scholarly journals Characterizing the i-band variability of YSOs over six orders of magnitude in time-scale

2019 ◽  
Vol 491 (4) ◽  
pp. 5035-5055 ◽  
Author(s):  
Darryl J Sergison ◽  
Tim Naylor ◽  
S P Littlefair ◽  
Cameron P M Bell ◽  
C D H Williams
Keyword(s):  
Time Scales ◽  
Power Law ◽  
Time Scale ◽  
Fourier Transforms ◽  
Simulated Data ◽  
Class Ii ◽  
Structure Functions ◽  
Young Stellar Objects ◽  
Class Iii ◽  
Stellar Objects

ABSTRACT We present an i-band photometric study of over 800 young stellar objects in the OB association Cep OB3b, which samples time-scales from one minute to 10 yr. Using structure functions we show that on all time-scales (τ) there is a monotonic decrease in variability from Class I to Class II through the transition disc (TD) systems to Class III, i.e. the more evolved systems are less variable. The Class Is show an approximately power-law increase (τ0.8) in variability from time-scales of a few minutes to 10 yr. The Class II, TDs, and Class III systems show a qualitatively different behaviour with most showing a power-law increase in variability up to a time-scale corresponding to the rotational period of the star, with little additional variability beyond that time-scale. However, about a third of the Class IIs shows lower overall variability, but their variability is still increasing at 10 yr. This behaviour can be explained if all Class IIs have two primary components to their variability. The first is an underlying roughly power-law variability spectrum, which evidence from the infrared suggests is driven by accretion rate changes. The second component is approximately sinusoidal and results from the rotation of the star. We suggest that the systems with dominant longer time-scale variability have a smaller rotational modulation either because they are seen at low inclinations or have more complex magnetic field geometries. We derive a new way of calculating structure functions for large simulated data sets (the ‘fast structure function’), based on fast Fourier transforms.

scholarly journals VLA cm-wave survey of young stellar objects in the Oph A cluster: constraining extreme UV- and X-ray-driven disk photoevaporation

2019 ◽  
Vol 631 ◽  
pp. A58 ◽  
Author(s):  
A. Coutens ◽  
H. B. Liu ◽  
I. Jiménez-Serra ◽  
T. L. Bourke ◽  
J. Forbrich ◽  
...  
Keyword(s):  
Dust Emission ◽  
Class Ii ◽  
Field Of View ◽  
Young Stellar Objects ◽  
Continuum Emission ◽  
Class Iii ◽  
Stellar Objects ◽  
Very Large Array ◽  
X Ray ◽  
Extreme Uv

Observations of young stellar objects (YSOs) in centimeter bands can probe the continuum emission from growing dust grains, ionized winds, and magnetospheric activity that are intimately connected to the evolution of protoplanetary disks and the formation of planets. We carried out sensitive continuum observations toward the Ophiuchus A star-forming region, using the Karl G. Jansky Very Large Array (VLA) at 10 GHz over a field-of-view of 6′ and with a spatial resolution of θmaj ×θmin ~ 0.′′4 × 0.′′2. We achieved a 5 μJy beam−1 rms noise level at the center of our mosaic field of view. Among the 18 sources we detected, 16 were YSOs (three Class 0, five Class I, six Class II, and two Class III) and two were extragalactic candidates. We find that thermal dust emission generally contributed less than 30% of the emission at 10 GHz. The radio emission is dominated by other types of emission, such as gyro-synchrotron radiation from active magnetospheres, free–free emission from thermal jets, free–free emission from the outflowing photoevaporated disk material, and synchrotron emission from accelerated cosmic-rays in jet or protostellar surface shocks. These different types of emission could not be clearly disentangled. Our non-detections for Class II/III disks suggest that extreme UV-driven photoevaporation is insufficient to explain disk dispersal, assuming that the contribution of UV photoevaporating stellar winds to radio flux does not evolve over time. The sensitivity of our data cannot exclude photoevaporation due to the role of X-ray photons as an efficient mechanism for disk dispersal. Deeper surveys using the Square Kilometre Array (SKA) will have the capacity to provide significant constraints to disk photoevaporation.

scholarly journals Techniques and Results of Observations of Rapid and Ultrarapid Variable Stars

1971 ◽  
Vol 15 ◽  
pp. 144-159 ◽  
Author(s):  
Brian Warner
Keyword(s):  
Time Scales ◽  
Time Scale ◽  
Variable Stars ◽  
Observational Evidence ◽  
Short Time Scale ◽  
Dwarf Novae ◽  
Stellar Objects ◽  
Blue Object ◽  
Flare Stars ◽  
Short Time

The inception of the present era of interest in rapidly varying stars may be identified with M. F. WALKER’s discovery in 1954 (WALKER 1954) of changes in the brightness of the blue object MacRae+43° 1 (now known as MVLyr), on a time scale of a few minutes. Outside of flare stars, few other stellar objects were suspected to vary on such a short time scale. GREENSTEIN’s (1954) spectra showed that MV Lyr resembles an old nova, which stimulated WALKER to look for light fluctuations in old, recurrent, and dwarf novae and related stars. His observational evidence that all members of these classes show variations of various amplitudes and time scales is summarized in the IAU Colloquium on non-stable stars (WALKER 1957). Most spectacular of his discoveries was the existence of 71 sec periodic variations in the light from the remnant of Nova Herculis 1934 (DQ Her) (WALKER 1956).

scholarly journals Short- and long-term near-infrared spectroscopic variability of eruptive protostars from VVV

2019 ◽  
Vol 492 (1) ◽  
pp. 294-314 ◽  
Author(s):  
Zhen Guo (郭震) ◽  
P W Lucas ◽  
C Contreras Peña ◽  
R G Kurtev ◽  
L C Smith ◽  
...  
Keyword(s):  
Time Scales ◽  
Near Infrared ◽  
Accretion Rate ◽  
Young Stellar Objects ◽  
Stellar Objects ◽  
Molecular Outflow ◽  
Infrared Spectroscopic ◽  
Long Time ◽  
The One ◽  
H2 Flux

ABSTRACT Numerous eruptive variable young stellar objects (YSOs), mostly Class I systems, were recently detected by the near-infrared Vista Variables in the Via Lactea (VVV) survey. We present an exploratory near-infrared spectroscopic variability study of 14 eruptive YSOs. The variations were sampled over one-day and one-to-two-year intervals and analysed in combination with VVV light curves. CO overtone absorption features are observed on three objects with FUor-like spectra: all show deeper absorption when they are brighter. This implies stronger emission from the circumstellar disc with a steeper vertical temperature gradient when the accretion rate is higher. This confirms the nature of fast VVV FUor-like events, in line with the accepted picture for classical FUors. The absence of Brγ emission in a FUor-like object declining to pre-outburst brightness suggests that reconstruction of the stellar magnetic field is a slow process. Within the one-day time-scale, 60 per cent of H2-emitting YSOs show significant but modest variation, and 2/6 sources have large variations in Brγ. Over year-long time-scales, H2 flux variations remain modest despite up to 1.8 mag variation in Ks. This indicates that emission from the molecular outflow usually arises further from the protostar and is unaffected by relatively large changes in accretion rate on year-long time-scales. Two objects show signs of on/off magnetospheric accretion traced by Brγ emission. In addition, a 60 per cent inter-night brightening of the H2 outflow is detected in one YSO.

scholarly journals Triggered star formation in the isolated cluster CB 34?

2006 ◽  
Vol 2 (S237) ◽  
pp. 464-464
Author(s):  
Dawn E. Peterson ◽  
R. A. Gutermuth ◽  
M. F. Skrutskie ◽  
S. T. Megeath ◽  
J. L. Pipher ◽  
...  
Keyword(s):  
Star Formation ◽  
Class Ii ◽  
High Rate ◽  
Young Stellar Objects ◽  
Collapse Mechanism ◽  
Stellar Objects ◽  
Hii Region ◽  
Dark Clouds ◽  
Sky Survey ◽  
Bok Globules

AbstractBok globules, optically opaque small dark clouds, are classical examples of isolated star formation. However, the collapse mechanism for these cold, dense clouds of gas and dust is not well understood. Observations of Bok globules include some which appear to be starless while others harbor single stars, binaries and even small groups of forming stars. One example of a Bok globule forming a group of stars is CB 34, observed with both the IRAC and MIPS instruments as part of the Spitzer Young Cluster Survey. Based on initial analysis of 1-8 μm photometry from IRAC and the Two Micron All Sky Survey (2MASS), we identified 9 Class 0/I and 14 Class II young stellar objects within the small, 4.5′ × 4.5′ region encompassing CB 34. This unusually high number of protostars compared with Class II sources is intriguing because it implies a high rate of star formation. Therefore we have begun a larger study of this region in order to determine why and how CB 34 started forming stars at such a high rate. Is CB 34 embedded within a larger HII region which may have triggered its collapse or does it appear to have collapsed in isolation from outside influences?

The IGRINS YSO Survey II: Veiling Spectra of Pre-main-sequence Stars in Taurus-Auriga

2021 ◽  
Vol 922 (1) ◽  
pp. 27
Author(s):  
Benjamin Kidder ◽  
Gregory Mace ◽  
Ricardo López-Valdivia ◽  
Kimberly Sokal ◽  
Victoria E. Catlett ◽  
...  
Keyword(s):  
Near Infrared ◽  
Thermal Emission ◽  
Young Stellar Objects ◽  
Continuum Emission ◽  
Class Iii ◽  
Infrared Excess ◽  
Stellar Objects ◽  
Star Forming ◽  
Infrared Spectrometer ◽  
Low Mass

Abstract We present measurements of the H- and K-band veiling for 141 young stellar objects (YSOs) in the Taurus-Auriga star-forming region using high-resolution spectra from the Immersion Grating Near-Infrared Spectrometer. In addition to providing measurements of r H and r K , we produce low-resolution spectra of the excess emission across the H and K bands. We fit temperatures to the excess spectra of 46 members of our sample and measure near-infrared excess temperatures ranging from 1200–2200 K, with an average of 1575 ± 225 K. We compare the luminosity of the excess continuum emission in Class II and Class III YSOs and find that a number of Class III sources display a significant amount of excess flux in the near-infrared. We conclude that the mid-infrared SED slope, and therefore young stellar object classification, is a poor predictor of the amount of near-infrared veiling. If the veiling arises in thermal emission from dust, its presence implies a significant amount of remaining inner-disk (<1 au) material in these Class III sources. We also discuss the possibility that the veiling effects could result from massive photospheric spots, unresolved binary companions, or accretion emission. Six low-mass members of our sample contain a prominent feature in their H-band excess spectra that is consistent with veiling from cool photospheric spots.

scholarly journals What’s Behind the Elephant’s Trunk? Identifying Young Stellar Objects on the Outskirts of IC 1396*

2021 ◽  
Vol 162 (6) ◽  
pp. 279
Author(s):  
Steven M. Silverberg ◽  
Hans Moritz Günther ◽  
Jinyoung Serena Kim ◽  
David A. Principe ◽  
Scott J. Wolk
Keyword(s):  
Cluster Core ◽  
Young Stellar Objects ◽  
Class Iii ◽  
Disk System ◽  
Distance Measurements ◽  
Stellar Objects ◽  
X Ray ◽  
Star Forming ◽  
Radiation Fields ◽  
Significant Difference

Abstract Empirically, the estimated lifetime of a typical protoplanetary disk is <5–10 Myr. However, the disk lifetimes required to produce a variety of observed exoplanetary systems may exceed this timescale. Some hypothesize that this inconsistency is due to estimating disk fractions at the cores of clusters, where radiation fields external to a star–disk system can photoevaporate the disk. To test this, we have observed a field on the western outskirts of the IC 1396 star-forming region with XMM-Newton to identify new Class III YSO cluster members. Our X-ray sample is complete for YSOs down to 1.8 M ⊙. We use a subset of these X-ray sources that have near- and mid-infrared counterparts to determine the disk fraction for this field. We find that the fraction of X-ray-detected cluster members that host disks in the field we observe is 17 − 7 + 10 % (1σ), comparable with the 29 − 3 + 4 % found in an adjacent field centered on the cometary globule IC 1396A. We reevaluate YSO identifications in the IC 1396A field using Gaia parallaxes compared to previous color-cut-only identifications, finding that incorporating independent distance measurements provides key additional constraints. Given the existence of at least one massive star producing an external radiation field in the cluster core, the lack of a statistically significant difference in disk fraction in each observed field suggests that disk lifetimes remain consistent as a function of distance from the cluster core.

scholarly journals On a new type of fractional difference operators on h-step isolated time scales

2021 ◽  
Vol 1 (1) ◽  
pp. 46-74
Author(s):  
Fahd Jarad ◽  
Thabet Abdeljawad ◽  
Abdon Atangana ◽  
Pshtiwan Othman Mohammed
Keyword(s):  
Time Scales ◽  
Power Law ◽  
Time Scale ◽  
Difference Operators ◽  
Step Size ◽  
Fractional Difference ◽  
Forward Difference ◽  
New Type

In this article, a new type of fractional sums and differences called the discrete weighted fractionaloperators are presented. The weighted backward and forward difference operators are defined on anisolated time scale with arbitrary step size and they obey the power law.

scholarly journals Investigation of the stellar content in the IRAS 05168+3634 star-forming region

2019 ◽  
Vol 622 ◽  
pp. A38
Author(s):  
N. M. Azatyan
Keyword(s):  
Molecular Cloud ◽  
Class Ii ◽  
Class I ◽  
Young Stellar Objects ◽  
Spectral Energy ◽  
Evolutionary Stage ◽  
Stellar Content ◽  
Infrared Excess ◽  
Stellar Objects ◽  
Star Forming

Aims. We report the investigation results of the structure and content of a molecular cloud surrounding the source IRAS 05168+3634 (also known as Mol 9). Methods. We present a photometric analysis using the data of J, H, K UKIDSS, [3.6], [4.5] μm Spitzer-IRAC and 3.4, 4.6, 12, 22 μm WISE databases. A multi-color criteria was used to identify the candidates of young stellar objects (YSOs) in the molecular cloud; in addition to IRAS 05168+3634, there are four IRAS sources embedded in the same molecular cloud. Color–magnitude diagrams and the K luminosity function (KLF) were used to determine the basic parameters of stellar objects (spectral classes, masses, ages). To study the YSOs with longer wavelength photometry the radiative transfer models were used. Results. Based on color–color and color–magnitude diagrams, we identified a rich population of embedded YSO candidates with infrared excess (Class 0/I and Class II) and their characteristics in a quite large molecular cloud located in a region of 24 arcmin radius. The molecular cloud includes 240 candidates of YSOs within the radii of subregions around five IRAS sources. The local distribution of identified YSOs in the molecular cloud frequently shows elongation and subclustering. The observed young subregions and parental molecular cloud morphologies are similar, especially when only the youngest Class I/0 sources are considered. The color–magnitude diagrams of the subregions suggest a very young stellar population. We construct the KLF of the subregions except for the IRAS 05162+3639 region and it shows unusually low values for α slope: 0.12–0.21. According to the values of the slopes of the KLFs, the age of the subregions can be estimated at 0.1–3 Myr. The spectral energy distributions (SEDs) are constructed for 45 Class I and 75 Class II evolutionary stage YSOs and the received parameters of these YSOs are well correlated with the results obtained by other methods. According to the results of SED fitting tool, the sources IRAS 05184+3635, IRAS 05177+3636, and IRAS 05162+3639 can be classified as Class I evolutionary stage objects. IRAS 05168+3634 and IRAS 05156+3643 can be classified as flat-spectrum objects.

scholarly journals 44-GHz class I methanol maser survey towards 6.7-GHz class II methanol masers

2012 ◽  
Vol 8 (S287) ◽  
pp. 284-285 ◽  
Author(s):  
Do-Young Byun ◽  
Kee-Tae Kim ◽  
Jae-Han Bae
Keyword(s):  
Class Ii ◽  
Class I ◽  
Young Stellar Objects ◽  
High Mass ◽  
Radio Telescopes ◽  
Stellar Objects ◽  
Methanol Maser ◽  
Peak Flux ◽  
Methanol Masers ◽  
Water Maser

AbstractThe Class II 6.7-GHz methanol maser is a tracer of high mass young stellar objects. We present results of a 44-GHz class I methanol maser and 22-GHz water maser survey using the KVN (Korean VLBI Network) 21-m single dish radio telescopes towards 284 6.7-GHz maser sites. Class I methanol maser and water maser emission is detected towards 116 (41%) and 136 (48%) sources, respectively. About 50 sources have a peak flux density higher than 10 Jy at 44-GHz. They are candidates for VLBI studies using the KVN.

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