Shaila Akhter - Academia.edu (original) (raw)
Papers by Shaila Akhter
Giant molecular clouds are the hosts for massive star formation, which occurs within dense conden... more Giant molecular clouds are the hosts for massive star formation, which occurs within dense condensation within the more diffuse gas. The HOPS Mopra telescope survey of the southern Galactic plane identified many regions of dense gas using transitions of the molecule ammonia. We are now examining these regions in greater depth, and here we describe how we are decomposing the areas of ammonia emission into 3-dimensional clumps. While 3-D clump finding is now routinely undertaken, the hyperfine structure of ammonia presents extra difficulties, and we discuss ways of getting around this problem. This study is a first step in understanding the clumping and multiplicity of dense (possibly) protostellar clumps throughout differing regions of the Milky Way Galaxy.
Monthly Notices of the Royal Astronomical Society, 2019
We present the first release of the data and compact-source catalogue for the JCMT Large Program ... more We present the first release of the data and compact-source catalogue for the JCMT Large Program SCUBA-2 Continuum Observations of Pre-protostellar Evolution (SCOPE). SCOPE
The Astrophysical Journal Supplement Series, 2018
The low dust temperatures (<14 K) of Planck Galactic Cold Clumps (PGCCs) make them ideal targets ... more The low dust temperatures (<14 K) of Planck Galactic Cold Clumps (PGCCs) make them ideal targets to probe the initial conditions and very early phase of star formation. "TOP-SCOPE" is a joint survey program targeting ∼2000 PGCCs in J=1-0 transitions of CO isotopologues and ∼1000 PGCCs in 850 µm continuum emisison. The objective of the "TOP-SCOPE" survey and the joint surveys (SMT 10-m, KVN 21-m and NRO 45-m) is to statistically study the initial conditions occurring during star formation and the evolution of molecular clouds, across a wide range of environments. The observations, data analysis and example science cases for these surveys are introduced with an exemplar source, PGCC G26.53+0.17 (G26), which is a filamentary infrared dark cloud (IRDC). The total mass, the length and the mean line-mass (M/L) of the G26 filament are ∼6200 M , ∼12 pc and ∼500 M pc −1 , respectively. Ten massive clumps including eight starless ones are found along the filament. The most massive Clump as a whole may be still in global collapse while its denser part seems to be undergoing expansion due to outflow feedback. The fragmentation in G26 filament from cloud scale to clump scale is in agreement with gravitational fragmentation of an isothermal, non-magnetized, and turbulent supported cylinder. A bimodal behavior in dust emissivity spectral index (β) distribution is found in G26, suggesting grain growth along the filament. The G26 filament may be formed due to large-scale compression flows evidenced by the temperature and velocity gradients across its natal cloud.
The physical and chemical properties of prestellar cores, especially massive ones, are still far ... more The physical and chemical properties of prestellar cores, especially massive ones, are still far from being well understood due to the lack of a large sample. The low dust temperature (<14 K) of Planck cold clumps makes them promising candidates for prestellar objects or for sources at the very initial stages of protostellar collapse. We have been conducting a series of observations toward Planck cold clumps (PCCs) with ground-based radio telescopes. In general, when compared with other star forming samples (e.g. infrared dark clouds), PCCs are more quiescent, suggesting that most of them may be in the earliest phase of star formation. However, some PCCs are associated with protostars and molecular outflows, indicating that not all PCCs are in a prestellar phase. We have identified hundreds of starless dense clumps from the mapping survey with the Purple Mountain Observatory (PMO) 13.7-m telescope. Follow-up observations suggest that these dense clumps are ideal targets to search...
A current important research topic in modern Astrophysics is to understand the formation process ... more A current important research topic in modern Astrophysics is to understand the formation process of galaxies like the Milky Way. The Milky Way Stellar Halo preserves the most useful information about the Galaxy evolution. Despite a number of detailed studies over many years, the shape and extent of the Galactic Halo is still debated and knowledge about it is incomplete. The existence of the halo stellar streams provides important clues to the hierarchical formation of our galaxy. We study these substructures through the characteristics of various stellar populations. RR Lyraes (RRLs) are very good Galactic halo tracers because of their relatively high intrinsic and well established absolute magnitudes. RRLs are metal poor and very old and can hold information of galaxy evolution. Their characteristic colors and light curves made them easy to be identified. Keller et al. (2008) found that the power-law slope of the RR Lyrae space density distribution is steepened beyond the Galactocentric radius 45 kpc. They identified 2016 RRL candidates derived from the analysis of archival observations of the Southern Edgeworth-Kuiper Belt Object (SEKBO) survey. We have investigated this result by following up on a subset of 137 candidates with a range of magnitudes (V{u0303}14-20) using the Faulkes Telescope (FT) database and confirmed 57 candidates as real RRLs. A cross-match between SEKBO RRL survey and Sloan Digital Sky Survey Data Release-7 (SDSS DR-7) revealed 272 RRL candidates in common. Applying the color selection criteria proposed by Ivezic' et al. (2005) resulted in 193 likely RRLs. The completeness as a function of magnitude was calculated empirically from the combined set of SEKBO RRL candidates from current FT data, SDSS cross-matched data, and the Prior et al. (2009) catalog. This resulted in a spatial density distribution characterized by two power laws with a break radius R within a range between 45 kpc and 50 kpc, similar to the results of Keller et al. We find the power-law slopes for the inner and the outer [...]
Monthly Notices of the Royal Astronomical Society, 2020
We used NH$\rm _3$(1,1) data from the H$\rm _2$O Southern Galactic Plane Survey (HOPS) between −6... more We used NH$\rm _3$(1,1) data from the H$\rm _2$O Southern Galactic Plane Survey (HOPS) between −60○ < l < −2.7○ and 3.9○ < l < 30○ and |b| < 0.5○ to identify dense molecular regions/clumps of the Milky Way Galaxy that are likely to form high-mass stars. We identified ∼500 such clumps and in this paper we report on 14 of these clumps near the Central Molecular Zone that show broad emission spectral lines (with observed velocity widths between 19.8 and 47.6 km/s corresponding to intrinsic velocity widths between 7.1 and 25.2 km/s). We find that these clumps are grouped into three clusters of dense molecular cores centred at l ≈ 5.4○, −5.4○ and −10○. We name them ‘Cluster-1’, ‘Cluster-2’ (also known as ‘Bania’s Clump 1’) and ‘Cluster-3’. We find that the same clumps exhibit broad emission spectra for other molecular lines - NH$\rm _3$(2,2), (3,3) and HC3N(3−2). The anti-symmetry of Cluster-1 and Cluster-2 in the l − b plane, and the large velocity dispersion of each clum...
Monthly Notices of the Royal Astronomical Society, 2018
We have analysed the full suite of Australia Telescope Compact Array data for the Stingray planet... more We have analysed the full suite of Australia Telescope Compact Array data for the Stingray planetary nebula. Data were taken in the 4-23 GHz range of radio frequencies between 1991 and 2016. The radio flux density of the nebula generally declined during that period, but between 2013 and 2016 it shows signs of halting that decline. We produced the first spatially resolved radio images of the Stingray nebula from data taken in 2005. A ring structure, which appears to be associated with the ring seen in HST images, was visible. In addition, we found a narrow extension to the radio emission towards the eastern and western edges of the nebula. We derived the emission measure of the nebula-this decreased between 1992 and 2011, suggesting that the nebula is undergoing recombination. The radio spectral index is broadly consistent with a free-free emission mechanism, however a single data point hints that a steeper spectral index has possibly emerged since 2013, which could indicate the presence of synchrotron emission. If a non-thermal component has emerged, such as one associated with a region that is launching a jet or outflow, we predict that it would intensify in the years to come.
Publications of The Korean Astronomical Society, 2015
Proceedings of the International Astronomical Union, 2015
Massive stars are some of the most important objects in the Universe, shaping the evolution of ga... more Massive stars are some of the most important objects in the Universe, shaping the evolution of galaxies, creating chemical elements, and hence shaping the evolution of the Universe. However, the processes by which they form, and how they shape their environment during their birth processes, are not well understood. We are using NH3 data from the “The H2O Southern Galactic Plane Survey” (HOPS) to define the positions of dense cores/clumps of gas in the southern Galactic plane that are likely to form stars. We did a comparative study with different methods for finding clumps and found Fellwalker to be the best for this dataset. We detected ~ 500 clumps with mean kinetic temperature ~ 20 K and virial mass ~ 680 solar masses.
Publications of The Korean Astronomical Society, 2015
Publications of the Astronomical Society of Australia, 2013
One of the major science goals of the SkyMapper survey of the Southern Hemisphere sky is the dete... more One of the major science goals of the SkyMapper survey of the Southern Hemisphere sky is the determination of the shape and extent of the halo of the Galaxy. In this paper, we quantify the likely efficiency and completeness of the survey as regards the detection of RR Lyrae variable stars, which are excellent tracers of the halo stellar population. We have accomplished this via observations of the RR Lyrae-rich globular cluster NGC 3201. We find that for single-epochuvgriobservations followed by two further epochs ofg,rimaging, as per the intended three-epoch survey strategy, we recover known RR Lyraes with a completeness exceeding 90%. We also investigate boundaries in the gravity-sensitive single-epoch two-colour diagram that yield high completeness and high efficiency (i.e., minimal contamination by non-RR Lyraes) and the general usefulness of this diagram in separating populations.
The Astrophysical Journal, 2012
We investigate the steepening of the power-law slope beyond the Galactocentric radius of R ∼ 45 k... more We investigate the steepening of the power-law slope beyond the Galactocentric radius of R ∼ 45 kpc in the RR Lyrae (RRL) space density distribution found by Keller et al. They identified 2016 RRL candidates derived from the analysis of archival observations of the Southern Edgeworth-Kuiper Belt Object (SEKBO) survey. Our aim is to verify the completeness of RRLs and ultimately the space density distribution function given by Keller et al. We followed up on a subset of 137 candidates with a range of magnitudes (V ∼ 14-20) using the Faulkes Telescope (FT) database and confirmed 57 candidates as real RRLs. A cross-match between SEKBO RRL survey and Sloan Digital Sky Survey Data Release-7 (SDSS DR-7) revealed 272 RRL candidates in common. Applying the color selection criteria proposed by Ivezić et al. resulted in 193 likely RRLs. The completeness as a function of magnitude was calculated empirically from the combined set of SEKBO RRL candidates from current FT data, SDSS cross-matched data, and the Prior et al. catalog. This resulted in a spatial density distribution characterized by two power laws with a break radius R within a range between 45 kpc and 50 kpc, similar to the results of Keller et al. We find the power-law slopes for the inner halo as n inner = −2.78 ± 0.02 and for the outer halo as n outer = −5.0 ± 0.2.
Publications of The Korean Astronomical Society, 2015
Giant molecular clouds are the hosts for massive star formation, which occurs within dense conden... more Giant molecular clouds are the hosts for massive star formation, which occurs within dense condensation within the more diffuse gas. The HOPS Mopra telescope survey of the southern Galactic plane identified many regions of dense gas using transitions of the molecule ammonia. We are now examining these regions in greater depth, and here we describe how we are decomposing the areas of ammonia emission into 3-dimensional clumps. While 3-D clump finding is now routinely undertaken, the hyperfine structure of ammonia presents extra difficulties, and we discuss ways of getting around this problem. This study is a first step in understanding the clumping and multiplicity of dense (possibly) protostellar clumps throughout differing regions of the Milky Way Galaxy.
Monthly Notices of the Royal Astronomical Society, 2019
We present the first release of the data and compact-source catalogue for the JCMT Large Program ... more We present the first release of the data and compact-source catalogue for the JCMT Large Program SCUBA-2 Continuum Observations of Pre-protostellar Evolution (SCOPE). SCOPE
The Astrophysical Journal Supplement Series, 2018
The low dust temperatures (<14 K) of Planck Galactic Cold Clumps (PGCCs) make them ideal targets ... more The low dust temperatures (<14 K) of Planck Galactic Cold Clumps (PGCCs) make them ideal targets to probe the initial conditions and very early phase of star formation. "TOP-SCOPE" is a joint survey program targeting ∼2000 PGCCs in J=1-0 transitions of CO isotopologues and ∼1000 PGCCs in 850 µm continuum emisison. The objective of the "TOP-SCOPE" survey and the joint surveys (SMT 10-m, KVN 21-m and NRO 45-m) is to statistically study the initial conditions occurring during star formation and the evolution of molecular clouds, across a wide range of environments. The observations, data analysis and example science cases for these surveys are introduced with an exemplar source, PGCC G26.53+0.17 (G26), which is a filamentary infrared dark cloud (IRDC). The total mass, the length and the mean line-mass (M/L) of the G26 filament are ∼6200 M , ∼12 pc and ∼500 M pc −1 , respectively. Ten massive clumps including eight starless ones are found along the filament. The most massive Clump as a whole may be still in global collapse while its denser part seems to be undergoing expansion due to outflow feedback. The fragmentation in G26 filament from cloud scale to clump scale is in agreement with gravitational fragmentation of an isothermal, non-magnetized, and turbulent supported cylinder. A bimodal behavior in dust emissivity spectral index (β) distribution is found in G26, suggesting grain growth along the filament. The G26 filament may be formed due to large-scale compression flows evidenced by the temperature and velocity gradients across its natal cloud.
The physical and chemical properties of prestellar cores, especially massive ones, are still far ... more The physical and chemical properties of prestellar cores, especially massive ones, are still far from being well understood due to the lack of a large sample. The low dust temperature (<14 K) of Planck cold clumps makes them promising candidates for prestellar objects or for sources at the very initial stages of protostellar collapse. We have been conducting a series of observations toward Planck cold clumps (PCCs) with ground-based radio telescopes. In general, when compared with other star forming samples (e.g. infrared dark clouds), PCCs are more quiescent, suggesting that most of them may be in the earliest phase of star formation. However, some PCCs are associated with protostars and molecular outflows, indicating that not all PCCs are in a prestellar phase. We have identified hundreds of starless dense clumps from the mapping survey with the Purple Mountain Observatory (PMO) 13.7-m telescope. Follow-up observations suggest that these dense clumps are ideal targets to search...
A current important research topic in modern Astrophysics is to understand the formation process ... more A current important research topic in modern Astrophysics is to understand the formation process of galaxies like the Milky Way. The Milky Way Stellar Halo preserves the most useful information about the Galaxy evolution. Despite a number of detailed studies over many years, the shape and extent of the Galactic Halo is still debated and knowledge about it is incomplete. The existence of the halo stellar streams provides important clues to the hierarchical formation of our galaxy. We study these substructures through the characteristics of various stellar populations. RR Lyraes (RRLs) are very good Galactic halo tracers because of their relatively high intrinsic and well established absolute magnitudes. RRLs are metal poor and very old and can hold information of galaxy evolution. Their characteristic colors and light curves made them easy to be identified. Keller et al. (2008) found that the power-law slope of the RR Lyrae space density distribution is steepened beyond the Galactocentric radius 45 kpc. They identified 2016 RRL candidates derived from the analysis of archival observations of the Southern Edgeworth-Kuiper Belt Object (SEKBO) survey. We have investigated this result by following up on a subset of 137 candidates with a range of magnitudes (V{u0303}14-20) using the Faulkes Telescope (FT) database and confirmed 57 candidates as real RRLs. A cross-match between SEKBO RRL survey and Sloan Digital Sky Survey Data Release-7 (SDSS DR-7) revealed 272 RRL candidates in common. Applying the color selection criteria proposed by Ivezic' et al. (2005) resulted in 193 likely RRLs. The completeness as a function of magnitude was calculated empirically from the combined set of SEKBO RRL candidates from current FT data, SDSS cross-matched data, and the Prior et al. (2009) catalog. This resulted in a spatial density distribution characterized by two power laws with a break radius R within a range between 45 kpc and 50 kpc, similar to the results of Keller et al. We find the power-law slopes for the inner and the outer [...]
Monthly Notices of the Royal Astronomical Society, 2020
We used NH$\rm _3$(1,1) data from the H$\rm _2$O Southern Galactic Plane Survey (HOPS) between −6... more We used NH$\rm _3$(1,1) data from the H$\rm _2$O Southern Galactic Plane Survey (HOPS) between −60○ < l < −2.7○ and 3.9○ < l < 30○ and |b| < 0.5○ to identify dense molecular regions/clumps of the Milky Way Galaxy that are likely to form high-mass stars. We identified ∼500 such clumps and in this paper we report on 14 of these clumps near the Central Molecular Zone that show broad emission spectral lines (with observed velocity widths between 19.8 and 47.6 km/s corresponding to intrinsic velocity widths between 7.1 and 25.2 km/s). We find that these clumps are grouped into three clusters of dense molecular cores centred at l ≈ 5.4○, −5.4○ and −10○. We name them ‘Cluster-1’, ‘Cluster-2’ (also known as ‘Bania’s Clump 1’) and ‘Cluster-3’. We find that the same clumps exhibit broad emission spectra for other molecular lines - NH$\rm _3$(2,2), (3,3) and HC3N(3−2). The anti-symmetry of Cluster-1 and Cluster-2 in the l − b plane, and the large velocity dispersion of each clum...
Monthly Notices of the Royal Astronomical Society, 2018
We have analysed the full suite of Australia Telescope Compact Array data for the Stingray planet... more We have analysed the full suite of Australia Telescope Compact Array data for the Stingray planetary nebula. Data were taken in the 4-23 GHz range of radio frequencies between 1991 and 2016. The radio flux density of the nebula generally declined during that period, but between 2013 and 2016 it shows signs of halting that decline. We produced the first spatially resolved radio images of the Stingray nebula from data taken in 2005. A ring structure, which appears to be associated with the ring seen in HST images, was visible. In addition, we found a narrow extension to the radio emission towards the eastern and western edges of the nebula. We derived the emission measure of the nebula-this decreased between 1992 and 2011, suggesting that the nebula is undergoing recombination. The radio spectral index is broadly consistent with a free-free emission mechanism, however a single data point hints that a steeper spectral index has possibly emerged since 2013, which could indicate the presence of synchrotron emission. If a non-thermal component has emerged, such as one associated with a region that is launching a jet or outflow, we predict that it would intensify in the years to come.
Publications of The Korean Astronomical Society, 2015
Proceedings of the International Astronomical Union, 2015
Massive stars are some of the most important objects in the Universe, shaping the evolution of ga... more Massive stars are some of the most important objects in the Universe, shaping the evolution of galaxies, creating chemical elements, and hence shaping the evolution of the Universe. However, the processes by which they form, and how they shape their environment during their birth processes, are not well understood. We are using NH3 data from the “The H2O Southern Galactic Plane Survey” (HOPS) to define the positions of dense cores/clumps of gas in the southern Galactic plane that are likely to form stars. We did a comparative study with different methods for finding clumps and found Fellwalker to be the best for this dataset. We detected ~ 500 clumps with mean kinetic temperature ~ 20 K and virial mass ~ 680 solar masses.
Publications of The Korean Astronomical Society, 2015
Publications of the Astronomical Society of Australia, 2013
One of the major science goals of the SkyMapper survey of the Southern Hemisphere sky is the dete... more One of the major science goals of the SkyMapper survey of the Southern Hemisphere sky is the determination of the shape and extent of the halo of the Galaxy. In this paper, we quantify the likely efficiency and completeness of the survey as regards the detection of RR Lyrae variable stars, which are excellent tracers of the halo stellar population. We have accomplished this via observations of the RR Lyrae-rich globular cluster NGC 3201. We find that for single-epochuvgriobservations followed by two further epochs ofg,rimaging, as per the intended three-epoch survey strategy, we recover known RR Lyraes with a completeness exceeding 90%. We also investigate boundaries in the gravity-sensitive single-epoch two-colour diagram that yield high completeness and high efficiency (i.e., minimal contamination by non-RR Lyraes) and the general usefulness of this diagram in separating populations.
The Astrophysical Journal, 2012
We investigate the steepening of the power-law slope beyond the Galactocentric radius of R ∼ 45 k... more We investigate the steepening of the power-law slope beyond the Galactocentric radius of R ∼ 45 kpc in the RR Lyrae (RRL) space density distribution found by Keller et al. They identified 2016 RRL candidates derived from the analysis of archival observations of the Southern Edgeworth-Kuiper Belt Object (SEKBO) survey. Our aim is to verify the completeness of RRLs and ultimately the space density distribution function given by Keller et al. We followed up on a subset of 137 candidates with a range of magnitudes (V ∼ 14-20) using the Faulkes Telescope (FT) database and confirmed 57 candidates as real RRLs. A cross-match between SEKBO RRL survey and Sloan Digital Sky Survey Data Release-7 (SDSS DR-7) revealed 272 RRL candidates in common. Applying the color selection criteria proposed by Ivezić et al. resulted in 193 likely RRLs. The completeness as a function of magnitude was calculated empirically from the combined set of SEKBO RRL candidates from current FT data, SDSS cross-matched data, and the Prior et al. catalog. This resulted in a spatial density distribution characterized by two power laws with a break radius R within a range between 45 kpc and 50 kpc, similar to the results of Keller et al. We find the power-law slopes for the inner halo as n inner = −2.78 ± 0.02 and for the outer halo as n outer = −5.0 ± 0.2.
Publications of The Korean Astronomical Society, 2015