Abstract
Variability in young stellar objects (YSOs) can be caused by various time-dependent phenomena associated with star formation, including accretion rates, geometric changes in the ...circumstellar disks, stochastic hydromagnetic interactions between stellar surfaces and inner-disk edges, reconnections within the stellar magnetosphere, and hot/cold spots on stellar surfaces. We uncover and characterize ∼1700 variables from a sample of ∼5400 YSOs in nearby low-mass star-forming regions using mid-IR light curves obtained from the 6.5 yr Near-Earth Object Wide-field Infrared Survey Explorer All Sky Survey. The mid-IR variability traces a wide range of dynamical, physical, and geometrical phenomenon. We classify six types of YSO mid-IR variability based on their light curves: secular variability (linear, curved, and periodic) and stochastic variability (burst, drop, and irregular). YSOs in earlier evolutionary stages have higher fractions of variables and higher amplitudes for the variability, with the recurrence timescale of FUor-type outbursts (defined here as ΔW1 or ΔW2 > 1 mag followed by inspection of candidates) of ∼1000 yr in the early embedded protostellar phase. Known eruptive young stars and subluminous objects show fractions of variables similar to the fraction (∼55%) found in typical protostars, suggesting that these two distinct types are not distinct in variability over the 6.5 yr timescale. Along with brightness variability, we also find a diverse range of secular color variations, which can be attributed to a competitive interplay between the variable accretion luminosity of the central source and the variable extinction by material associated with the accretion process.
Abstract
Protostars likely accrete material at a highly time-variable rate, but measurements of accretion variability from the youngest protostars are rare, as they are still deeply embedded within ...their envelopes. Submillimeter/millimeter observations can trace the thermal response of dust in the envelope to accretion luminosity changes, allowing variations in the accretion rate to be quantified. In this paper, we present contemporaneous submillimeter/millimeter light curves of variable protostars in Serpens Main, as observed by the ALMA Atacama Compact Array (ACA), the Submillimeter Array (SMA), and the James Clerk Maxwell Telescope (JCMT). The most recent outburst of EC 53 (V371 Ser), an ∼18 month periodic variable, is well sampled in the SMA and JCMT observations. The SMA light curve of EC 53 is observed to peak weeks earlier and exhibit a stronger amplitude than at the JCMT. Stochastic variations in the ACA observations are detected for SMM 10 IR, with an amplitude a factor of ∼2 greater than that seen by the JCMT. We develop a toy model of the envelope response to accretion outbursts to show that EC 53's light curves are plausibly explained by the delay associated with the light travel time across the envelope and the additional dilution of the JCMT response, due to the incorporation of cold envelope material in the beam. The larger JCMT beam can also wash out the response to rapid variations, which may be occurring for SMM 10 IR. Our work thus provides a valuable proof of concept for the use of submillimeter/millimeter observations as a means of probing both the underlying accretion luminosity variations and the protostellar environment.
Multigenerational Star Formation in L1551 Moriarty-Schieven, Gerald H; Johnstone, Doug; Bally, John ...
The Astrophysical journal,
07/2006, Volume:
645, Issue:
1
Journal Article
Peer reviewed
The L1551 molecular cloud is undergoing a long and sustained period of relatively high efficiency star formation. It contains two small clusters of Class 0 and I protostars, as well as a halo of more ...evolved Class II and III YSOs, indicating a current and at least one past burst of star formation. We present here new, sensitive maps of 850 and 450 km dust emission covering most of the L1551 cloud; new CO J= 2-1 data of the molecular cloud; and a new, deep, optical image of S II emission (6730 A). Compact submillimeter emitters are concentrated in two subclusters: L1551 IRS5 and L1551 NE, and the HL Tauri group. Both stellar groups show significant extended emission and outflow/jet activity. A jet, terminating at HH 265 and with a very weak associated molecular outflow, may originate from LkHa 358 or from a binary companion to another member of the HL Tauri group. Several Herbig-Haro objects associated with L1551 IRS5/L1551 NE were clearly detected in the submillimeter, as were faint ridges of emission tracing outflow cavity walls. We confirm a large-scale molecular outflow originating from L1551 NE, parallel to that from L1551 IRS5, and suggest that the "hollow shell" morphology is more likely due to two interacting outflows. We confirm the presence of a prestellar core (L1551 MC) of mass 2-3 M sub( )northwest of L1551 IRS5. The next-generation cluster may be forming in this core. The L1551 cloud appears cometary in morphology and appears to be illuminated and eroded from the direction of Orion, perhaps explaining the multiple episodes of star formation.
The James Clerk Maxwell Telescope (JCMT) Gould Belt Survey (GBS) was one of the first legacy surveys with the JCMT in Hawaii, mapping 47 deg2 of nearby (<500 pc) molecular clouds in dust continuum ...emission at 850 and 450 m, as well as a more limited area in lines of various CO isotopologues. While molecular clouds and the material that forms stars have structures on many size scales, their larger-scale structures are difficult to observe reliably in the submillimeter regime using ground-based facilities. In this paper, we quantify the extent to which three subsequent data reduction methods employed by the JCMT GBS accurately recover emission structures of various size scales, in particular, dense cores, which are the focus of many GBS science goals. With our current best data reduction procedure, we expect to recover 100% of structures with Gaussian sizes of ≤30″ and intensity peaks of at least five times the local noise for isolated peaks of emission. The measured sizes and peak fluxes of these compact structures are reliable (within 15% of the input values), but source recovery and reliability both decrease significantly for larger emission structures and fainter peaks. Additional factors such as source crowding have not been tested in our analysis. The most recent JCMT GBS data release includes pointing corrections, and we demonstrate that these tend to decrease the sizes and increase the peak intensities of compact sources in our data set, mostly at a low level (several percent), but occasionally with notable improvement.
A photometric and spectral study of the variable star V2494 Cyg in the L 1003 dark cloud is presented. The brightness of the star, formerly known as HH 381 IRS, increased by 2.5 mag in R (probably in ...the 1980s) and since then has remained nearly constant. Since the brightness increase, V2494 Cyg has illuminated a bipolar cometary nebula. The stellar spectrum has several features typical of the FU Ori (FUor) type, plus it exhibits very strong Hα and forbidden emission lines with high-velocity components. These emission lines originate in the Herbig-Haro (HH) jet near the star. The kinematic age of the jet is consistent with it forming at the time of the outburst leading to the luminosity increase. V2494 Cyg also produces a rather extended outflow; it is the first known FUor with both an observed outburst and a parsec-sized HH flow. The nebula, illuminated by V2494 Cyg, possesses similar morphological and spectral characteristics to Hubble's variable nebula (R Monocerotis/NGC 2261).
► Very strong nightside peaks in temperature and CO abundance appear and disappear in the Venus lower thermosphere on several week timescales. ► These variations do not correlate with variations in ...the large-scale zonal and sub-solar-to-antisolar circulation fields. ► Temporal variations in a smaller-scale nightside residual circulation may account for the lower thermospheric CO and temperature variabilities. ► Variations in global average temperatures and CO abundances in the Venus mesosphere and lower thermosphere occur over several year timescales.
Sub-millimeter
12CO (346
GHz) and
13CO (330
GHz) line absorptions, formed in the mesosphere and lower thermosphere of Venus (70–120
km), have been mapped across the nightside Venus disk during 2001–2009 inferior conjunctions, employing the James Clerk Maxwell Telescope (JCMT). Radiative transfer analysis of these thermal line absorptions supports temperature and CO mixing profile retrievals, as well as Doppler wind fields (described in the companion paper,
Clancy et al., 2012). Temporal sampling over the hourly, daily, weekly and interannual timescales was obtained over 2001–2009. On timescales inferred as several weeks, we observe changes between very distinctive CO and temperature nightside distributions. Retrieved nightside CO, temperature distributions for January 2006 and August 2007 observations display strong local time, latitudinal gradients consistent with early morning (2–3
am), low-to-mid latitude (0–40NS) peaks of 100–200% in CO and 20–30
K in temperature. The temperature increases are most pronounced above 100
km altitudes, whereas CO variations extend from 105
km (top altitude of retrieval) down to below 80
km in the mesosphere. In contrast, the 2004 and 2009 periods of observation display modest temperature (5–10
K) and CO (30–60%) increases, that are centered on antisolar (midnight) local times and equatorial latitudes. Doppler wind derived global (zonal and should be SSAS) circulations from the same data do not exhibit variations correlated with these CO, temperature short-term variations. However, large-scale residual wind fields not fit by the zonal, SSAS circulations are observed in concert with the strong temperature, CO gradients observed in 2006 and 2007 (Clancy et al., 2010). These short term variations in nightside CO, temperature distributions may also be related to observed nightside variations in O
2 airglow (Hueso, H., Sánchez-Lavega, A., Piccioni, G., Drossart, P., Gérard, J.C., Khatuntsev, I., Zasova, L., Migliorini, A. 2008. J. Geophys. Res. 113, E00B02.
doi:10.1029/2008JE003081) and upper mesospheric SO and SO
2 layers (Sandor, B.J., Clancy, R.T., Moriarty-Schieven, G.H., Mills, F.P. 2010. Icarus 208, 49–60).
The retrieved temperature profiles also exhibit 20
K long-term (2001–2009) variations in nightside (whole disk) average mesospheric (80–95
km) temperatures, similar to 1982–1991 variations identified in previous millimeter CO line observations (Clancy et al., 1991). Global average diurnal variations in lower thermospheric temperatures and mesospheric CO abundances decreased by a factor-of-two between 2000–2002 versus 2007–2009 periods of combined dayside and nightside observations. The infrequency and still limited temporal extent of the observations make it difficult to assign specific timescales to such longer term variations, which may be associated with longer term variations observed for cloud top SO
2 (Esposito, L.W., Bertaux, J.-L., Krasnopolsky, V., Moroz, V.I., Zasova, L.V. 1997. Chemistry of lower atmosphere and clouds. In: Bougher, S.W., Hunten, D.M., Phillips, R.J. (Eds.), VENUS II, 1362pp) and mesospheric water vapor (Sandor, B.J., Clancy, R.T. 2005. Icarus 177, 129–143) abundances.
First measurements of SO2 and SO in the Venus mesosphere (70-100km) are reported. This altitude range is distinctly above the a1460-70km range to which nadir-sounding IR and UV investigations are ...sensitive. Since July 2004, use of ground-based sub-mm spectroscopy has yielded multiple discoveries. Abundance of each molecule varies strongly on many timescales over the entire sub-Earth Venus hemisphere. Diurnal behavior is evident, with more SO2, and less SO, at night than during the day. Non-diurnal variability is also present, with measured SO2 and SO abundances each changing by up to 2A- or more between observations conducted on different dates, but at fixed phase, hence identical sub-Earth Venus local times. Change as large and rapid as a 5Ief doubling of SO on a one-week timescale is seen. The sum of SO2 and SO abundances varies by an order of magnitude or more, indicating at least one additional sulfur reservoir must be present, and that it must function as both a sink and source for these molecules. The ratio SO2/SO varies by nearly two orders of magnitude, with both diurnal and non-diurnal components. In contrast to the strong time dependence of molecular abundances, their altitude distributions are temporally invariant, with far more SO2 and SO at 85-100km than at 70-85km. The observed increase of SO2 mixing ratio with altitude requires that the primary SO2 source be upper mesospheric photochemistry, contrary to atmospheric models which assert upward transport as the only source of above-cloud SO2. Abundance of upper mesospheric aerosol, with assumption that it is composed primarily of sulfuric acid, is at least sufficient to provide the maximum gas phase (SO+SO2) sulfur reported in this study. Sulfate aerosol is thus a plausible source of upper mesospheric SO2.
► The global circulation of the Venus upper atmosphere was measured over 2001–2009. ► Distinctly different types of global circulation transition over timescales as short as a day. ► Simple diurnal ...and zonal circulations typically account for only 50% of the observed circulation.
Sub-millimeter
12CO (346
GHz) and
13CO (330
GHz) line absorptions, formed within the mesospheric to lower thermospheric altitude (70–120
km) region of the Venus atmosphere, have been mapped across the nightside disk of Venus during 2001–2009 inferior conjunctions, employing the James Clerk Maxwell Telescope (JCMT). Radiative transfer analysis of these thermal line absorptions supports temperature and CO mixing profile retrievals, as described in a companion paper (
Clancy et al., 2012). Here, we consider the analysis of the sharp line absorption cores of these CO spectra in terms of accurate Doppler wind profile measurements at 95–115
km altitudes versus local time (∼8
pm–4
am) and latitude (∼60N–60S). These Doppler wind measurements support determinations of the nightside zonal and subsolar-to-antisolar (SSAS) circulation components over a variety of timescales. The average behavior fitted from 21 retrieved maps of
12CO Doppler winds (obtained over hourly, daily, weekly, and interannual intervals) indicates stronger average zonal (85
m/s retrograde) versus SSAS (65
m/s) circulation at the 1
μbar pressure (108–110
km altitude) level. However, the absolute and relative magnitudes of these circulation components exhibit extreme variability over daily to weekly timescales. Furthermore, the individual Doppler wind measurements within each nightside mapping observation generally show significant deviations (20–50
m/s, averaged over 5000
km horizontal scales) from the simple zonal/SSAS solution, with distinct local time and latitudinal characters that are also time variable. These large scale residual circulations contribute 30–70% of the observed nightside Doppler winds at any given time, and may be most responsible for global variations in nightside lower thermospheric trace composition and temperatures, as coincidentally retrieved CO abundance and temperature distributions do not correlate with solution retrograde zonal and SSAS winds (see companion paper,
Clancy et al., 2012). Limited comparisons of these nightside submillimeter results with dayside infrared Doppler wind measurements suggest distinct dayside versus nightside circulations, in terms of zonal winds in particular. Combined
12CO and
13CO Doppler wind mapping observations obtained since 2004 indicate that the average zonal and SSAS wind components increase by 50–100% between altitudes of 100 and 115
km. If gravity waves originating from the cloud levels are responsible for the extension of zonal winds into the thermosphere (Alexander, M.J. 1992. Geophys. Res. Lett. 19, 2207–2210), such waves deposit substantial momentum (i.e., break) in the lower nightside thermosphere.
PDF
