Artis Aberfelds - Academia.edu (original) (raw)

Papers by Artis Aberfelds

Proceedings of the International Astronomical Union, Dec 1, 2022

Monthly Notices of the Royal Astronomical Society

We present the results of a 5-yr monitoring program of 42 sources targeted at 6.7-GHz methanol ma... more We present the results of a 5-yr monitoring program of 42 sources targeted at 6.7-GHz methanol masers, conducted from 2017 March to 2022 October using the Irbene 32- and 16-m radio telescopes. Sources were observed with irregular intervals where time between two consecutive observations ranged from 24 h to 35 d. We found that more than 55 per cent of the sources showed significant variability, but often only one or a few spectral features were varying significantly. Numerous type of variability were found in our sample: low-variable, periodic, irregular, synchronized, and anticorrelated between features and steadily raising or falling flux. Our analysis techniques also uncover new variability trends for several sources. The maser monitoring program is one of the first single-dish science initiatives at the Irbene radio telescope complex, initiated shortly after the instrument’s reconstruction and upgrades. Our findings unequivocally demonstrate its suitability for maser research pur...

Monthly Notices of the Royal Astronomical Society

The variability study of 6.7 GHz methanol masers has become a useful way to improve our understan... more The variability study of 6.7 GHz methanol masers has become a useful way to improve our understanding of the physical conditions in high-mass star-forming regions. Based on the single-dish monitoring using the Irbene telescopes, we selected three sources with close sky positions. We imaged them using the European very long baseline interferometry (VLBI) Network and searched available data on VLBI archives to follow detailed changes in their structures and single maser spot variability. All three targets show a few groups of maser cloudlets of a typical size of 3.5 mas and the majority of them show linear or arched structures with velocity gradients of order 0.22 km s−1 mas−1. The cloudlets and overall source morphologies are remarkably stable on time-scales of 7–15 yr, supporting a scenario of variability due to changes in the maser pumping rate.

Publications of the Astronomical Society of Japan

In 2019 September, a sudden flare of the 6.7 GHz methanol maser was observed toward the high-mass... more In 2019 September, a sudden flare of the 6.7 GHz methanol maser was observed toward the high-mass young stellar object (HMYSO) G24.33+0.14. This may represent the fourth detection of a transient mass accretion event in an HMYSO after S255IR NIRS3, NGC 6334I-MM1, and G358.93−0.03-MM1. G24.33+0.14 is unique among these sources as it clearly shows a repeating flare with an 8 yr interval. Using the Atacama Large Millimeter/submillimeter Array (ALMA), we observed the millimeter continuum and molecular lines toward G24.33+0.14 in the pre-flare phase in 2016 August (ALMA Cycle 3) and the mid-flare phase in 2019 September (ALMA Cycle 6). We identified three continuum sources in G24.33+0.14, and the brightest source, C1, which is closely associated with the 6.7 GHz maser emission, shows only a marginal increase in flux density with a flux ratio (Cycle 6$/$Cycle 3) of 1.16 ± 0.01, considering an additional absolute flux calibration uncertainty of 1010\%10. We identified 26 transitions from 13 m...

Veikta augstas izšķirtspējas absorbcijas spektra analīze zvaigznei BS17569-0011, kas literatūrā i... more Veikta augstas izšķirtspējas absorbcijas spektra analīze zvaigznei BS17569-0011, kas literatūrā ir klasificētā kā metālnabadzīga vecākās paaudzes zvaigzne ar anomālu enerģijas sadalījumu spektrā. Noteikts masas centra radiālas ātrums, atmosfēras efektīvā temperatūra, virsmas gravitācija, mikroturbulences ātrums un piecu ķīmisko elementu koncentrācijas. Zvaigznes spektrā identificētas intensīvas TiO līnijas un emisija Balmera H līnijā. Atmosfēras parametri ir tipiski sarkanajiem milžiem, Teff = 4250 K, log(g)=1,0 (cgs), = 3,0 km/s. Dzelzs atomu koncentrācija pētāmās zvaigznes atmosfērā ir apmēram 16 reizes zemāka nekā Saules atmosfērā. Elementu koncentrācijas, kas veidojas neitronu satveršanas procesos, ir relatīvi paaugstināta apmēram 10 reizes. Iegūtie rezultāti liecina, ka BS17569-0011 pieder Galaktikas II populācijas zvaigznēm un atrodas sarkanā milža evolūcijas stadijā. Atslēgvārdi: BS17569-0011 – atmosfēras ķīmiskais sastāvs – augstas izšķirtspējas spektroskopija – zvaigžņu evo...

Veikta zvaigznes BS17569–0011 analīze izmantojot augstas izšķirtspējas absorbcijas spektru un atm... more Veikta zvaigznes BS17569–0011 analīze izmantojot augstas izšķirtspējas absorbcijas spektru un atmosfēru modeļu metodi. Literatūrā zvaigzne klasificēta kā Galaktikas vecākās paaudzes zvaigzne ar ļoti zemu metālu koncentrāciju atmosfērā un anomālu enerģijas sadalījumu spektrā. Veikta zvaigznes integrālā starojuma analīze un konstatēts mainīgums ar dominējošo periodu, P ~ 50–75 dienas. BS17569–0011 spektrā tika identificētas ļoti intensīvas TiO molekulas absorbcijas līnijas, kas liecina par oglekļa un skābekļa attiecību C/O < 1, un emisija Balmera H un H līnijās, kas norāda uz masas zaudēšanas procesiem. Darbā noteikts zvaigznes masas centra radiālais ātrums, precizēta efektīvā temperatūra, virsmas gravitācija, mikroturbulences ātrums un aprēķināta 23 ķīmisko elementu koncentrācijas atmosfērā. Aprēķinātie atmosfēras parametri ir raksturīgi aukstajām zvaigznēm ar plašām un retinātām atmosfērām, Teff = 3300 K, log(g) = 0.0 (cgs), = 3.0 km/s.During this scientific work star’s BS17569-0...

Proceedings of European VLBI Network Mini-Symposium and Users' Meeting 2021 — PoS(EVN2021), Mar 30, 2022

The Maser Monitoring Organisation (M2O) is a research community of telescope operators, astronomy... more The Maser Monitoring Organisation (M2O) is a research community of telescope operators, astronomy researchers and maser theoreticians pursuing a joint goal of reaching a deeper understanding of maser emission and exploring its variety of uses as tracers of astrophysical events. These proceedings detail the origin, motivations and current status of the M2O, as was introduced at the 2021 EVN symposium.

Proceedings of European VLBI Network Mini-Symposium and Users' Meeting 2021 — PoS(EVN2021), 2022

Sciences (VeUAS) has developed an 800 m single-baseline interferometer consisting of 32 m (IR) an... more Sciences (VeUAS) has developed an 800 m single-baseline interferometer consisting of 32 m (IR) and 16 m (IB) radio telescopes with 15 arcseconds of angular resolution. Irbene radio telescopes are operating at C/M/X-band (4.5-8.8 GHz). Raw telescope data are correlated with the Software FX Correlator (SFXC) [1] and output visibilities are calibrated with a ParselTongue * pipeline. Data processing techniques provide a velocity resolution of 0.088 −1 and spectral resolution of 0.002 MHz and enabling observations to approach the theoretical sensitivity limit of the interferometer, at about 6 mJy for a 120 s integration. We propose to use the Irbene interferometer for discoveries in a new age of transient astrophysics. In this paper, we will show correlation and calibration schema and initial monitoring results.

Astronomy & Astrophysics, 2021

Aims.Molecular masers, including methanol and hydroxyl masers, and in particular the ones in exci... more Aims.Molecular masers, including methanol and hydroxyl masers, and in particular the ones in excited rotational states (ex-OH masers), are one of the most informative tools for studying star-forming regions. So, the discovery, of new maser sources in these regions is of great importance. Many studies and surveys of ex-OH maser sources have been carried out in the southern celestial hemisphere, but only a few have been done in the northern hemisphere. The specific aim of this work is to close this gap.Methods.The star-forming regions in the northern hemisphere with known active methanol masers were observed to search for new ex-OH maser sources with the 32 m and 16 m radio telescopes of the Ventspils International Radio Astronomy Centre (VIRAC).Results.Three OH maser lines in the excited state at the 6035 MHz in three northern hemisphere star-forming regions are detected. The maser 189.030+0.783 was previously known, but we suggest this maser is a possible variable. We confirm recent...

Modern scientific research particularly radio astronomi- cal spectroscopic observations cannot be... more Modern scientific research particularly radio astronomi- cal spectroscopic observations cannot be imagined without appropriate software suite. That is necessary because of two reasons 1) data are in digital form and 2) data processing is a complex multi-step process. In this paper created in VIRAC data processing suite for a single radio telescope, space maser observations are presented. Software Defined Radio (SDR) backend data processing is described. Implementation of the frequency switching algorithm, acquisition of observation data and their format for storing are discussed. Multiple ways to display the observation results are highlighted.

Proceedings of 14th European VLBI Network Symposium & Users Meeting — PoS(EVN2018), 2019

Ventspils International Radio Astronomy Centre (VIRAC, Latvia) operates two radio telescopes, RT-... more Ventspils International Radio Astronomy Centre (VIRAC, Latvia) operates two radio telescopes, RT-16 and RT-32 accordingly, with fully steerable 16-and 32-m Cassegrain type antennas. The main receiving systems of the both telescopes are cryogenic receivers with 4.5-8.8 GHz frequency range. Additionally, in 2019 radio telescope RT-32 will be equipped with a new L band receiver. Data registration units of the both antennas are suitable for interferometric observations. The Nicolaus Copernicus University Department of Radio Astronomy in Torun, Poland, operates a 32m radio telescope RT-4, which also works in similar bands-L, C and Mand regularly participates in the VLBI observations. Moreover, VIRAC possesses a high-performance computer cluster with SFXC software correlator developed at JIVE. In the spring 2018, VIRAC team with support from Torun group conducted several VLBI experiments in the baseline Irbene-Torun using the Irbene RT-16 radio telescope and Torun RT-4 radio telescope. Successful fringes and FITS files with obtained uv data implies that data correlation and post processing facilities established in VIRAC allow to run small-scale interferometric tasks.

Proceedings of the International Astronomical Union, 2017

We report another 6.7 GHz methanol maser burst in the high mass star region G33.641-0.228. The fl... more We report another 6.7 GHz methanol maser burst in the high mass star region G33.641-0.228. The flare is in its second component at vLSR = 59.6 km s−1 and was observed in August-September 2016 by VIRAC radio telescope RT-32 in Irbene, Latvia. Several bursts of the second spatial component of G33.641-0.228 have been reported previously by Fujisawa et al. The maximum peak flux density of the source was measured to reach 343 Jy that is 13 times increase from its ground level. Significant oscillations were discovered during the decay phase indicating a more complex burst mechanism that cannot be explained by a simple heating of the region.

Proceedings of the International Astronomical Union, 2017

Ventspils International Radio Astronomy Centre (VIRAC) has two fully steerable Cassegrean System ... more Ventspils International Radio Astronomy Centre (VIRAC) has two fully steerable Cassegrean System 32 and 16 m radio telescopes. After renovation and modernization program the Galactic masers, particularly CH3OH research and monitoring program became one of the most important realized on these telescopes. Both telescopes are equipped with broadband cryogenic receivers covering 4.5-8.8 GHz frequency band. Digital backend consisting from DBBC-2 (Digital Base Band Convertor developed by HAT-LAB, Italy) and FLEXBUFF (data storage system based on commercially available server system) is used for data digitalization and registration. A special program complex for spectral line data reduction and correction was developed and implemented.

Proceedings of the International Astronomical Union, 2017

The first long-term maser (mainly methanol) monitoring program is under way with the radio telesc... more The first long-term maser (mainly methanol) monitoring program is under way with the radio telescopes of Ventspils International Radio Astronomy Center. The first activity of this program was to develop an observations methodology and data registration and reduction software for the Ventspils telescopes. The developed routines are to be used for maser variability monitoring, investigating short bursts of intensity and a search for new, previously unknown, maser sources. Currently the program consists of 41 methanol masers observed at 6.7 GHz, while new ones are periodically added. The maser sources are observed at 3 – 5 day intervals. It was found that most the sources display a significant level of variability with time, ranging from a few days, up to several months and, perhaps, years. In addition to non-varying masers, several types of maser variability behavior were observed, including: monotonic increases or decreases, un-periodical, quasi-periodic and periodic variations.

Proceedings of the International Astronomical Union, Dec 1, 2022

Monthly Notices of the Royal Astronomical Society

We present the results of a 5-yr monitoring program of 42 sources targeted at 6.7-GHz methanol ma... more We present the results of a 5-yr monitoring program of 42 sources targeted at 6.7-GHz methanol masers, conducted from 2017 March to 2022 October using the Irbene 32- and 16-m radio telescopes. Sources were observed with irregular intervals where time between two consecutive observations ranged from 24 h to 35 d. We found that more than 55 per cent of the sources showed significant variability, but often only one or a few spectral features were varying significantly. Numerous type of variability were found in our sample: low-variable, periodic, irregular, synchronized, and anticorrelated between features and steadily raising or falling flux. Our analysis techniques also uncover new variability trends for several sources. The maser monitoring program is one of the first single-dish science initiatives at the Irbene radio telescope complex, initiated shortly after the instrument’s reconstruction and upgrades. Our findings unequivocally demonstrate its suitability for maser research pur...

Monthly Notices of the Royal Astronomical Society

The variability study of 6.7 GHz methanol masers has become a useful way to improve our understan... more The variability study of 6.7 GHz methanol masers has become a useful way to improve our understanding of the physical conditions in high-mass star-forming regions. Based on the single-dish monitoring using the Irbene telescopes, we selected three sources with close sky positions. We imaged them using the European very long baseline interferometry (VLBI) Network and searched available data on VLBI archives to follow detailed changes in their structures and single maser spot variability. All three targets show a few groups of maser cloudlets of a typical size of 3.5 mas and the majority of them show linear or arched structures with velocity gradients of order 0.22 km s−1 mas−1. The cloudlets and overall source morphologies are remarkably stable on time-scales of 7–15 yr, supporting a scenario of variability due to changes in the maser pumping rate.

Publications of the Astronomical Society of Japan

In 2019 September, a sudden flare of the 6.7 GHz methanol maser was observed toward the high-mass... more In 2019 September, a sudden flare of the 6.7 GHz methanol maser was observed toward the high-mass young stellar object (HMYSO) G24.33+0.14. This may represent the fourth detection of a transient mass accretion event in an HMYSO after S255IR NIRS3, NGC 6334I-MM1, and G358.93−0.03-MM1. G24.33+0.14 is unique among these sources as it clearly shows a repeating flare with an 8 yr interval. Using the Atacama Large Millimeter/submillimeter Array (ALMA), we observed the millimeter continuum and molecular lines toward G24.33+0.14 in the pre-flare phase in 2016 August (ALMA Cycle 3) and the mid-flare phase in 2019 September (ALMA Cycle 6). We identified three continuum sources in G24.33+0.14, and the brightest source, C1, which is closely associated with the 6.7 GHz maser emission, shows only a marginal increase in flux density with a flux ratio (Cycle 6$/$Cycle 3) of 1.16 ± 0.01, considering an additional absolute flux calibration uncertainty of 1010\%10. We identified 26 transitions from 13 m...

Veikta augstas izšķirtspējas absorbcijas spektra analīze zvaigznei BS17569-0011, kas literatūrā i... more Veikta augstas izšķirtspējas absorbcijas spektra analīze zvaigznei BS17569-0011, kas literatūrā ir klasificētā kā metālnabadzīga vecākās paaudzes zvaigzne ar anomālu enerģijas sadalījumu spektrā. Noteikts masas centra radiālas ātrums, atmosfēras efektīvā temperatūra, virsmas gravitācija, mikroturbulences ātrums un piecu ķīmisko elementu koncentrācijas. Zvaigznes spektrā identificētas intensīvas TiO līnijas un emisija Balmera H līnijā. Atmosfēras parametri ir tipiski sarkanajiem milžiem, Teff = 4250 K, log(g)=1,0 (cgs), = 3,0 km/s. Dzelzs atomu koncentrācija pētāmās zvaigznes atmosfērā ir apmēram 16 reizes zemāka nekā Saules atmosfērā. Elementu koncentrācijas, kas veidojas neitronu satveršanas procesos, ir relatīvi paaugstināta apmēram 10 reizes. Iegūtie rezultāti liecina, ka BS17569-0011 pieder Galaktikas II populācijas zvaigznēm un atrodas sarkanā milža evolūcijas stadijā. Atslēgvārdi: BS17569-0011 – atmosfēras ķīmiskais sastāvs – augstas izšķirtspējas spektroskopija – zvaigžņu evo...

Veikta zvaigznes BS17569–0011 analīze izmantojot augstas izšķirtspējas absorbcijas spektru un atm... more Veikta zvaigznes BS17569–0011 analīze izmantojot augstas izšķirtspējas absorbcijas spektru un atmosfēru modeļu metodi. Literatūrā zvaigzne klasificēta kā Galaktikas vecākās paaudzes zvaigzne ar ļoti zemu metālu koncentrāciju atmosfērā un anomālu enerģijas sadalījumu spektrā. Veikta zvaigznes integrālā starojuma analīze un konstatēts mainīgums ar dominējošo periodu, P ~ 50–75 dienas. BS17569–0011 spektrā tika identificētas ļoti intensīvas TiO molekulas absorbcijas līnijas, kas liecina par oglekļa un skābekļa attiecību C/O < 1, un emisija Balmera H un H līnijās, kas norāda uz masas zaudēšanas procesiem. Darbā noteikts zvaigznes masas centra radiālais ātrums, precizēta efektīvā temperatūra, virsmas gravitācija, mikroturbulences ātrums un aprēķināta 23 ķīmisko elementu koncentrācijas atmosfērā. Aprēķinātie atmosfēras parametri ir raksturīgi aukstajām zvaigznēm ar plašām un retinātām atmosfērām, Teff = 3300 K, log(g) = 0.0 (cgs), = 3.0 km/s.During this scientific work star’s BS17569-0...

Proceedings of European VLBI Network Mini-Symposium and Users' Meeting 2021 — PoS(EVN2021), Mar 30, 2022

The Maser Monitoring Organisation (M2O) is a research community of telescope operators, astronomy... more The Maser Monitoring Organisation (M2O) is a research community of telescope operators, astronomy researchers and maser theoreticians pursuing a joint goal of reaching a deeper understanding of maser emission and exploring its variety of uses as tracers of astrophysical events. These proceedings detail the origin, motivations and current status of the M2O, as was introduced at the 2021 EVN symposium.

Proceedings of European VLBI Network Mini-Symposium and Users' Meeting 2021 — PoS(EVN2021), 2022

Sciences (VeUAS) has developed an 800 m single-baseline interferometer consisting of 32 m (IR) an... more Sciences (VeUAS) has developed an 800 m single-baseline interferometer consisting of 32 m (IR) and 16 m (IB) radio telescopes with 15 arcseconds of angular resolution. Irbene radio telescopes are operating at C/M/X-band (4.5-8.8 GHz). Raw telescope data are correlated with the Software FX Correlator (SFXC) [1] and output visibilities are calibrated with a ParselTongue * pipeline. Data processing techniques provide a velocity resolution of 0.088 −1 and spectral resolution of 0.002 MHz and enabling observations to approach the theoretical sensitivity limit of the interferometer, at about 6 mJy for a 120 s integration. We propose to use the Irbene interferometer for discoveries in a new age of transient astrophysics. In this paper, we will show correlation and calibration schema and initial monitoring results.

Astronomy & Astrophysics, 2021

Aims.Molecular masers, including methanol and hydroxyl masers, and in particular the ones in exci... more Aims.Molecular masers, including methanol and hydroxyl masers, and in particular the ones in excited rotational states (ex-OH masers), are one of the most informative tools for studying star-forming regions. So, the discovery, of new maser sources in these regions is of great importance. Many studies and surveys of ex-OH maser sources have been carried out in the southern celestial hemisphere, but only a few have been done in the northern hemisphere. The specific aim of this work is to close this gap.Methods.The star-forming regions in the northern hemisphere with known active methanol masers were observed to search for new ex-OH maser sources with the 32 m and 16 m radio telescopes of the Ventspils International Radio Astronomy Centre (VIRAC).Results.Three OH maser lines in the excited state at the 6035 MHz in three northern hemisphere star-forming regions are detected. The maser 189.030+0.783 was previously known, but we suggest this maser is a possible variable. We confirm recent...

Modern scientific research particularly radio astronomi- cal spectroscopic observations cannot be... more Modern scientific research particularly radio astronomi- cal spectroscopic observations cannot be imagined without appropriate software suite. That is necessary because of two reasons 1) data are in digital form and 2) data processing is a complex multi-step process. In this paper created in VIRAC data processing suite for a single radio telescope, space maser observations are presented. Software Defined Radio (SDR) backend data processing is described. Implementation of the frequency switching algorithm, acquisition of observation data and their format for storing are discussed. Multiple ways to display the observation results are highlighted.

Proceedings of 14th European VLBI Network Symposium & Users Meeting — PoS(EVN2018), 2019

Ventspils International Radio Astronomy Centre (VIRAC, Latvia) operates two radio telescopes, RT-... more Ventspils International Radio Astronomy Centre (VIRAC, Latvia) operates two radio telescopes, RT-16 and RT-32 accordingly, with fully steerable 16-and 32-m Cassegrain type antennas. The main receiving systems of the both telescopes are cryogenic receivers with 4.5-8.8 GHz frequency range. Additionally, in 2019 radio telescope RT-32 will be equipped with a new L band receiver. Data registration units of the both antennas are suitable for interferometric observations. The Nicolaus Copernicus University Department of Radio Astronomy in Torun, Poland, operates a 32m radio telescope RT-4, which also works in similar bands-L, C and Mand regularly participates in the VLBI observations. Moreover, VIRAC possesses a high-performance computer cluster with SFXC software correlator developed at JIVE. In the spring 2018, VIRAC team with support from Torun group conducted several VLBI experiments in the baseline Irbene-Torun using the Irbene RT-16 radio telescope and Torun RT-4 radio telescope. Successful fringes and FITS files with obtained uv data implies that data correlation and post processing facilities established in VIRAC allow to run small-scale interferometric tasks.

Proceedings of the International Astronomical Union, 2017

We report another 6.7 GHz methanol maser burst in the high mass star region G33.641-0.228. The fl... more We report another 6.7 GHz methanol maser burst in the high mass star region G33.641-0.228. The flare is in its second component at vLSR = 59.6 km s−1 and was observed in August-September 2016 by VIRAC radio telescope RT-32 in Irbene, Latvia. Several bursts of the second spatial component of G33.641-0.228 have been reported previously by Fujisawa et al. The maximum peak flux density of the source was measured to reach 343 Jy that is 13 times increase from its ground level. Significant oscillations were discovered during the decay phase indicating a more complex burst mechanism that cannot be explained by a simple heating of the region.

Proceedings of the International Astronomical Union, 2017

Ventspils International Radio Astronomy Centre (VIRAC) has two fully steerable Cassegrean System ... more Ventspils International Radio Astronomy Centre (VIRAC) has two fully steerable Cassegrean System 32 and 16 m radio telescopes. After renovation and modernization program the Galactic masers, particularly CH3OH research and monitoring program became one of the most important realized on these telescopes. Both telescopes are equipped with broadband cryogenic receivers covering 4.5-8.8 GHz frequency band. Digital backend consisting from DBBC-2 (Digital Base Band Convertor developed by HAT-LAB, Italy) and FLEXBUFF (data storage system based on commercially available server system) is used for data digitalization and registration. A special program complex for spectral line data reduction and correction was developed and implemented.

Proceedings of the International Astronomical Union, 2017

The first long-term maser (mainly methanol) monitoring program is under way with the radio telesc... more The first long-term maser (mainly methanol) monitoring program is under way with the radio telescopes of Ventspils International Radio Astronomy Center. The first activity of this program was to develop an observations methodology and data registration and reduction software for the Ventspils telescopes. The developed routines are to be used for maser variability monitoring, investigating short bursts of intensity and a search for new, previously unknown, maser sources. Currently the program consists of 41 methanol masers observed at 6.7 GHz, while new ones are periodically added. The maser sources are observed at 3 – 5 day intervals. It was found that most the sources display a significant level of variability with time, ranging from a few days, up to several months and, perhaps, years. In addition to non-varying masers, several types of maser variability behavior were observed, including: monotonic increases or decreases, un-periodical, quasi-periodic and periodic variations.