Can We See More Through the Eyes of GAIA?

This post has been written by Arnab Chowhan, who is a 4th-year Integrated MSc Physics student at the Centre for Excellence in Basic Sciences, University of Mumbai. He likes to solve physics and maths problems using computational techniques. Specifically, he is drawn to astrophysical problems studying stellar evolution, interstellar medium, and the AGNs. Outside of research, he enjoys playing chess, cricket, and badminton.

Introduction:

Stars are continuously being formed in our universe through a fundamental process that has intrigued us for centuries. From the formation of our own solar system to the exploration of exoplanet populations, and the study of stellar evolution, understanding the intricacies of star formation is a prerequisite for unlocking the mysteries of the cosmos. With the help of modern telescopes such as GAIA, we can now identify individual Young Stellar Objects (YSOs) and determine how evolved they are. We get the distance to each of the sources through their parallax as well as track their proper motion across the sky. Further, we have other telescopes such as 2-MASS and WISE which work in the Infra-Red range (we will come to why IR later in this section). We have the magnitudes of every source in several bands. By studying the population statistics of YSOs within a star-forming region, we can gain insights into the rate at which stars form and the factors that govern this process. 

We used the Spectral Energy Distribution (SED), a graphical representation of the distribution of the electromagnetic energy emitted by an astronomical object as a function of wavelength or frequency, as a confirmatory indicator of YSO. If the YSO is in a phase of active accretion, the SED will show strong emission from the surrounding disk, while if the YSO has entered the main sequence, the SED will show only emission from the star itself. The shape and features of the SED can be used to determine the object’s temperature, luminosity, and dust content. 

However, unlike main sequence stars, Young Stellar Objects (YSOs) do not follow the blackbody radiation curve. Instead, they exhibit an infrared excess, which is a higher level of flux in the infrared region than what would be expected based on a typical star. This excess is attributed to the presence of cooler circumstellar matter that is heated by the core and re-radiates at higher wavelengths. A thorough understanding of the nature of this excess and the variables that influence it is necessary for identifying YSOs and comprehending their properties. As the YSO continues to evolve towards a star, the circumstellar disk becomes smaller, leading to a curve that more closely resembles the blackbody radiation curve. This concept has been mathematically quantified through the use of the spectral index, which is calculated based on the flux density in the infrared region, and has resulted in the classification of YSOs into four different classes (0, I, II, III). The figure above shows the SED for various classes. A detailed explanation can be found here.

At the surface of the Local Bubble, we have a large star-forming region called the Chamaeleon Cloud Complex (CCC) in the Chamaeleon constellation. It is a prime location for studying the early stages of star formation due to its relatively close proximity, which allows for detailed observations of young stellar objects (YSOs) within the complex. In the present study, we used GAIA, 2-MASS, and WISE telescope data for YSO identification in the CCC as all three are sensitive in the IR region. And SED of the sources was used as a confirmatory indicator.

Methodology:

This research is focused on young stellar objects (YSOs) in the CCC. It is aimed at making a refined list of YSOs by using the GAIA parameters (parallax and proper motion). YSOs in CCC should have similar parallax and proper motion since they are likely to have formed from the same molecular cloud and to have similar ages and distances. By having similar parallax and proper motion, it suggests that these YSOs belong to a single kinematic population and have a common origin.

Also, an extended area of the sky was studied to look for more possible YSOs which fall within the parallax and proper motion cuts. The study began with the collection of data from GAIA, 2-MASS, and WISE for a region of (6X6) degrees around the center of the CCC. All the lists of YSOs known till date were combined and 259 known YSOs were found in the CCC.

Figure-1: Grey: Area covered by our study, Black: Already reported YSOs

With median parallax as 5.4, median proper motion RA and Dec as (-23,0) mas/yr, we made a cut of radius 2.5 and 6 mas/yr respectively. We got 3 of the 259 known YSOs as outliers in both cuts.

Next, we used literature methods to frame our own list of YSOs. We identified 202 YSOs in the CCC, 86 of which matched the reported ones. >50% of the detected ones were not reported earlier as we had used a bigger region of analysis. To filter the other 106 sources, we used cuts on GAIA parameters as before as the deciding factor. This resulted in only one source surviving the cuts. It was new and was never reported in previous studies.

Results:

I discovered that 3 previously known YSOs do not satisfy the GAIA parameter cuts, and thus may not be in the CCC. And in a region of (6X6) degrees around the center, we got 87 YSOs (86 already known and 1 brand new).

Figure-2: Yellow: The newly discovered YSO, Black: Already reported YSOs

This source lies in the location of CCC. It survived the WISE and 2-MASS selections (colour-colour diagrams described in the literature) and also our GAIA cuts. Moreover, when the SED, i.e. spectral energy distribution (flux density times wavelength vs wavelength) was plotted, it displayed the characteristics of a YSO (Class II). 

This new YSO, along with the discovery that three previously reported YSOs were misclassified as per GAIA parameters, highlights the value of using GAIA data to refine the list of YSOs in the CCC. We also discovered approximately 100 sources in WISE and 2-MASS catalogs that did not have GAIA DR3 counterparts. So, even though they were selected via the WISE and 2MASS techniques as YSOs, GAIA was of no help in confirming or rejecting them. So, these sources, though they follow WISE and 2MASS constraints, have not been reported as YSOs (probably because these lie in that part of the region which has not been studied yet), and even GAIA has not been able to help us decipher if they are YSOs.

Figure-3: The SED of the newly discovered YSO. Its SED is similar to that of a Class II YSO.

Big Picture:

GAIA data has allowed us to refine the list of Young Stellar Objects (YSOs) in the Chamaeleon Cloud Complex (CCC) and discover a new YSO that was previously undiscovered. Future studies could analyze a larger area around the CCC center using GAIA data, as well as incorporating additional data and methods such as molecular line data or radial velocity. By fitting the spectral energy distribution (SED), we could also determine the class I and II sources more explicitly. Using other telescopes such as the James Webb Space Telescope (JWST), we can also obtain dust masses and molecular emission data. Overall, these advancements will aid in our understanding of YSOs and their role in star formation processes.

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