Well-widened, high-dispersion s p e c t r a were used to obtain the identifications and equivalen... more Well-widened, high-dispersion s p e c t r a were used to obtain the identifications and equivalent widths for over 1500 lines between 3700 f o r the relatively cool Ap star, HD 204411. these data was performed.
We have detected the bright molecular bar in M83 standing out as a 100% enhancement of molecular ... more We have detected the bright molecular bar in M83 standing out as a 100% enhancement of molecular emission with respect to the off-bar emission at the same radii. We compare the spatial variations in the star formation efficiency, as traced by Ha emission and the surface density of the interstellar gas_ in M83 and M51. Both the central bar of M83 and the spiral arms of M51 are regions characterized by high massive star formation rates. For M83, we ascribe the fact that both the gas surface density and the star formation efficiency are high to the hydrodynamics of the central region.
Model atmospheres have been computed i n the range 6000 5 Teff 5 7500°K Convection is found to pl... more Model atmospheres have been computed i n the range 6000 5 Teff 5 7500°K Convection is found to play a significant with 2. 0 5 log g 5 3. 5. obtain f o r stars i n the RR L y r a e gap. role i n the influence of the emergent fluxes predicted by these models. Temperature determinations based on purely radiative models a r e systematically too low by as much as 2 0 0 "-3 5 0 " K; gravity determinations are not affected. The method of predicting (B-V), and thus reddening, f r o m the strength of the Balmer lines is found to give l e s s than desirable results.
We review the present knowledge of disk accretion in young low mass stars, and in particular, the... more We review the present knowledge of disk accretion in young low mass stars, and in particular, the mass accretion rate ˙M and its evolution with time. The methods used to obtain mass accretion rates from ultraviolet excesses and emission lines are described, and the ...
s of recently accepted papers T Chamaeleontis: a ”weak-line” YY Orionis star? J.M. Alcalá , E. Co... more s of recently accepted papers T Chamaeleontis: a ”weak-line” YY Orionis star? J.M. Alcalá , E. Covino,M. Franchini , J. Krautter ,L. Terranegra 2,4 and R. Wichmann 1 Landessternwarte-Königstuhl, D-6900 Heidelberg,Germany 2 Osservatorio Astronomico di Capodimonte, Via Moiariello 16, I-80131 Napoli, Italy 3 Osservatorio Astronomico di Trieste, Via G. Tiepolo 11, I-34131 Trieste, Italy 4 Instituto Nacional de Astrof́ısica, Optica y Electrónica, A.P. 51 y 216 C.P. 72000 Puebla, Mexico We present new low and high resolution spectroscopy and optical photometry of the star T Cha. From the low resolution spectroscopy we classify T Cha as a G8 type star. We also identify T Cha as the optical counterpart of the ROSAT all-sky survey X-ray source RX J1157.2-7921. On the basis of the physical association with the Chamaeleon cloud complex, the erratic photometric variability and colour behaviour, the presence of Hα emission and of Li λ 6708 absorption line, as well as its position on the HR diagr...
0 , 6°). It has been classified by Morgan as a moderately strong metallic line cluster of class V... more 0 , 6°). It has been classified by Morgan as a moderately strong metallic line cluster of class V. In this classification, NGC 6712 lies between the strong line nucleus clusters like NGC 6356 of class VI and the very weak-line clusters like Ml5 and M92 of class I. This investigation shows that the CM diagram is also intermediate in its several characteristics. Photographic plates and a photoelectric sequence rangingfrom 12.5 to 17.5, 5=13.0 to 5=18.5 were obtained with the 200-inch telescope. The CM diagram shows a well-defined giant branch, a long horizontal branch starting from 5-F= 1.35, F=16.15 and going to 5-F=0.35, F=16.5 which contains the variable star gap whose edges are at 5-F=0.60 and 5-F=0.88, and the beginning of the subgiant sequence. We have terminated the photometry at V-\7 to avoid the extreme crowding problem in this difficult field. The reddening, determined from the position of the variable star gap is 5(5-F)= 0.44 with the normal assumptions. A lower limit of 5(5-F) =0.27 was found by UBV photometry of field stars closer than NGC 6712. If M v = + 1.0 for the RR Lyrae stars, then the apparent modulus is mM = 15.15 and the true modulus is 13.8. The CM diagram shows that the majority of the horizontal branch stars lie redward of the variable star gap although there are a few stars on the blue side. This agrees with previous results which suggested that the very strong metallic line clusters (NGC 6356, 47 Tue) have their horizontal branch stars entirely on the red side of the gap while stars in very weak line clusters (Ml5, M92) are almost entirely on the blue side. Furthermore, the magnitude difference between the horizontal branch and the giant branch, read at (5-F) = 1.4, is AF=2.0 instead of 3.0 as in the low metal-abundance halo clusters. Therefore, the present data strengthens the conclusion that the M v of the brightest stars in globular clusters depends on the chemical composition (Sandage, A., and Wallerstein, G., Astrophys. /. 131, 598, 1960).
ABSTRACT It is shown how flux from the Galactic magnetic field radically alters the problems of e... more ABSTRACT It is shown how flux from the Galactic magnetic field radically alters the problems of equilibrium, collapse, and fragmentation of self-gravitating gas clouds involved in star formation. The roles are examined of Alfvenic turbulent motion, magnetic transport of angular momentum, flux leakage and the magnetic field and associated turbulence in the formation of the first optically thick fragments that will form stars. The masses of the resulting protostars are discussed.
Symposium - International Astronomical Union, 1965
To assist in the interpretation of ultraviolet continuum observations, we present observational f... more To assist in the interpretation of ultraviolet continuum observations, we present observational features of a grid of non-grey model atmosphères in strict radiative equilibrium which have effective temperatures in the range 10000 °K to 20000 °K and surface gravities 103 and 104. For three models, we add the blended wings of the higher Balmer and Lyman hydrogen lines and include in an approximate way further effects of line blanketing in the ultraviolet. Finally, we present a preliminary calculation of the effect of departures from L. T. E. on the continuous flux.To examine the validity of the models, we compare the predicted continuous flux and predicted line profiles with the corresponding observed quantities for Vega and Sirius.We consider the question of discrepancies between theory and observations in the ultraviolet from the viewpoint of these new calculations, and find generally good agreement with recent observations made by T. Stecher.
JKM results, main sequence stars only 110 4.10 JKM results, the full IRTF data set Ill 4.11 JKN r... more JKM results, main sequence stars only 110 4.10 JKM results, the full IRTF data set Ill 4.11 JKN results, the full IRTF data set 112 4.12 Third order polynomial fit to JKM MS colors 113 4.13 JKM model results, Tdest = 1800, a = 2 115 5.1 Spot models, spots 1000 K cooler than photosphere xviii 5.2 Spot models, spots 2000 K cooler than photosphere 124 5.3 Spot models, 500 K spots 5.4
Through existing studies of star-forming regions, Spitzer has created rich databases which have a... more Through existing studies of star-forming regions, Spitzer has created rich databases which have already profoundly influenced our ability to understand the star and planet formation process on micro and macro scales. However, it is essential to note that Spitzer observations to date have focused largely on deep observations of regions of recent star formation associated directly with well-known molecular clouds located within 500 pc. What has not been done is to explore to sufficient depth or breadth a representative sample of the much larger regions surrounding the more massive of these molecular clouds. Also, while there have been targeted studies of specific distant star forming regions, in general, there has been little attention devoted to mapping and characterizing the stellar populations and star-forming histories of the surrounding giant molecular clouds (GMCs). As a result, we have yet to develop an understanding of the major physical processes that control star formation on the scale or spiral arms. Doing so will allow much better comparison of star-formation in our galaxy to the star-forming complexes that dominate the spiral arms of external galaxies. The power of Spitzer in the Warm Mission for studies of star formation is its ability to carry out large-scale surveys unbiased by prior knowledge of ongoing star formation or the presence of molecular clouds. The Spitzer Warm Mission will provide two uniquely powerful capabilities that promise equally profound advances : high sensitivity and efficient coverage of many hundreds of square degrees, and angular resolution sufficient to resolve dense groups and clusters of YSOs and to identify contaminating background galaxies whose colors mimic those of young stars. In this contribution, we describe two major programs: a survey of the outer regions of selected nearby OB associations, and a study of distant GMCs and star formation on the scale of a spiral arm.
We describe a 'point design' for a 30m Giant Segmented Mirror Telescope (GSMT) aimed at meeting a... more We describe a 'point design' for a 30m Giant Segmented Mirror Telescope (GSMT) aimed at meeting a set of initial science goals developed over a period of two years by working groups comprised of more than 60 astronomers. The paper summarizes these goals briefly, captures the top-level performance requirements that follow from them, and describes a plausible, first-cut technical solution developed as part of an overall systems-level analysis. The key features of the point design are: (1) a fast (f/1) primary; (2) an adaptive secondary that serves both to compensate for the effects of wind buffeting and as the first stage of three adaptive optics systems: (i) multi-conjugate AO; (ii) high-performance on-axis AO; (iii) ground-level seeing compensation; (3) a radio telescope structure; (4) multiple instrument ports (prime focus; Nasmyth foci; direct Cass); (5) an hierarchical control system comprising multiple active and adaptive elements.
Proceedings of the International Astronomical Union, 2005
A key science driver for future ELTs is to chronicle the complete formation and evolutionary hist... more A key science driver for future ELTs is to chronicle the complete formation and evolutionary histories of a meaningful number of nearby galaxies through their resolved stars. The goal will be to measure the entire star formation and chemical enrichment histories of a sample of galaxies that includes all Hubble types and covers all of their components, demanding photometry of stars in regions with high surface brightness at distances of up to 20Mpc. We present simulations that compare the abilities of 20, 30, 50, and 100m telescopes to recover the correct stellar population mix represented in field star color-magnitude diagrams observed with J, H, and K filters. As input, our simulations use scenes containing stars drawn from a mix of model isochrones with differing ages and metallicities, with surface densities set to match that found in the M31 bulge and at the effective radius of NGC 3379. We convolve these scenes with PSFs corresponding to the projected performance of MCAO systems containing two deformable mirrors, including the effect of realistic variations in the atmospheric turbulence profile over the span of the observations. These simulations provide a way to evaluate the scientific advances enabled by ELTs of differing apertures in the area of extragalactic stellar populations.
Optical/infrared (O/IR) astronomy in the 1990's is reviewed. The following subject areas are ... more Optical/infrared (O/IR) astronomy in the 1990's is reviewed. The following subject areas are included: research environment; science opportunities; technical development of the 1980's and opportunities for the 1990's; and ground-based O/IR astronomy outside the U.S. Recommendations are presented for: (1) large scale programs (Priority 1: a coordinated program for large O/IR telescopes); (2) medium scale programs (Priority 1: a coordinated
The following topics were discussed: (1) current observation evidence for the presence of circums... more The following topics were discussed: (1) current observation evidence for the presence of circumstellar disks associated with solar type pre-main sequence (PMS) stars; (2) the properties of such disks; and (3) the disk environment.
Based on a panel discussion at the meeting "New Light on Young Stars: Spitzer's View of ... more Based on a panel discussion at the meeting "New Light on Young Stars: Spitzer's View of Circumstellar Disks", we provide some definitions of common usage of terms describing disks and related objects.
Well-widened, high-dispersion s p e c t r a were used to obtain the identifications and equivalen... more Well-widened, high-dispersion s p e c t r a were used to obtain the identifications and equivalent widths for over 1500 lines between 3700 f o r the relatively cool Ap star, HD 204411. these data was performed.
We have detected the bright molecular bar in M83 standing out as a 100% enhancement of molecular ... more We have detected the bright molecular bar in M83 standing out as a 100% enhancement of molecular emission with respect to the off-bar emission at the same radii. We compare the spatial variations in the star formation efficiency, as traced by Ha emission and the surface density of the interstellar gas_ in M83 and M51. Both the central bar of M83 and the spiral arms of M51 are regions characterized by high massive star formation rates. For M83, we ascribe the fact that both the gas surface density and the star formation efficiency are high to the hydrodynamics of the central region.
Model atmospheres have been computed i n the range 6000 5 Teff 5 7500°K Convection is found to pl... more Model atmospheres have been computed i n the range 6000 5 Teff 5 7500°K Convection is found to play a significant with 2. 0 5 log g 5 3. 5. obtain f o r stars i n the RR L y r a e gap. role i n the influence of the emergent fluxes predicted by these models. Temperature determinations based on purely radiative models a r e systematically too low by as much as 2 0 0 "-3 5 0 " K; gravity determinations are not affected. The method of predicting (B-V), and thus reddening, f r o m the strength of the Balmer lines is found to give l e s s than desirable results.
We review the present knowledge of disk accretion in young low mass stars, and in particular, the... more We review the present knowledge of disk accretion in young low mass stars, and in particular, the mass accretion rate ˙M and its evolution with time. The methods used to obtain mass accretion rates from ultraviolet excesses and emission lines are described, and the ...
s of recently accepted papers T Chamaeleontis: a ”weak-line” YY Orionis star? J.M. Alcalá , E. Co... more s of recently accepted papers T Chamaeleontis: a ”weak-line” YY Orionis star? J.M. Alcalá , E. Covino,M. Franchini , J. Krautter ,L. Terranegra 2,4 and R. Wichmann 1 Landessternwarte-Königstuhl, D-6900 Heidelberg,Germany 2 Osservatorio Astronomico di Capodimonte, Via Moiariello 16, I-80131 Napoli, Italy 3 Osservatorio Astronomico di Trieste, Via G. Tiepolo 11, I-34131 Trieste, Italy 4 Instituto Nacional de Astrof́ısica, Optica y Electrónica, A.P. 51 y 216 C.P. 72000 Puebla, Mexico We present new low and high resolution spectroscopy and optical photometry of the star T Cha. From the low resolution spectroscopy we classify T Cha as a G8 type star. We also identify T Cha as the optical counterpart of the ROSAT all-sky survey X-ray source RX J1157.2-7921. On the basis of the physical association with the Chamaeleon cloud complex, the erratic photometric variability and colour behaviour, the presence of Hα emission and of Li λ 6708 absorption line, as well as its position on the HR diagr...
0 , 6°). It has been classified by Morgan as a moderately strong metallic line cluster of class V... more 0 , 6°). It has been classified by Morgan as a moderately strong metallic line cluster of class V. In this classification, NGC 6712 lies between the strong line nucleus clusters like NGC 6356 of class VI and the very weak-line clusters like Ml5 and M92 of class I. This investigation shows that the CM diagram is also intermediate in its several characteristics. Photographic plates and a photoelectric sequence rangingfrom 12.5 to 17.5, 5=13.0 to 5=18.5 were obtained with the 200-inch telescope. The CM diagram shows a well-defined giant branch, a long horizontal branch starting from 5-F= 1.35, F=16.15 and going to 5-F=0.35, F=16.5 which contains the variable star gap whose edges are at 5-F=0.60 and 5-F=0.88, and the beginning of the subgiant sequence. We have terminated the photometry at V-\7 to avoid the extreme crowding problem in this difficult field. The reddening, determined from the position of the variable star gap is 5(5-F)= 0.44 with the normal assumptions. A lower limit of 5(5-F) =0.27 was found by UBV photometry of field stars closer than NGC 6712. If M v = + 1.0 for the RR Lyrae stars, then the apparent modulus is mM = 15.15 and the true modulus is 13.8. The CM diagram shows that the majority of the horizontal branch stars lie redward of the variable star gap although there are a few stars on the blue side. This agrees with previous results which suggested that the very strong metallic line clusters (NGC 6356, 47 Tue) have their horizontal branch stars entirely on the red side of the gap while stars in very weak line clusters (Ml5, M92) are almost entirely on the blue side. Furthermore, the magnitude difference between the horizontal branch and the giant branch, read at (5-F) = 1.4, is AF=2.0 instead of 3.0 as in the low metal-abundance halo clusters. Therefore, the present data strengthens the conclusion that the M v of the brightest stars in globular clusters depends on the chemical composition (Sandage, A., and Wallerstein, G., Astrophys. /. 131, 598, 1960).
ABSTRACT It is shown how flux from the Galactic magnetic field radically alters the problems of e... more ABSTRACT It is shown how flux from the Galactic magnetic field radically alters the problems of equilibrium, collapse, and fragmentation of self-gravitating gas clouds involved in star formation. The roles are examined of Alfvenic turbulent motion, magnetic transport of angular momentum, flux leakage and the magnetic field and associated turbulence in the formation of the first optically thick fragments that will form stars. The masses of the resulting protostars are discussed.
Symposium - International Astronomical Union, 1965
To assist in the interpretation of ultraviolet continuum observations, we present observational f... more To assist in the interpretation of ultraviolet continuum observations, we present observational features of a grid of non-grey model atmosphères in strict radiative equilibrium which have effective temperatures in the range 10000 °K to 20000 °K and surface gravities 103 and 104. For three models, we add the blended wings of the higher Balmer and Lyman hydrogen lines and include in an approximate way further effects of line blanketing in the ultraviolet. Finally, we present a preliminary calculation of the effect of departures from L. T. E. on the continuous flux.To examine the validity of the models, we compare the predicted continuous flux and predicted line profiles with the corresponding observed quantities for Vega and Sirius.We consider the question of discrepancies between theory and observations in the ultraviolet from the viewpoint of these new calculations, and find generally good agreement with recent observations made by T. Stecher.
JKM results, main sequence stars only 110 4.10 JKM results, the full IRTF data set Ill 4.11 JKN r... more JKM results, main sequence stars only 110 4.10 JKM results, the full IRTF data set Ill 4.11 JKN results, the full IRTF data set 112 4.12 Third order polynomial fit to JKM MS colors 113 4.13 JKM model results, Tdest = 1800, a = 2 115 5.1 Spot models, spots 1000 K cooler than photosphere xviii 5.2 Spot models, spots 2000 K cooler than photosphere 124 5.3 Spot models, 500 K spots 5.4
Through existing studies of star-forming regions, Spitzer has created rich databases which have a... more Through existing studies of star-forming regions, Spitzer has created rich databases which have already profoundly influenced our ability to understand the star and planet formation process on micro and macro scales. However, it is essential to note that Spitzer observations to date have focused largely on deep observations of regions of recent star formation associated directly with well-known molecular clouds located within 500 pc. What has not been done is to explore to sufficient depth or breadth a representative sample of the much larger regions surrounding the more massive of these molecular clouds. Also, while there have been targeted studies of specific distant star forming regions, in general, there has been little attention devoted to mapping and characterizing the stellar populations and star-forming histories of the surrounding giant molecular clouds (GMCs). As a result, we have yet to develop an understanding of the major physical processes that control star formation on the scale or spiral arms. Doing so will allow much better comparison of star-formation in our galaxy to the star-forming complexes that dominate the spiral arms of external galaxies. The power of Spitzer in the Warm Mission for studies of star formation is its ability to carry out large-scale surveys unbiased by prior knowledge of ongoing star formation or the presence of molecular clouds. The Spitzer Warm Mission will provide two uniquely powerful capabilities that promise equally profound advances : high sensitivity and efficient coverage of many hundreds of square degrees, and angular resolution sufficient to resolve dense groups and clusters of YSOs and to identify contaminating background galaxies whose colors mimic those of young stars. In this contribution, we describe two major programs: a survey of the outer regions of selected nearby OB associations, and a study of distant GMCs and star formation on the scale of a spiral arm.
We describe a 'point design' for a 30m Giant Segmented Mirror Telescope (GSMT) aimed at meeting a... more We describe a 'point design' for a 30m Giant Segmented Mirror Telescope (GSMT) aimed at meeting a set of initial science goals developed over a period of two years by working groups comprised of more than 60 astronomers. The paper summarizes these goals briefly, captures the top-level performance requirements that follow from them, and describes a plausible, first-cut technical solution developed as part of an overall systems-level analysis. The key features of the point design are: (1) a fast (f/1) primary; (2) an adaptive secondary that serves both to compensate for the effects of wind buffeting and as the first stage of three adaptive optics systems: (i) multi-conjugate AO; (ii) high-performance on-axis AO; (iii) ground-level seeing compensation; (3) a radio telescope structure; (4) multiple instrument ports (prime focus; Nasmyth foci; direct Cass); (5) an hierarchical control system comprising multiple active and adaptive elements.
Proceedings of the International Astronomical Union, 2005
A key science driver for future ELTs is to chronicle the complete formation and evolutionary hist... more A key science driver for future ELTs is to chronicle the complete formation and evolutionary histories of a meaningful number of nearby galaxies through their resolved stars. The goal will be to measure the entire star formation and chemical enrichment histories of a sample of galaxies that includes all Hubble types and covers all of their components, demanding photometry of stars in regions with high surface brightness at distances of up to 20Mpc. We present simulations that compare the abilities of 20, 30, 50, and 100m telescopes to recover the correct stellar population mix represented in field star color-magnitude diagrams observed with J, H, and K filters. As input, our simulations use scenes containing stars drawn from a mix of model isochrones with differing ages and metallicities, with surface densities set to match that found in the M31 bulge and at the effective radius of NGC 3379. We convolve these scenes with PSFs corresponding to the projected performance of MCAO systems containing two deformable mirrors, including the effect of realistic variations in the atmospheric turbulence profile over the span of the observations. These simulations provide a way to evaluate the scientific advances enabled by ELTs of differing apertures in the area of extragalactic stellar populations.
Optical/infrared (O/IR) astronomy in the 1990's is reviewed. The following subject areas are ... more Optical/infrared (O/IR) astronomy in the 1990's is reviewed. The following subject areas are included: research environment; science opportunities; technical development of the 1980's and opportunities for the 1990's; and ground-based O/IR astronomy outside the U.S. Recommendations are presented for: (1) large scale programs (Priority 1: a coordinated program for large O/IR telescopes); (2) medium scale programs (Priority 1: a coordinated
The following topics were discussed: (1) current observation evidence for the presence of circums... more The following topics were discussed: (1) current observation evidence for the presence of circumstellar disks associated with solar type pre-main sequence (PMS) stars; (2) the properties of such disks; and (3) the disk environment.
Based on a panel discussion at the meeting "New Light on Young Stars: Spitzer's View of ... more Based on a panel discussion at the meeting "New Light on Young Stars: Spitzer's View of Circumstellar Disks", we provide some definitions of common usage of terms describing disks and related objects.
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Papers by Stephen Strom