analysed the lines in 6251-6263 A and found core distance varies which shows a relation with the great dimming event. This type of variation could be caused by rising and sinking cool plumes as a temporary spill-over of convection on alpha Ori. Based on our study, we conclude that the cause for the great dimming is located in the photosphere. Furthermore, the long-term spectroscopic and photometric time series suggests that this great dimming does not appear to be a unique phenomenon, but rather that such dimmings do occur more frequently, which motivates further monitoring of alpha Ori with facilities such as TIGRE.

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arXiv2022

Eight years of TIGRE robotic spectroscopy: Operational experience and selected scientific results

TIGRE (Telescopio Internacional de Guanajuato Rob\'otico Espectrosc\'opico) has been operating in fully robotic mode in the Observatory La Luz (Guanajuato, Mexico) since the end of 2013. With its sole instrument, HEROS, an \'echelle spectrograph with a spectral resolution R~20000, TIGRE has collected more than 48000 spectra of 1151 different sources with a total exposure time of more than 11000 hours in these eight years. Here we briefly describe the system and the upgrades performed during the last years. We present the statistics of the weather conditions at the La Luz Observatory, emphasizing the characteristics that affect the astronomical observations. We evaluate the performance and efficiency of TIGRE, both optical and operational, and describe the improvements of the system implemented to optimize the telescope's performance and meet the requirements of the astronomer in terms of timing constraints for the observations and the quality of the spectra. We describe the actions taken to slow down the optical efficiency loss due to the aging of the optical surfaces as well as the upgrades of the scheduler and the observing procedures to minimize the time lost due to interrupted observations or observations that do not reach the required quality. Finally, we highlight a few of the main scientific results obtained with TIGRE data.

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arXiv2022

On the physical nature of the Wilson-Bappu effect: revising the gravity and temperature dependence

We present a sample of 32 stars of spectral types G and K, and luminosity classes I to V, with moderate activity levels, covering four orders of magnitude of surface gravity and a representative range of effective temperature. For each star we obtained high S/N TIGRE-HEROS spectra with a spectral resolving power of $R\approx20,000$ and have measured the Ca II K line-widths of interest, $W_0$ and $W_1$. The main physical parameters are determined by means of iSpec synthesis and Gaia EDR3 parallaxes. Mass estimates are based on matching to evolution models. Using this stellar sample, that is highly uniform in terms of spectral quality and assessment, we derive the best-fit relation between emission line width and gravity $g$, including a notable dependence on effective temperature $T_{\rm eff}$, of the form $W_1 \propto g^{-0.229} T_{\rm eff}^{+2.41}$. This result confirms the physical interpretation of the Wilson-Bappu effect as a line saturation and photon redistribution effect in the chromospheric Ca II column density, under the assumption of hydrostatic equilibrium at the bottom of the chromosphere. While the column density (and so $W_1$) increases towards lower gravities, the observed temperature dependence is then understood as a simple ionization effect -- in cooler stars, Ca II densities decrease in favor of Ca I.

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arXiv2022

Yet another star in the Albireo system -- The discovery of Albireo Ad

Context. Albireo is a well-known bright visual double star. It is still unclear if the components A and B form a gravitationally bound system. The component Albireo A is itself a binary star. The orbital parameters of the Albireo Aa, Ac system have been determined only recently. Thus, Albireo is still of interest for current research. Aims. We aim to present evidence for the detection of a new member in the Albireo system. Furthermore, we aim to determine the orbital parameters and to find further conclusions for the Albireo system. Methods. We used spectroscopic observations of Albireo A obtained with the TIGRE telescope and determined the radial velocities during a period of over three years. We analyzed the radial velocity curve with RadVel to determine the orbital parameters. In addition, we determined the stellar parameters of Albireo Aa with iSpec. Results. We found clear evidence for yet another star in the Albireo system orbiting Albireo Aa with a period of about $P = 371$ days. Several alternative explanations for the periodic radial velocity signal could be discarded. The new companion Albireo Ad is a low mass star of about $0.085~M_\odot$. Conclusions. We conclude that Albireo is a hierarchical multiple star system and remains an interesting object for future observations and studies.

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arXiv2021

Connecting the formation of stars and planets. I -- Spectroscopic characterization of host stars with TIGRE

In search for a connection between the formation of stars and the formation of planets, a new semi-automatic spectral analysis method using \textsf{iSpec} was developed for the TIGRE telescope installed in Guanajuato, Mexico. TIGRE is a 1.2m robotic telescope, equipped with an Echelle spectrograph (HEROS), with a resolution R $\simeq 20000$. \textsf{iSpec} is a synthetic spectral fitting program for stars that allows to determine in an homogeneous way their fundamental parameters: effective temperature, $T_{\rm eff}$, surface gravity, $\log g$, metallicities, $[M/H]$ and $[Fe/H]$, and rotational velocity, $V \sin i$. In this first article we test our method by analysing the spectra of 46 stars, host of exoplanets, obtained with the TIGRE.

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arXiv2021

Connecting the formation of stars and planets. II: coupling the angular momentum of stars with the angular momentum of planets

A sample of 46 stars, host of exoplanets, is used to search for a connection between their formation process and the formation of the planets rotating around them. Separating our sample in two, stars hosting high-mass exoplanets (HMEs) and low-mass exoplanets (LMEs), we found the former to be more massive and to rotate faster than the latter. We also found the HMEs to have higher orbital angular momentum than the LMEs and to have lost more angular momentum through migration. These results are consistent with the view that the more massive the star and higher its rotation, the more massive was its protoplanetarys disk and rotation, and the more efficient the extraction of angular momentum from the planets.

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arXiv2019

A catalogue of spectroscopic binary candidate stars derived from a comparison of Gaia DR2 with other radial velocity catalogues

Using the recently published Gaia second data release which includes measurements of the mean radial velocity of about 7.2 million stars, we performed a systematic comparison with other existing radial velocity catalogues in order to search for variations in the radial velocity measurements, with the goal that detected differences may indicate that these stars are possibly spectroscopic binaries stars with only one visible component (SB1). We present a spectroscopic binary candidate stars catalogue containing 35,246 stars, compiled to encourage follow-up observations obtaining spectra at different epochs of these stars orbits in order to verify their binarity and to study these systems using radial velocity curves. Comparing the Gaia DR2 database with the K-M dwarf catalogue we found 16 stars that show radial velocity variations. In a comparison with the Pulkovo radial velocity catalogue of Hipparcos stars we identified a total of 539 SB1 candidate stars. In the largest radial velocity catalogue available, the RAVE catalogue, we found a total of 34,691 stars that show radial velocity variations when compared to the Gaia DR2 data.

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arXiv2017

Carrington cycle 24: The solar chromospheric emission in a historical and stellar perspective

We present the solar S-index record of cycle 24, obtained by the TIGRE robotic telescope facility and its high-resolution spectrograph HEROS (R$\approx$20,000), which measures the solar chromospheric Ca II H&K line emission by using moonlight. Our calibration process uses the same set of standard stars as introduced by the Mt. Wilson team, thus giving us a direct comparison with their huge body of observations taken between 1966 and 1992, as well as with other cool stars. Carrington cycle 24 activity started from the unusually deep and long minimum 2008/09, with an S-index average of only 0.154, by 0.015 deeper than the one of 1986 ( =0.169). In this respect,the chromospheric radiative losses differ remarkably from the variation of the coronal radio flux F10.7cm and the sunspot numbers. In addition, the cycle 24 S-amplitude remained small, 0.022 (cycles 21 and 22 averaged: 0.024), and so resulted a very low 2014 maximum of =0.176 (cycles 21 and 22 averaged: 0.193). We argue that this find is significant, since the Ca II H&K line emission is a good proxy for the solar far-UV flux, which plays an important role in the heating of the Earth's stratosphere, and we further argue that the solar far-UV flux changes change s with solar activity much more strongly than the total solar output.

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arXiv2017

Study of the variability of Nova V5668 Sgr, based on high resolution spectroscopic monitoring

We present results of our dense spectroscopic monitoring of Nova V5668 Sgr. Starting on March 19 in 2015, only a few days after discovery, we have obtained a series of spectra with the TIGRE telescope and its HEROS echelle spectrograph which offers a resolution of R = 20,000 and covers the optical wavelength range from 3800 to 8800 {\AA}. We performed a line identification of the discernible features for four spectra which are representative for the respective phases in the light curve evolution of that nova. By simultaneously analysing the variations in the visual light curve and the corresponding spectra of Nova V5668 Sgr, we found that during the declining phases of the nova the absorption features in all hydrogen and many other lines had shifted to higher expansion velocities of -2000 km s^-1. Conversely, during the rise towards the following maximum, these observed absorption features had returned to lower expansion velocities.We found that the absorption features of some Fe II lines displayed the same behaviour, but in addition disappeared for a few days during some declining phases. Features of several N I lines also disappeared while new N II lines appeared in emission for a few days during some of the declining phases of the light curve of Nova V5668 Sgr. The shape of the emission features is changing during the evolution and shows a clear double peak structure after the deep minimum.

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arXiv2015

Time series of high resolution spectra of SN 2014J observed with the TIGRE telescope

We present a time series of high resolution spectra of the Type Ia supernova 2014J, which exploded in the nearby galaxy M82. The spectra were obtained with the HEROS echelle spectrograph installed at the 1.2 m TIGRE telescope. We present a series of 33 spectra with a resolution of R = 20, 000, which covers the important bright phases in the evolution of SN 2014J during the period from January 24 to April 1 of 2014. The spectral evolution of SN 2014J is derived empirically. The expansion velocities of the Si II P-Cygni features were measured and show the expected decreasing behaviour, beginning with a high velocity of 14,000 km/s on January 24. The Ca II infrared triplet feature shows a high velocity component with expansion velocities of > 20, 000 km/s during the early evolution apart from the normal component showing similar velocities as Si II. Further broad P-Cygni profiles are exhibited by the principal lines of Ca II, Mg II and Fe II. The TIGRE SN 2014J spectra also resolve several very sharp Na I D doublet absorption components. Our analysis suggests interesting substructures in the interstellar medium of the host galaxy M82, as well as in our Milky Way, confirming other work on this SN. We were able to identify the interstellar absorption of M82 in the lines of Ca II H & K at 3933 and 3968 A as well as K I at 7664 and 7698 A. Furthermore, we confirm several Diffuse Interstellar Bands, at wavelengths of 6196, 6283, 6376, 6379 and 6613 A and give their measured equivalent widths.

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arXiv2015

Identification of the feature that causes the I-band secondary maximum of a type Ia supernova

We obtained a time series of spectra covering the secondary maximum in the I-band of the bright Type Ia supernova 2014J in M82 with the TIGRE telescope. Comparing the observations with theoretical models calculated with the time dependent extension of the PHOENIX code, we identify the feature that causes the secondary maximum in the I-band light curve. Fe II 3d6(3D)4s-3d6(5D)4p and similar high excitation transitions produce a blended feature at 7500 {\AA}, which causes the rise of the light curve towards the secondary maximum. The series of observed spectra of SN 2014J and archival data of SN 2011fe confirm this conclusion. We further studied the plateau phase of the Rband light curve of SN 2014J and searched for features which contribute to the flux. The theoretical models do not clearly indicate a new feature that may cause the Rband plateau phase. However, Co II features in the range of 6500 - 7000 {\AA} and the Fe II feature of the I-band are clearly seen in the theoretical spectra, but do not appear to provide all of the flux necessary for the R-band plateau.

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arXiv2013

Habitability around F-type Stars

We explore the general astrobiological significance of F-type main-sequence stars with masses between 1.2 and 1.5 Msun. Special consideration is given to stellar evolutionary aspects due to nuclear main-sequence evolution. DNA is taken as a proxy for carbon-based macromolecules following the paradigm that extraterrestrial biology may be most likely based on hydrocarbons. Consequently, the DNA action spectrum is utilized to represent the impact of the stellar UV radiation. Planetary atmospheric attenuation is taken into account based on parameterized attenuation functions. We found that the damage inflicted on DNA for planets at Earth-equivalent positions is between a factor of 2.5 and 7.1 higher than for solar-like stars, and there are intricate relations for the time-dependence of damage during stellar main-sequence evolution. If attenuation is considered, smaller factors of damage are obtained in alignment to the attenuation parameters. This work is motivated by earlier studies indicating that the UV environment of solar-type stars is one of the most decisive factors in determining the suitability of exosolar planets and exomoons for biological evolution and sustainability.

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arXiv2012

A 3D radiative transfer framework IX. Time dependence

Context. Time-dependent, 3D radiation transfer calculations are important for the modeling of a variety of objects, from supernovae and novae to simulations of stellar variability and activity. Furthermore, time-dependent calculations can be used to obtain a 3D radiative equilibrium model structure via relaxation in time. Aims. We extend our 3D radiative transfer framework to include direct time dependence of the radiation field; i.e., the $\partial I/ \partial t$ terms are fully considered in the solution of radiative transfer problems. Methods. We build on the framework that we have described in previous papers in this series and develop a subvoxel method for the $\partial I/\partial t$ terms. Results. We test the implementation by comparing the 3D results to our well tested 1D time dependent radiative transfer code in spherical symmetry. A simple 3D test model is also presented. Conclusions. The 3D time dependent radiative transfer method is now included in our 3D RT framework and in PHOENIX/3D.

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arXiv2011

Theoretical light curves of type Ia supernovae

Aims. We present the first theoretical SN Ia light curves calculated with the time-dependent version of the general purpose model atmosphere code PHOENIX. Our goal is to produce light curves and spectra of hydro models of all types of supernovae. Methods. We extend our model atmosphere code PHOENIX to calculate type Ia supernovae light curves. A simple solver was imple- mented which keeps track of energy conservation in the atmosphere during the free expansion phase. Results. The correct operation of the new additions to PHOENIX were verified in test calculations. Furthermore, we calculated theo- retical light curves and compared them to the observed SN Ia light curves of SN 1999ee and SN 2002bo. We obtained LTE as well as NLTE model light curves. Conclusions. We have verified the correct operation of our extension into the time domain. We have calculated the first SN Ia model light curves using PHOENIX in both LTE and NLTE. For future work the infrared model light curves need to be further investigated.

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arXiv2009

Time-dependent radiative transfer with PHOENIX

Aims. We present first results and tests of a time-dependent extension to the general purpose model atmosphere code PHOENIX. We aim to produce light curves and spectra of hydro models for all types of supernovae. Methods. We extend our model atmosphere code PHOENIX to solve time-dependent non-grey, NLTE, radiative transfer in a special relativistic framework. A simple hydrodynamics solver was implemented to keep track of the energy conservation of the atmosphere during free expansion. Results. The correct operation of the new additions to PHOENIX were verified in test calculations. Conclusions. We have shown the correct operation of our extension to time-dependent radiative transfer and will be able to calculate supernova light curves and spectra in future work.

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