Jose Carlos Guirado - Academia.edu (original) (raw)

Papers by Jose Carlos Guirado

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

Radio observations allow us to probe the nature of low mass objects, including ultracool dwarfs. ... more Radio observations allow us to probe the nature of low mass objects, including ultracool dwarfs. Additionally, very long baseline interferometric observations provide the highest angular resolution in astronomy to date, which opens a window to measure dynamical masses of these objects in order to benchmark evolutionary models. We present here a multi-epoch, multi-frequency study of the substellar triple system VHS 1256−1257. We have found radio emission at 6 GHz and 33 GHz coincident with the expected position of the central binary of VHS 1256−1257. No emission is detected at higher frequencies (230 GHz and 345 GHz) nor at 5 GHz with VLBI arrays. The emission appears to be stable over almost 3 years at 6 GHz. These results can be well explained by non-thermal gyrosynchrotron emission originating at radiation belts present in both components and viewed equatorially. We also share preliminary results from our VLA observations of 5 low-mass binary systems and EVN observations of 4 ultracool dwarfs (including a binary).

Astronomy & Astrophysics, 2018

We have studied a complete radio sample of active galactic nuclei with the very-long-baseline-int... more We have studied a complete radio sample of active galactic nuclei with the very-long-baseline-interferometry (VLBI) technique and for the first time successfully obtained high-precision phase-delay astrometry atQband (43 GHz) from observations acquired in 2010. We have compared our astrometric results with those obtained with the same technique atUband (15 GHz) from data collected in 2000. The differences in source separations among all the source pairs observed in common at the two epochs are compatible at the 1σlevel betweenUandQbands. With the benefit of quasi-simultaneousUandQband observations in 2010, we have studied chromatic effects (core-shift) at the radio source cores with three different methods. The magnitudes of the core-shifts are of the same order (about 0.1 mas) for all methods. However, some discrepancies arise in the orientation of the core-shifts determined through the different methods. In some cases these discrepancies are due to insufficient signal for the meth...

ESO Astrophysics Symposia European Southern Observatory

capjournal.org

Our team has developed a variety of astronomical activities specifically intended for people with... more Our team has developed a variety of astronomical activities specifically intended for people with special needs. In particular, we have created a series of interactive talks, software with astronomical content, hands-on activities, and a planetarium show, among others, to communicate astronomy to people with different kinds of physical and cognitive disabilities. The main goal of this work is to stress the fact that working with this kind of audience is not as difficult as it might seem a priori. Moreover, the experience is formative and very rewarding for everyone involved.

Astronomische Nachrichten, 2005

We expand upon the results of Close et al. 2005 regarding the young, low-mass object AB Dor C and... more We expand upon the results of Close et al. 2005 regarding the young, low-mass object AB Dor C and its role as a calibration point for theoretical tracks. We present an improved spectral reduction and a new orbital solution with two additional epochs. Our improved analysis confirms our spectral type of M8 (±1) and mass of 0.090 ±0.003 M ⊙ for AB Dor C. Comparing the results for AB Dor C with other young, low-mass objects with dynamical masses we find a general trend where current evolutionary models tend to over-predict the temperature (or under-predict the mass) for low mass stars and brown dwarfs. Given our precision, there is a ∼99% chance that the mass of AB Dor C is underestimated by the DUSTY tracks in the HR diagram.

Astronomy and …, 1999

From simultaneous 8.4 and 2.3 GHz VLBA observations we have studied the morphology and properties... more From simultaneous 8.4 and 2.3 GHz VLBA observations we have studied the morphology and properties of the compact components of the gravitational lens PKS 1830-211. We have found that the size of the southwest component increases as λ 2 , most probably due to scattering by interstellar plasma, while the size of the northeast component does not increase with such wavelength dependence. The small separation between both lensed components, compared with the typical size of scattering clumps in our Galaxy, suggests that the scattering material is located in the lensing galaxy, rather than in the Milky Way. Within the uncertainties, the flux-density ratios of the brightest feature in the two compact components for the two frequency bands agree with each other, as expected for images of the same background feature.

Astronomy and Astrophysics, 1998

From two sets of very-long-baseline interferometry (VLBI) observations, one each at 5 GHz (1985.7... more From two sets of very-long-baseline interferometry (VLBI) observations, one each at 5 GHz (1985.77) and at 2.3 and 8.4 GHz simultaneously (1988.83), of the quasar 1928+738 phase-referenced to the BL Lac object 2007+777, we infer a change in their relative position of 0.74 ±0.22 milliarcseconds (mas) along a position angle of –9 ◦±25◦, where the quoted standard errors account for errors in our models of the geometry of the interferometric array, the atmosphere, and the source structure. This change is interpreted as an (inadvertent) shift of the reference point selected in the structure of 1928+738, due to the presence of an emerging component; different opacity effects in the three observing frequency bands may also be a significant contributor.

Precise determinations of dynamical masses of pre-main-sequence (PMS) stars are necessary to cali... more Precise determinations of dynamical masses of pre-main-sequence (PMS) stars are necessary to calibrate PMS stellar evolutionary models, whose predictions are in disagreement with measurements for masses below 1.2 solar masses. Binary stars in young, nearby loose associations (moving groups) are particularly good candidates, primarily because all members share a common age. Belonging to the AB Doradus moving group, we have observed the binary AB Dor Ba/Bb, 0.06" separation, with the Australian Long Baseline Array at 8.4 GHz. We have detected the two components Ba/Bb, which facilitates (i) a measurement of the relative orbital motion through subsequent radio maps, and (ii) an estimate of the orbital parameters, once combined the radio information with infrared relative astrometry. Our preliminary analysis shows that best-fit orbit corresponds to that with a period of 1.1 yr and semi major axis of 0.068". The sum of the masses AB Dor Ba/Bb is 0.3 +/- 0.1 solar masses. The stu...

We present 5 GHz space-VLBI observations of the quasar pair B 1342+662/B 1342+663 which show the ... more We present 5 GHz space-VLBI observations of the quasar pair B 1342+662/B 1342+663 which show the feasibility of using phase-referencing techniques with the HALCA satellite. From residual referenced phases we derive an upper bound of 10 meters to the uncertainty of the HALCA's orbit reconstruction. An analysis of the phase-reference maps of the observed sources also suggests that the above mentioned uncertainty is very likely as small as 3 meters. With errors of this magnitude, HALCA is a useful tool for astrometric studies of close pairs of radiosources.

Astronomy and Astrophysics, 1996

... Subsequent VLBI observations made in 1990, 1991 and 1992 did not show any evidence of motions... more ... Subsequent VLBI observations made in 1990, 1991 and 1992 did not show any evidence of motions in 3C 395 (Lara et al. ... the peak of brightness of component A, or as even an apparent deceleration of component C due to phase effects caused by Kelvin-Helmholtz instabilities ...

Experimental Astronomy, 2021

This paper presents the ESA Voyage 2050 White Paper for a concept of TeraHertz Exploration and Zo... more This paper presents the ESA Voyage 2050 White Paper for a concept of TeraHertz Exploration and Zooming-in for Astrophysics (THEZA). It addresses the science case and some implementation issues of a space-borne radio interferometric system for ultra-sharp imaging of celestial radio sources at the level of angular resolution down to (sub-) microarcseconds. THEZA focuses at millimetre and sub-millimetre wavelengths (frequencies above sim\sim sim ∼ 300 GHz), but allows for science operations at longer wavelengths too. The THEZA concept science rationale is focused on the physics of spacetime in the vicinity of supermassive black holes as the leading science driver. The main aim of the concept is to facilitate a major leap by providing researchers with orders of magnitude improvements in the resolution and dynamic range in direct imaging studies of the most exotic objects in the Universe, black holes. The concept will open up a sizeable range of hitherto unreachable parameters of observation...

Proceedings of Wide Field Astronomy & Technology for the Square Kilometre Array — PoS(SKADS 2009), 2011

60°L at=-32°Lat=-23°F ig. 6: Loci of constant declination plotted in a polar plot as a function o... more 60°L at=-32°Lat=-23°F ig. 6: Loci of constant declination plotted in a polar plot as a function of polar coordinates.

Proceedings of the International Astronomical Union, 2005

We expand upon the results of Close et al. 2005 regarding the young, low-mass object AB Dor C and... more We expand upon the results of Close et al. 2005 regarding the young, low-mass object AB Dor C and its role as a calibration point for theoretical tracks. We argue for a new 70±30 Myr age estimate and present two additional detections of C with the Simultaneous Differential Imaging (SDI) camera. Our improved analysis (Nielsen et al. 2005) confirms our spectral type of M8 (±1) and mass of 0.090 ±0.003 M sun for AB Dor C. However, Luhman & Potter (2006) argue for a hotter spectral type (M6±1). Here we adopt the consistent spectral range of M7±1 in this paper as a final spectral type. Plotting AB Dor C (and all other young (<0.12 Myr), lowmass (0.3 − 0.05Msun) objects with accurate dynamical masses) on the HR diagram suggests a trend where current evolutionary models tend to over-predict the temperature (or under-predict the mass) for young low-mass stars and high-mass brown dwarfs. With our uncertainties, there is a ∼90% chance that the mass of AB Dor C is underestimated by the DUSTY tracks in the HR diagram.

International Astronomical Union Colloquium, 1998

The link of the Hipparcos and VLBI extragalactic reference frames has been achieved with a precis... more The link of the Hipparcos and VLBI extragalactic reference frames has been achieved with a precision of 0.0005″ in global orientation at the epoch of the catalogue (1991.25) and of 0.0003″/yr in rate of rotation by VLBI observations of 12 radio-emitting stars.

Extragalactic Radio Sources, 1996

Astronomy and Astrophysics

ABSTRACT

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

Radio observations allow us to probe the nature of low mass objects, including ultracool dwarfs. ... more Radio observations allow us to probe the nature of low mass objects, including ultracool dwarfs. Additionally, very long baseline interferometric observations provide the highest angular resolution in astronomy to date, which opens a window to measure dynamical masses of these objects in order to benchmark evolutionary models. We present here a multi-epoch, multi-frequency study of the substellar triple system VHS 1256−1257. We have found radio emission at 6 GHz and 33 GHz coincident with the expected position of the central binary of VHS 1256−1257. No emission is detected at higher frequencies (230 GHz and 345 GHz) nor at 5 GHz with VLBI arrays. The emission appears to be stable over almost 3 years at 6 GHz. These results can be well explained by non-thermal gyrosynchrotron emission originating at radiation belts present in both components and viewed equatorially. We also share preliminary results from our VLA observations of 5 low-mass binary systems and EVN observations of 4 ultracool dwarfs (including a binary).

Astronomy & Astrophysics, 2018

We have studied a complete radio sample of active galactic nuclei with the very-long-baseline-int... more We have studied a complete radio sample of active galactic nuclei with the very-long-baseline-interferometry (VLBI) technique and for the first time successfully obtained high-precision phase-delay astrometry atQband (43 GHz) from observations acquired in 2010. We have compared our astrometric results with those obtained with the same technique atUband (15 GHz) from data collected in 2000. The differences in source separations among all the source pairs observed in common at the two epochs are compatible at the 1σlevel betweenUandQbands. With the benefit of quasi-simultaneousUandQband observations in 2010, we have studied chromatic effects (core-shift) at the radio source cores with three different methods. The magnitudes of the core-shifts are of the same order (about 0.1 mas) for all methods. However, some discrepancies arise in the orientation of the core-shifts determined through the different methods. In some cases these discrepancies are due to insufficient signal for the meth...

ESO Astrophysics Symposia European Southern Observatory

capjournal.org

Our team has developed a variety of astronomical activities specifically intended for people with... more Our team has developed a variety of astronomical activities specifically intended for people with special needs. In particular, we have created a series of interactive talks, software with astronomical content, hands-on activities, and a planetarium show, among others, to communicate astronomy to people with different kinds of physical and cognitive disabilities. The main goal of this work is to stress the fact that working with this kind of audience is not as difficult as it might seem a priori. Moreover, the experience is formative and very rewarding for everyone involved.

Astronomische Nachrichten, 2005

We expand upon the results of Close et al. 2005 regarding the young, low-mass object AB Dor C and... more We expand upon the results of Close et al. 2005 regarding the young, low-mass object AB Dor C and its role as a calibration point for theoretical tracks. We present an improved spectral reduction and a new orbital solution with two additional epochs. Our improved analysis confirms our spectral type of M8 (±1) and mass of 0.090 ±0.003 M ⊙ for AB Dor C. Comparing the results for AB Dor C with other young, low-mass objects with dynamical masses we find a general trend where current evolutionary models tend to over-predict the temperature (or under-predict the mass) for low mass stars and brown dwarfs. Given our precision, there is a ∼99% chance that the mass of AB Dor C is underestimated by the DUSTY tracks in the HR diagram.

Astronomy and …, 1999

From simultaneous 8.4 and 2.3 GHz VLBA observations we have studied the morphology and properties... more From simultaneous 8.4 and 2.3 GHz VLBA observations we have studied the morphology and properties of the compact components of the gravitational lens PKS 1830-211. We have found that the size of the southwest component increases as λ 2 , most probably due to scattering by interstellar plasma, while the size of the northeast component does not increase with such wavelength dependence. The small separation between both lensed components, compared with the typical size of scattering clumps in our Galaxy, suggests that the scattering material is located in the lensing galaxy, rather than in the Milky Way. Within the uncertainties, the flux-density ratios of the brightest feature in the two compact components for the two frequency bands agree with each other, as expected for images of the same background feature.

Astronomy and Astrophysics, 1998

From two sets of very-long-baseline interferometry (VLBI) observations, one each at 5 GHz (1985.7... more From two sets of very-long-baseline interferometry (VLBI) observations, one each at 5 GHz (1985.77) and at 2.3 and 8.4 GHz simultaneously (1988.83), of the quasar 1928+738 phase-referenced to the BL Lac object 2007+777, we infer a change in their relative position of 0.74 ±0.22 milliarcseconds (mas) along a position angle of –9 ◦±25◦, where the quoted standard errors account for errors in our models of the geometry of the interferometric array, the atmosphere, and the source structure. This change is interpreted as an (inadvertent) shift of the reference point selected in the structure of 1928+738, due to the presence of an emerging component; different opacity effects in the three observing frequency bands may also be a significant contributor.

Precise determinations of dynamical masses of pre-main-sequence (PMS) stars are necessary to cali... more Precise determinations of dynamical masses of pre-main-sequence (PMS) stars are necessary to calibrate PMS stellar evolutionary models, whose predictions are in disagreement with measurements for masses below 1.2 solar masses. Binary stars in young, nearby loose associations (moving groups) are particularly good candidates, primarily because all members share a common age. Belonging to the AB Doradus moving group, we have observed the binary AB Dor Ba/Bb, 0.06" separation, with the Australian Long Baseline Array at 8.4 GHz. We have detected the two components Ba/Bb, which facilitates (i) a measurement of the relative orbital motion through subsequent radio maps, and (ii) an estimate of the orbital parameters, once combined the radio information with infrared relative astrometry. Our preliminary analysis shows that best-fit orbit corresponds to that with a period of 1.1 yr and semi major axis of 0.068". The sum of the masses AB Dor Ba/Bb is 0.3 +/- 0.1 solar masses. The stu...

We present 5 GHz space-VLBI observations of the quasar pair B 1342+662/B 1342+663 which show the ... more We present 5 GHz space-VLBI observations of the quasar pair B 1342+662/B 1342+663 which show the feasibility of using phase-referencing techniques with the HALCA satellite. From residual referenced phases we derive an upper bound of 10 meters to the uncertainty of the HALCA's orbit reconstruction. An analysis of the phase-reference maps of the observed sources also suggests that the above mentioned uncertainty is very likely as small as 3 meters. With errors of this magnitude, HALCA is a useful tool for astrometric studies of close pairs of radiosources.

Astronomy and Astrophysics, 1996

... Subsequent VLBI observations made in 1990, 1991 and 1992 did not show any evidence of motions... more ... Subsequent VLBI observations made in 1990, 1991 and 1992 did not show any evidence of motions in 3C 395 (Lara et al. ... the peak of brightness of component A, or as even an apparent deceleration of component C due to phase effects caused by Kelvin-Helmholtz instabilities ...

Experimental Astronomy, 2021

This paper presents the ESA Voyage 2050 White Paper for a concept of TeraHertz Exploration and Zo... more This paper presents the ESA Voyage 2050 White Paper for a concept of TeraHertz Exploration and Zooming-in for Astrophysics (THEZA). It addresses the science case and some implementation issues of a space-borne radio interferometric system for ultra-sharp imaging of celestial radio sources at the level of angular resolution down to (sub-) microarcseconds. THEZA focuses at millimetre and sub-millimetre wavelengths (frequencies above sim\sim sim ∼ 300 GHz), but allows for science operations at longer wavelengths too. The THEZA concept science rationale is focused on the physics of spacetime in the vicinity of supermassive black holes as the leading science driver. The main aim of the concept is to facilitate a major leap by providing researchers with orders of magnitude improvements in the resolution and dynamic range in direct imaging studies of the most exotic objects in the Universe, black holes. The concept will open up a sizeable range of hitherto unreachable parameters of observation...

Proceedings of Wide Field Astronomy & Technology for the Square Kilometre Array — PoS(SKADS 2009), 2011

60°L at=-32°Lat=-23°F ig. 6: Loci of constant declination plotted in a polar plot as a function o... more 60°L at=-32°Lat=-23°F ig. 6: Loci of constant declination plotted in a polar plot as a function of polar coordinates.

Proceedings of the International Astronomical Union, 2005

We expand upon the results of Close et al. 2005 regarding the young, low-mass object AB Dor C and... more We expand upon the results of Close et al. 2005 regarding the young, low-mass object AB Dor C and its role as a calibration point for theoretical tracks. We argue for a new 70±30 Myr age estimate and present two additional detections of C with the Simultaneous Differential Imaging (SDI) camera. Our improved analysis (Nielsen et al. 2005) confirms our spectral type of M8 (±1) and mass of 0.090 ±0.003 M sun for AB Dor C. However, Luhman & Potter (2006) argue for a hotter spectral type (M6±1). Here we adopt the consistent spectral range of M7±1 in this paper as a final spectral type. Plotting AB Dor C (and all other young (<0.12 Myr), lowmass (0.3 − 0.05Msun) objects with accurate dynamical masses) on the HR diagram suggests a trend where current evolutionary models tend to over-predict the temperature (or under-predict the mass) for young low-mass stars and high-mass brown dwarfs. With our uncertainties, there is a ∼90% chance that the mass of AB Dor C is underestimated by the DUSTY tracks in the HR diagram.

International Astronomical Union Colloquium, 1998

The link of the Hipparcos and VLBI extragalactic reference frames has been achieved with a precis... more The link of the Hipparcos and VLBI extragalactic reference frames has been achieved with a precision of 0.0005″ in global orientation at the epoch of the catalogue (1991.25) and of 0.0003″/yr in rate of rotation by VLBI observations of 12 radio-emitting stars.

Extragalactic Radio Sources, 1996

Astronomy and Astrophysics

ABSTRACT