arXiv subjects

Priya Hasan

Publications and source records attributed to Priya Hasan.

26 records · Page 2Linked to original sources

arXiv2021

The Phosphene Controversy: Is it Phosphene? Is there life on Venus?

On 14th September 2020, the Royal Astronomical Society made an official statement coupled with a webminar on the discovery of phosphine on Venus. Single-line millimetre-waveband spectral detections of phosphine (with a signal-to-noise ratio of $\approx$ 15$σ$) from the JCMT and ALMA telescopes indicated a phosphine abundance of 20 ppb (parts per billion), 1000 times more than that on the Earth. Phosphine is an important biomarker and immediate speculation in the media about indicators of life being found on Venus followed. This article presents an analysis of the study and the results on the observation of the spectral absorption feature of phosphine in the clouds of Venus, thus implying as a potential biosignature. If phosphine is produced through biotic, as opposed to abiotic pathways, the discovery could imply a significant biomass in the Venusian atmosphere. The discovery led to a major controversy with criticism of the analysis and results and responses to it. The issue remains unresolved, leading to a fresh interest in the study of Venus including ground-based observations as well as space-probes that can answer these questions conclusively.

physics.pop-ph↗

arXiv2021

Membership of Stars in Open Clusters using Random Forest with Gaia Data

Membership of stars in open clusters is one of the most crucial parameters in studies of star clusters. Gaia opened a new window in the estimation of membership because of its unprecedented 6-D data. In the present study, we used published membership data of nine open star clusters as a training set to find new members from Gaia DR2 data using a supervised random forest model with a precision of around 90\%. The number of new members found is often double the published number. Membership probability of a larger sample of stars in clusters is a major benefit in determination of cluster parameters like distance, extinction and mass functions. We also found members in the outer regions of the cluster and found sub-structures in the clusters studied. The color magnitude diagrams are more populated and enriched by the addition of new members making their study more promising.

astro-ph.SR↗

arXiv2018

Spectroscopic Study of NGC 281 West

NGC 281 is a complex region of star formation at 2.8 kpc. This complex is situated 300 pc above the Galactic plane, and appears to be part of a 270 pc diameter ring of atomic and molecular clouds expanding at 22 km/s (Megeath et al. 2003). It appears that two modes of triggered star formation are at work here: an initial supernova to trigger the ring complex and the initial O~stars and the subsequent triggering of low mass star formation by photoevaporation driven molecular core compression. To get a complete census of the young stellar population, we use \textbf{observations from} Chandra ACIS 100 ksec coupled with data from 2MASS and Spitzer. The Master X-ray catalog has 446 sources detected in different bandpasses. We present the spatial distribution of Class~I, II and III sources to study the progress of star formation. We also determine the gas to dust ratio $N_H/A_K$ to be 1.93 $\pm$ 0.47 $\times 10^{22}$ cm$^{-2}$ mag$^{-1}$ for this region. \textbf{In this article, we present NGC 281 as a good target to study with the 3.6-m Devasthal Optical Telescope (DOT) in spectroscopy. With these spectra, we look for evidence for the pre-main-sequence (PMS) nature of the objects, study the properties of the detected emission lines as a function of evolutionary class, and obtain spectral types for the observed young stellar objects (YSOs). The temperatures implied by the spectral types can be combined with luminosities determined from the near-infrared (NIR) photometry to construct Hertzsprung--Russell (HR) diagrams for the clusters. By comparing the positions of the YSOs in the HR diagrams with the PMS tracks, we can determine the ages of the embedded sources and study the relative ages of the YSOs with and without optically thick circumstellar disks.

astro-ph.SR↗

arXiv2018

The Enigma of Star Formation at High Galactic Latitudes

Molecular clouds at very high latitudes ($b > 60^o$) away from the Galactic plane are considered rare and not conventional sites of star formation. Contrary to this, the recent discovery of high latitude embedded Clusters can possibly change our understanding of the Galaxy formation, evolution and dynamics and the role of the halo in the Galactic evolutionary process. This article reviews a study of nine embedded clusters (ECs) reported in recent literature with ages less than 5 Myr and vertical distances from the galactic disc ranging from 1.8 to 5 kpc. It discusses the processes that could cause star formation within low density and extraplanar environments in the halo and discuss the possible origins of these clusters. Are these episodic events or is star cluster formation a systematic phenomenon in the Galactic halo? Two of these objects will be observed by us with Astrosat, so we shall comment on the possible results we expect to get with Astrostat UV and Xray data.

astro-ph.GA↗

arXiv2011

Mass segregation in diverse environments

In this paper, using 2MASS photometry, we study the mass functions $ϕ(M) = dN/dM \propto M^{-α}$ of a sample of nine clusters of ages varying from 4 Myr--1.2 Gyr and Galactocentric distances from 6--12 kpc. We look for evidence of mass segregation in these clusters by tracing the variation in the value of $α$ in different regions of the cluster as a function of the parameter $τ= t_{age}/t_{relax}$ (where $t_{age}$ is the age of the cluster and $t_{relax}$ is the relaxation time of the cluster), Galactocentric distance, age and size of the cluster. The value of $α$ value increases with age and $τ$ and fits straight lines with slopes $m$ and y-intercepts $c$ given by $m=0.40\pm0.03$, $c=-1.86\pm0.27$ and $m=0.01\pm0.001$, $c=-0.85\pm0.02$, respectively and is a clear indicator of the dynamical processes involved. The confidence level of the Pearson's product-moment correlation of $α$ with age is 0.76 with p=0.002 and with $τ$ is 0.71 with p=0.007. The value of $α$ also increases with Galactocentric distance, indicating the presence of a larger relative number of low mass stars in clusters at larger Galactocentric distances. We find two clusters, viz. IC 1805 and NGC 1893, with evidence of primordial or early dynamical mass segregation. Implications of primordial mass segregation on the formation of massive stars and recent results supporting early dynamical mass segregation are discussed.

astro-ph.GA↗

arXiv2008

The structure and dynamics of young star clusters: King 16, NGC 1931, NGC 637 and NGC 189

In this paper, using 2MASS photometry, we study the structural and dynamical properties of four young star clusters viz. King 16, NGC 1931, NGC 637 and NGC 189. For the clusters King 16, NGC 1931, NGC 637 and NGC 189, we obtain the limiting radii of 7', 12', 6' and 5' which correspond to linear radii of 3.6 pc, 8.85 pc, 3.96 pc and 2.8 pc respectively. The reddening values $E(B-V)$ obtained for the clusters are 0.85, 0.65--0.85, 0.6 and 0.53 and their true distances are 1786 pc, 3062 pc, 2270 pc and 912 pc respectively. Ages of the clusters are 6 Myr, 4 Myr, 4 Myr and 10 Myr respectively. We compare their structures, luminosity functions and mass functions ($ϕ(M) = dN/dM \propto M^{-(1+χ)}$) to the parameter $τ= t_{age}/t_{relax}$ to study the star formation process and the dynamical evolution of these clusters. We find that, for our sample, mass seggregation is observed in clusters or their cores only when the ages of the clusters are comparable to their relaxation times ($τ\geq 1$). These results suggest mass seggregation due to dynamical effects. The values of $χ$, which characterise the overall mass functions for the clusters are 0.96 $\pm$ 0.11, 1.16 $\pm$ 0.18, 0.55 $\pm$ 0.14 and 0.66 $\pm$ 0.31 respectively. The change in $χ$ as a function of radius is a good indicator of the dynamical state of clusters.

astro-ph↗

arXiv2007

Evolutionary studies of the Young Star Clusters: \\NGC 1960, NGC 2453 and NGC 2384

We report the analysis of the young star clusters NGC 1960, NGC 2453 and NGC 2384 observed in the $J$ (1.12 $μ$m), $H$ (1.65 $μ$m) and $K^{'}$ (2.2 $μ$m) bands. Estimates of reddening, distance and age as $E(B-V)=0.25$, $d= 1380$ pc and $t=31.6$ to 125 Myr for NGC 1960, $E(B-V)=0.47$, $d=3311$ pc and $t=40$ to 200 Myr for NGC 2453 and $E(B-V)=0.25$, $d=3162$ pc and $t=55$ to 125 Myr for NGC 2384 have been obtained. Also, we have extended the color--magnitude diagrams of these clusters to the fainter end and thus extended the luminosity functions to fainter magnitudes. The evolution of the main sequence and luminosity functions of these clusters have been compared with themselves as well as Lyngå2 and NGC 1582.

astro-ph↗

arXiv2007

Morphologies of AGN host galaxies using HST/ACS in the CDFS-GOODS field

Using HST/ACS images in four bands F435W, F606W, F775W and F850LP, we identify optical counterparts to the X-ray sources in the Chandra Deep Field South in the GOODS South field. A detailed study has been made of these sources to study their morphological types. We use methods like decomposition of galaxy luminosity profiles, color maps and visual inspection of 192 galaxies which are identified as possible optical counterparts of Chandra X-ray sources in the CDFS-GOODS field. We find that most moderate luminosity AGN hosts are bulge dominated in the redshift range (z \approx 0.4-1.3), but not merging/interacting galaxies. This implies probable fueling of the moderate luminosity AGN by mechanisms other than those merger driven.

astro-ph↗