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Register a new userSpectrophotometric Distances to Galactic H,{sc{ii}} Regions
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Added by
Alessandro Mois\\'es
Publication date
2010
fields
Physics
and research's language is
English
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We present a near infrared study of the stellar content of 35 H,{sc{ii}} regions in the Galactic plane. In this work, we have used the near infrared domain $J-$, $H-$ and $K_{s}-$ band color images to visually inspect the sample. Also, color-color and color-magnitude diagrams were used to indicate ionizing star candidates, as well as, the presence of young stellar objects such as classical TTauri Stars (CTTS) and massive young stellar objects (MYSOs). We have obtained {it Spitzer} IRAC images for each region to help further characterize them. {it Spitzer} and near infrared morphology to place each cluster in an evolutionary phase of development. {it Spitzer} photometry was also used to classify the MYSOs. Comparison of the main sequence in color-magnitude diagrams to each observed cluster was used to infer whether or not the cluster kinematic distance is consistent with brightnesses of the stellar sources. We find qualitative agreement for a dozen of the regions, but about half the regions have near infrared photometry that suggests they may be closer than the kinematic distance. A significant fraction of these already have spectrophotometric parallaxes which support smaller distances. These discrepancies between kinematic and spectrophotometric distances are not due to the spectrophotometric methodologies, since independent non-kinematic measurements are in agreement with the spectrophotometric results. For instance, trigonometric parallaxes of star-forming regions were collected from the literature and show the same effect of smaller distances when compared to the kinematic results. In our sample of H,{sc{ii}} regions, most of the clusters are evident in the near infrared images. Finally, it is possible to distinguish among qualitative evolutionary stages for these objects.
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The determination of accurate distances to star-forming regions are discussed in the broader historical context of astronomical distance measurements. We summarize recent results for regions within 1 kpc and present perspectives for the near and more distance future.
We use observations of quiet Sun (QS) regions in the H$alpha$ 6563 AA, Ca~{sc ii} 8542 AA, and Fe~{sc i} 6302 AA lines. We observe brightenings in the wings of the H$alpha$ and Ca~{sc ii} combined with observations of the interacting magnetic concentrations observed in the Stokes signals of Fe~{sc i}. These brightenings are similar to Ellerman bombs (EBs), i.e. impulsive bursts in the wings of the Balmer lines which leave the line cores unaffected. Such enhancements suggest that these events have similar formation mechanisms to the classical EBs found in active regions, with the reduced intensity enhancements found in the QS regions due to a weaker feeding magnetic flux. The observations also show that the quiet Sun Ellerman bombs (QSEBs) are formed at a higher height in the upper photosphere than the photospheric continuum level. Using simulations, we investigate the formation mechanism associated with the events and suggest that these events are driven by the interaction of magnetic field-lines in the upper photospheric regions. The results of the simulation are in agreement with observations when comparing the light-curves, and in most cases we found that the peak in the Ca~{sc ii} 8542 AA wing occurred before the peak in H$alpha$ wing. Moreover, in some cases, the line profiles observed in Ca~{sc ii} are asymmetrical with a raised core profile. The source of heating in these events is shown by the MURaM simulations and is suggested to occur 430 km above the photosphere.
Stellar feedback plays a fundamental role in shaping the evolution of galaxies. Here we explore the use of ionised gas kinematics in young, bipolar H II regions as a probe of early feedback in these star-forming environments. We have undertaken a multiwavelength study of a young, bipolar H II region in the Galactic disc, G$316.81-0.06$, which lies at the centre of a massive ($sim10^3$ M$_{odot}$) infrared-dark cloud filament. It is still accreting molecular gas as well as driving a $sim 0.2$ pc ionised gas outflow perpendicular to the filament. Intriguingly, we observe a large velocity gradient ($47.81 pm 3.21$ km s$^{-1}$ pc$^{-1}$) across the ionised gas in a direction perpendicular to the outflow. This kinematic signature of the ionised gas shows a reasonable correspondence with the simulations of young H II regions. Based on a qualitative comparison between our observations and these simulations, we put forward a possible explanation for the velocity gradients observed in G$316.81-0.06$. If the velocity gradient perpendicular to the outflow is caused by rotation of the ionised gas, then we infer that this rotation is a direct result of the initial net angular momentum in the natal molecular cloud. If this explanation is correct, this kinematic signature should be common in other young (bipolar) H II regions. We suggest that further quantitative analysis of the ionised gas kinematics of young H II regions, combined with additional simulations, should improve our understanding of feedback at these early stages.
A large number (67) of the compact/ultra-compact H II regions identified in the Coordinated Radio and Infrared Survey for High-Mass Star Formation catalogue were determined to be powered by a Lyman continuum flux in excess of what was expected given their corresponding luminosity. In this study we attempt to reasonably explain the Lyman excess phenomenon in as many of the 67 H II regions as possible through a variety of observational and astrophysical means including new luminosity estimates, new Herschel photometry, new distance determinations, the use of different models for dust and ionized gas covering factors, and the use of different stellar calibrations. This phenomenon has been observed before; however, the objects shown to exhibit this behaviour in the literature have decidedly different physical properties than the regions in our sample, and thus the origin of the excess is not the same. We find that the excess can be reproduced using OB stellar atmosphere models that have been slightly modified in the extreme ultraviolet. Though the exact mechanism producing the excess is still uncertain, we do find that a scaled up magnetospheric accretion model, often used to explain similar emission from T Tauri stars, is unable to match our observations. Our results suggest that the Lyman excess may be associated with younger H II regions, and that it is more commonly found in early B-type stars. Our refined sample of 24 Lyman excess H II regions provides an ideal sample for comparative studies with regular H II regions, and can act as the basis for the further detailed study of individual regions.
We report the observational findings of the Sh2-112 H{sc ii} region by using the multiwavelength data analysis ranging from optical to radio wavelengths. This region is powered by a massive O8V-type star BD +45 3216. The surface density distribution and minimum spanning tree analyses of the young stellar object (YSO) candidates in the region reveal their groupings toward the western periphery of the H{sc ii} region. A GMRT radio continuum emission peak is found toward the north-west boundary of the H{sc ii} region and is investigated as a compact/ultra-compact H{sc ii} region candidate powered by a B0-B0.5 type star. Toward the south-west direction, a prominent curved rim-like structure is found in the H$alpha$ image and GMRT radio continuum maps, where the H$_2$ and $^{13}$CO emission is also observed. These results suggest the existence of the ionized boundary layer (IBL) on the surface of the molecular cloud. This IBL is found to be over-pressurized with respect to the internal pressure of the surrounding molecular cloud. This implies that the shocks are propagating/ propagated into the molecular cloud and the young stars identified within it are likely triggered due to the massive star. It is also found that this region is ionization bounded toward the west-direction and density bounded toward the east-direction. Based on the distribution of the ionized gas, molecular material, and the YSO candidates; we propose that the Sh2-112 H{sc ii} region is a good candidate for the blister-type H{sc ii} region which has been evolved on the surface of a cylindrical molecular cloud.
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