#astromatter

[christinetheastrophysicist]

Hubble’s Legacy 

Title: First Results from the Hubble OPAL Program: Jupiter in 2015

Authors: A. A. Simon, M. H. Wong, G. S. Orton

For the remainder of Hubble’s lifetime, the Hubble 2020 Outer Planet Atmospheres Legacy (OPAL) program will produce new global maps of the giant planets every year. Hubble’s Wide Field Camera 3 (WFC3) will be used to observe the atmospheric phenomena of each planet, such as storm activity and aerosol distributions.

The program began by observing Jupiter during Hubble’s cycle 22, occurring between October 2014 and September 2015. In order to produce a global map of Jupiter, Hubble needs to complete 6-7 consecutive orbits, where each orbit provides about 52 minutes of visibility. All images were acquired with the Ultraviolet Imaging Spectrograph (UVIS) detector, making use of different filters to allow for a variety of features to become visible.

The data provides insight into many of Jupiter's atmospheric features. Jupiter's zonal wind flow, wind moving along latitudinal lines, or the west-east direction, were found to be similar to data obtained by the Voyager and Cassini missions. A feature not seen before in previous Hubble data is a small wave in the North Equatorial Belt (NEB), a region that is home to many small cyclonic cells. This particular wave feature is reminiscent of a baroclinic wave, which is responsible for shaping cyclones on Earth.

Jupiter’s most prominent feature, the Great Red Spot (GRS), is still orange. Another of Jupiter’s features, Oval BA, located southeast of the GRS, appears paler in color. Imaging in the violet wavelengths shows that the interior of the GRS is gradually shrinking.

Even though the program has just begun, the new Jupiter data has already been producing interesting results. The OPAL program will continue to study the giant planets for the remainder of Hubble’s and its camera’s lifetimes.

Image: Projections of the first two Jupiter rotations. This image is a combination of three Hubble filters. (Credit: Simon et al.)

[christinetheastrophysicist]

The Beginnings of a Supermassive Star? 

Title: MY Camelopardalis, a very massive merger progenitor

Authors: J. Lorenzo, I. Negueruela, A.K.F. Val Baker, M. García, S. Simón-Díaz, P. Pastor, M. Méndez Majuelos

At 13,000 light years away, MY Camelopardalis (MY Cam) is the brightest stellar object in the open cluster Alicante 1. Since its initial discovery, MY Cam was later identified to be a binary star, consisting of two O-type stars that orbit around each other. MY Cam is one of the most massive binary star systems, and studying it provides insight into the theory of how extremely massive single stars are formed.

The authors used spectroscopic data, obtained in 2004, and photometric data, obtained in 2008, to determine the properties of the two stars. The primary star is the largest and hottest of the two, with a temperature of 42,000K and a mass 37.7 times greater than our Sun. The secondary star is 39,000K and has a mass 31 times the Sun. They are both early O-type main sequence stars.

By studying the light curve, the authors found that MY Cam has a short period of a little over one day. With such a fast orbit, these massive stars must be so close together that they are touching. This is known as a contact binary, and MY Cam is one of the largest. In addition, the two stars eclipse one another from our point of view, making MY Cam an eclipsing binary.

It was found that these close binary stars were overfilling their Roche lobes, the region around a star where matter is gravitationally bound to it. As a result, mass from the smaller star is being transferred to the larger one. The fate of these two stars is unknown, but it is likely the two stars will merge into a single, very massive star. There isn’t much insight into massive stellar mergers. Observations are limited to non-massive binary systems and models of their evolution just don’t exist. But based on theoretical predictions, the authors suggest that this merger will result in a single massive star that will have very unusual properties.

Image: The open cluster Alicante 1. The binary star MY Cam is circled in blue. (Credit: Ignacio Negueruela and Javier Lorenzo)

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Using Neural Networks to Classify Galaxies 

Title: Rotation-invariant convolutional neural networks for galaxy morphology prediction

Author: Sander Dieleman, Kyle W. Willett, Joni Dambre

No two galaxies are exactly the same. They come in many different shapes, sizes, and colors. Studying these structural properties of galaxies provides a way to group similar looking galaxies together. Their morphology, the properties that describe what a galaxy looks like, is used to study their formation and evolution.

Traditionally, the classification of galaxies has been performed by someone visually inspecting each image. Now, with new large sky surveys producing extremely large amounts of images in one night, using the traditional galaxy classification inspection techniques will be time consuming and not very efficient.

To address this growing problem, the need for an automated classification approach that can handle large amounts of data is increasing. One solution lies in the field of machine learning. Training computers to see and analyze images would allow for more images to be classified in a timely manner. To do this, the Galaxy Zoo crowdsourcing project teamed up with Kaggle, an online predictive modeling platform, to create The Galaxy Challenge, a competition to build the best galaxy classification model.

Sander Dieleman, a PhD student at Ghent University in Belgium and first author of the paper, finished in first place with his model. He presents a convolutional neural network that classifies images of galaxies based on their morphology. A convolutional neural network (CNN) is a machine learning algorithm based on biological neural networks, such as the central nervous system. It uses artificial neurons to create connections to other artificial neurons based on the information it learns from the data.

The images provided by the competition hosts were separated into two groups: a training set and an evaluation set. The training set consisted of over 61,000 color images of galaxies that were annotated with morphological data. The evaluation set consisted of almost 80,000 images, but unlike the training set, had no morphological data associated with it. As their names suggest, the training set is used to train the model and the evaluation set is used to evaluate the model’s learning capabilities.

The goal of the model was to determine the probability that a galaxy belongs to a particular class. There were 37 possible classes, each one representing a possible answer that corresponds to the questions asked of Galaxy Zoo participants. The list of questions presented to Galaxy Zoo participants creates a decision tree, a tree-like graph that shows all possible decisions and outcomes, and is supposed to represent all classes described in the Hubble tuning fork classification scheme.

To increase the performance of the model, Dieleman used rotational symmetry, allowing the rules of the model to be applied multiple times to various versions of the images.

At the end of the competition, Dieleman’s model placed first out of 326 participants with near-perfect accuracy. Analyzing the results showed that there was agreement between the Galaxy Zoo participants’ answers and the model’s output. Due to the training, the model had difficulty identifying images with morphology features that appeared infrequently. An interesting outcome pertains to images that feature black lines running across them. The black lines are caused by dead pixels in the camera, which the model doesn’t know. As a result, it tries to classify those images as disturbed galaxies. The galaxies that were easiest to classify were elliptical galaxies.

Image: A schematic of the neural network architecture displaying the steps taken to reach a prediction. (Credit: Dieleman et al.)

[christinetheastrophysicist]

The Closest Star to Flyby the Solar System

Title: The Closest Known Flyby of a Star to the Solar System

Authors: E. E. Mamajek, S. A. Barenfeld, V. D. Ivanov, A. Y. Kniazev, P. Vaisanen, Y. Beletsky, H. M. J. Boffin

Long-period comets originate from the Oort cloud, but how do they come to travel into the planetary system? One hypothesis favors the existence of a companion to the Sun, but analysis shows that such a companion would be inconsistent with the observed periodic variations in long-period comets. It has also been proposed that stars pass near enough to the Oort cloud to alter the orbits of the comets towards the Sun.

A recently discovered star, WISE J072003.20-084651.2, also know as Scholz’s star or W0720, is about 23 light years away, near the Galactic Plane in the constellation Monoceros. The motion of the star makes it likely that it may have interacted with the Sun, or could in the future.

Scholz’s star is a binary system with a mass about 0.15 times the mass of the Sun and a stellar classification of M9.5+T5. The primary star (W0720A) is a red dwarf (M9.5) with a mass about 86 times the mass of Jupiter. The secondary (W0720B) is a brown dwarf (T5) with a mass about 65 times the mass of Jupiter. The velocity of Scholz’s star is similar to the large group of stars that make up the Hercules stream. The stream contains stars of different ages and compositions and by looking at stars with similar characteristics, it was determined that Scholz’s star is 3 to 10 billion years old.

The authors simulated 10,000 orbits of Scholz’s star and the Sun and saw that the closest pass to the Sun was 70,000 years ago at a distance of 0.8 light years, within the inner Oort cloud. For reference, the neighboring star Proxima Centauri is 4.24 light years away. In addition, 98% of the simulations resulted in a trajectory where Scholz’s star passed within the outer Oort cloud at a distance of 1.9 light years.

By comparing their results with other stars recently studied, the authors determined that Scholz’s star is the closest known star to flyby our system. At its closest approach of 0.8 light years, Scholz’s star would appear brighter than Proxima Centauri, but dimmer than naked eye stars. Scholz’s star is active, meaning that it experiences stellar flares. During a flare event, it is possible for Scholz’s star to be visible to the naked eye for a short period of time.

Further astrometry observations of Scholz’s star will provide more precise measurements about the star and the flyby.

Image: View containing Scholz’s star, circled in red. (Credit: WISE)

[christinetheastrophysicist]

A Group of Cepheids May Belong to an Unknown Galaxy

Title: Clustered Cepheid Variables 90 kiloparsec from the Galactic Center

Authors: Sukanya Chakrabarti, Roberto Saito, Alice Quillen, Felipe Gran, Christopher Klein, Leo Blitz

The regions near the galactic plane, where the majority of the Milky Way lies, is obscured by so much dust that it becomes difficult to study. In order to minimize dust obscuration, a team of astronomers led by Rochester Institute of Technology’s Sukanya Chakrabarti used near-infrared data from the VISTA Variables of the Via Lactea (VVV) public survey to search for stars at low latitudes near the center of the galaxy. With this survey, the team has discovered four Cepheid variable stars located almost 300,000 lightyears away.

Cepheid variables are pulsating stars. This pulsating results from the periodic change in brightness due to the expansion and contraction of the star's surface. It was discovered that a relationship existed between the Cepheid's luminosity and its period, and that it could be used to determine distances, both galactic and extragalactic. As a result, Cepheids are a type of ‘standard candles.’

The team focused on searching for faint variable stars with magnitudes greater than 15 in the infrared K-band and with periods greater than three days. They examined the stars’ light curves and the uncertainties in their periods. With this information, the team determined that the stars are Type I Cepheids. Also known known as classical Cepheids, they are the most common subclass of Cepheids.

These Cepheids are clustered together in such a way that suggests they may be associated with a dwarf galaxy. Previous research predicted that our galaxy may have interacted with a smaller dwarf galaxy 300 million years ago. Even though no dwarf galaxy with evidence of this interaction has been found, it is possible that a dwarf galaxy containing the Cepheids may exist in a region not studied by the survey. The team believes that it is possible to determine the structure of this object with further observations.

Image: V838 Monocerotis, a red supergiant variable star in the constellation Monoceros. (Credit: NASA, ESA and H.E. Bond (STScI))

[christinetheastrophysicist]

The Milky Way's Newest Neighbor

Title: A new isolated dSph galaxy near the Local Group

Authors: I. D. Karachentsev, L. N. Makarova, D. I. Makarov, R. B. Tully, L. Rizzi

The Local Group is a collection of galaxies that contains the Milky Way, Andromeda, and many other galaxies. Recently, astronomers have found a new member of our Local Group, an isolated dwarf galaxy located about 7 million light-years away.

KKs3 is a dwarf spheroidal galaxy (dSph) that was observed with Hubble's Advanced Camera for Surveys. This galaxy has a total stellar mass 23 million times the mass of the Sun and its total magnitude is 18.30. KKs3 is isolated from other known galaxies. The dwarf galaxy IC 3014 is its closest neighbor, located about 3.2 million light-years away.

Due to its isolation, KKs3 has exhausted most of its star forming fuel early on. The most active star formation occurred 12 to 14 billion years (Gyr) ago, producing around 74% of the galaxy's stellar mass. Since then, KKs3 experienced two smaller periods of star formation, 4 to 6 Gyr ago and 0.8 to 2 Gyr ago. These two periods produced around 14% and 12% of the rest of the stellar mass, respectively. As a result, KKs3 contains a very old and metal poor population of stars.

Image: Hubble image of dwarf spheroidal galaxy KKs3. The region highlighted with the white square contains a globular cluster, seen in the bottom right corner. (Credit: I. D. Karachentsev et al.)

[christinetheastrophysicist]

A Possible Thorne-Żytkow Object

Title: Discovery of a Thorne-Zytkow object candidate in the Small Magellanic Cloud

Authors: Emily M. Levesque, Philip Massey, Anna N. Zytkow, Nidia Morrell

In 1977, Kip Thorne and Anna Żytkow proposed that a neutron star inside of a red supergiant could exist. Such a star is known as a Thorne-Żytkow object (TŻO). This exotic object forms when the two massive stars collide.

At this point, the neutron star and the red supergiant's core begin to spiral in towards each other. Once they collide, the stars will either coalesce into a single neutron star surrounded by a gas envelope, or collapse into a black hole. A Thorne-Żytkow object would look like a normal red supergiant, but would have a different chemical signature due to the neutron star at its core.

Recently, a group of astronomers believe that they have found a candidate TŻO. HV 2112 is a star in the neighboring Small Magellanic Cloud. This star is classified as a red supergiant (M3 I) with a visual magnitude of about 13.7, a little bit dimmer than Pluto. It has a mass 15 times the mass of the Sun and a temperature of 3450 K.

What distinguishes TŻOs from red supergiants is the large amounts of unusual elements found in their spectra. It was first predicted that these elements were rubidium and molybdenum and it was later predicted to include lithium. When looking at HV 2112’s spectra, it showed abnormally large amounts of those same elements.

The abnormalities in HV 2112’s spectrum and its classification as a red supergiant agrees with the TŻO predictions, making HV 2112 a strong TŻO candidate. The authors do not rule out the possibility that HV 2112 may reside in the Milky Way, eliminating the possibility of it being a TŻO. Further studying of TŻOs will provide new models and predictions that could help identify whether HV 2112 is a real TŻO

Image: An image of HV 2112. (Credit: CDS Portal)

[christinetheastrophysicist]

A Mysterious Object on the Edge of Saturn's Ring

Title: The discovery and dynamical evolution of an object at the outer edge of Saturn's A ring

Authors: Carl D. Murray, Nicholas J. Cooper, Gareth A. Williams, Nicholas O. Attree, Jeffrey S. Boyer

Scientists studying images of Saturn's rings have found an object on the outermost edge of Saturn's brightest ring, known as the A ring.

The image, taken on April 15, 2013, showed an object 1200 km long, 10 km wide, and 20% brighter than the surrounding ring material. By looking through more images taken between May 2012 and November 2013, there were 107 detections of similar objects also found on the edge of the A ring.

The A ring's edge is maintained by the moon Janus and the inner Lindblad resonance (ILR). This orbital resonance provides ideal conditions for ring particles to coalesce into larger objects and, if the objects had a density greater than ice, could result in their ejection from the ring system. As a result, it is possible that some of these objects could escape into the region outside of the A ring.

By observing the formation and evolution of these ring particles, we could learn whether objects like this could be the beginning formation of some of Saturn's moons.

Image: Taken by Cassini, the enlargement shows the object on the edge of the A ring. (Credit: Murray et al.)