Top PDF 1-10 Myr-old low mass stars and brown dwarfs in nearby star forming regions

1-10 Myr-old low mass stars and brown dwarfs in nearby star forming regions

1-10 Myr-old low mass stars and brown dwarfs in nearby star forming regions

As discussed in chapter 6, our somewhat limited understanding of the intermediate mass stellar IMF is greatly overshadowed by our relative non-existent understand- ing of the lowest mass stellar and substellar end of the mass function. Almost as problematic is the high mass end of the IMF. Garnering a full census of high mass stars is difficult, due to the fact that they form, evolve, and die within just a few to tens of megayears, often before their lower mass counterparts have even reached the main sequence. Thus, deriving a field star IMF that extends to tens of solar masses is meaningless as the included high mass stars will necessarily be gigayears younger than the local intermediate and low mass field star population. A less biased view of the high mass IMF can be derived by examining a young, high mass cluster for which the highest mass stars have not yet evolved into supergiants. In this appendix I illustrate analysis of how an HR diagram can be used to assess the age and mass distributions of high mass clusters by describing a study I completed of the massive double cluster h & χ Persei. This work was carried out very early during my time at Caltech, prior to my work with the Quest-2 surveys of Taurus and USco and prior to
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1-10 Myr-old Low Mass Stars and Brown Dwarfs in Nearby Star Forming Regions

1-10 Myr-old Low Mass Stars and Brown Dwarfs in Nearby Star Forming Regions

While the environments of the ONC and NGC 2024 are similar, they are different from that of USco in that USco is a low stellar-density, OB association rather than a high stellar-density embedded cluster. One factor, however, that all three former regions have in common which IC 348 and Taurus do not share is the presence of hot, ionizing stars. The young regions of NGC 2024 and the ONC, both have an O-type member. The older region of USco currently has 49 B-type stars and its most massive member, the O7-type progenitor of pulsar PSR J1932+1059 exploded as a supernova ∼ 1.5 Myr ago (de Geus, 1992). Conversely, neither Taurus nor IC348 have (or show any evidence for having once had) a member earlier than ∼ B5 (Brice˜ no et al. 2002, Luhman et al. 2003b). It is possible that a prestellar core in the presence of an ionizing OB (i.e., with spectral type earlier than ∼ B0) star may ultimately become a lower mass star than it otherwise would have because much of its surrounding material is eroded away by ionizing radiation before it can be accreted. This scenario has been discussed previously in the literature both through observational arguments (e.g., Robberto et al. 2004; Luhman 2004b) and through quantitative theoretical analysis (e.g., Whitworth & Zinnecker 2004; Kroupa & Bouvier 2003). However, this process for generating low mass stars can only take place during the first ∼ 10 5 yr
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Testing the universality of star formation - II. Comparing separation distributions of nearby star-forming regions and the field

Testing the universality of star formation - II. Comparing separation distributions of nearby star-forming regions and the field

We have chosen to compare our separation distribution with that of Raghavan et al. (2010). The primary stars in the Raghavan et al. sample have spectral types from F6 to K3. To better match the young region sample we remove from their sample of 259 compan- ions, 10 L- and T-type brown dwarfs (to better match our sample of stellar companions) leaving 186 companions with measured sepa- rations, 48 with spectroscopically determined orbital elements and 15 with only estimates of the orbital period. For all of the Ragha- van et al. spectroscopic binaries, the periods are short enough that the instantaneous separation will always be smaller than the lower limit to our separation ranges (19 au) and for 13 of the 15 systems with estimated periods, those periods (or direct imaging) also im- ply maximum separations below 19 au. We neglect the two systems for which we have no useful information (HD 16673 Aa,Ab and HD 197214 Aa,Ab) and use the 186 companions with measured separations to make a comparison with our cluster samples. Ap- plying the same separation cuts to the Raghavan sample, we are left with 48, 60 and 97 companions in separation ranges 19–100, 62–620 and 19–774 au, respectively.
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Very low mass stars and brown dwarfs in Upper Scorpius using Gaia DR1 : mass function, disks, and kinematics

Very low mass stars and brown dwarfs in Upper Scorpius using Gaia DR1 : mass function, disks, and kinematics

Gaia published its first data release in 2017 (Gaia Collaboration et al. 2016a,b, henceforth Gaia DR1). While Gaia DR1 does not yet provide Gaia-internal parallaxes and has not yet reached optimum astrometric precision, it can already be used for improving current selection methods for young VLMOs and to refine the resulting samples, as we demonstrate in this paper for the nearest OB association Upper Scorpius. Using the combined parallaxes from the Tycho-Gaia Astrometric Solution (TGAS) for bright young stars, the estimates for distances, age and spatial depth for nearby star forming regions can be solidified. With the help of the Gaia DR1 astrometry, the scatter in proper motions from VLMOS in Upper Scorpius is now (for the first time) dominated by the kinematic spread of the region itself, not by the positional uncertainties. For this particular region later data releases are unlikely to significantly improve the member selection from proper motions. Upper Scorpius is a region mostly free from reddening, therefore follow-up spectroscopy is not as essential here as in other star forming regions.
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Testing the universality of star formation - I. Multiplicity in nearby star-forming regions

Testing the universality of star formation - I. Multiplicity in nearby star-forming regions

A census of the known stellar and substellar pre-main-sequence members of the Taurus–Auriga association was compiled by Kenyon, G´omez & Whitney (2008) and updated by Luhman et al. (2009). The completeness of this sample was investigated by Luhman et al. (2009) who reported that the regions covered by the XEST survey (G¨udel et al. 2007, where complimentary optical and IR surveys exist) are complete for class I and II stars and com- plete down to 0.02 M for class II brown dwarfs. Deep, wide-field, optical, near-IR (Brice˜no et al. 2002; Luhman 2004; Guieu et al. 2006) and Spitzer imaging surveys (Luhman et al. 2010) of Tau- rus provide a high level of completeness into the substellar regime across the region. Therefore, to provide an essentially complete stel- lar membership list we have removed objects with spectral types later than M6, corresponding to sources below the substellar limit in this ∼1–2 Myr old cluster, leaving 292 stars.
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A universal spin mass relation for brown dwarfs and planets

A universal spin mass relation for brown dwarfs and planets

While brown dwarfs show similarities to stars early in their lives, their spin evolutions are much more akin to those of planets. We have used light curves from the K2 mission to measure new rotation periods for 18 young brown dwarfs in the Taurus star-forming region. Our sample spans masses from 0.02 to 0.08 M e and has been characterized extensively in the past. To search for periods, we utilize three different methods ( autocorrelation, periodogram, Gaussian processes ) . The median period for brown dwarfs with disks is twice as long as for those without ( 3.1 versus 1.6 days ) , a signature of rotational braking by the disk, albeit with small numbers. With an overall median period of 1.9 days, brown dwarfs in Taurus rotate slower than their counterparts in somewhat older ( 3 – 10 Myr ) star-forming regions, consistent with spin-up of the latter due to contraction and angular momentum conservation, a clear sign that disk braking overall is inef fi cient and / or temporary in this mass domain. We con fi rm the presence of a linear increase of the typical rotation period as a function of mass in the substellar regime. The rotational velocities, when calculated forward to the age of the solar system, assuming angular momentum conservation, fi t the known spin – mass relation for solar system planets and extra-solar planetary-mass objects. This spin – mass trend holds over six orders of magnitude in mass, including objects from several different formation paths. Our result implies that brown dwarfs by and large retain their primordial angular momentum through the fi rst few Myr of their evolution.
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Emission line diagnostics for accretion and outflows in young very low mass stars and brown dwarfs

Emission line diagnostics for accretion and outflows in young very low mass stars and brown dwarfs

2M1207-39 is the most well-studied among the brown dwarfs in the nearby10 Myr old TW Hya association. It has a disk from which it is accreting [12], an outflow [13] and a planetary companion [14]. An almost two orders of magnitude variation of the accretion rate has been inferred in the past based on the Hα emission from different epochs [15]. This strong variability and the perception of 2M 1207-39 as a kind of prototype accreting brown dwarf have led us to include it in our X-Shooter survey.

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Punanova, Anna
  

(2017):


	Chemistry and kinematics in low-mass star-forming regions.


Dissertation, LMU München: Fakultät für Physik

Punanova, Anna (2017): Chemistry and kinematics in low-mass star-forming regions. Dissertation, LMU München: Fakultät für Physik

Out of the 13 cores in L1495 studied in Chapter 2 we chose one to study the kinematics of the central ∼4000 au and search for substructures within the core. Many of the previously well-studied cores are in isolation: L1521F (Crapsi et al. 2004), L1544 (Caselli et al. 2002b,c; Crapsi et al. 2007), H-MM1 (Parise et al. 2011; Harju et al. 2017), and L1451 (Pineda et al. 2011). Most stars, however, are formed in groups or clusters (see e.g. Gomez et al. 1993, for the distribution of young stars in Taurus). To study a typical example of low-mass star formation, we chose core 10, sitting within a chain of dense cores (see e.g. Fig. 2.1). The core is not directly impacted by any outows from the nearby protostars (Rebull et al. 2010; Wang et al. 2014), so that the kinematic information deduced from the observations will give us insights on the process of core accretion and evolution towards star formation. Core 10 is the most compact among the bright starless cores in L1495. Estimates of the core mass vary from 0.8 to 3.4 M (Marsh et al. 2014) while the Jeans mass for this core
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A Search for Pulsation in Young Brown Dwarfs and Very Low Mass Stars

A Search for Pulsation in Young Brown Dwarfs and Very Low Mass Stars

To amass a statistically significant sample of observations of BDs and VLMSs in several dif- ferent young clusters and star-forming regions, our photometric monitoring program drew on a number of small-to-medium-sized telescopes. The white noise simulations suggested that runs of approximately two weeks apiece at 5–10 minute cadences offered the best chance of probing variability to below the 0.01 magnitude level on sub-hour timescales in these objects. Choice of photometric band was more of an open question, as the wavelength dependence of pulsation amplitudes is unknown and cannot be effectively determined with- out complex three-dimensional stellar simulations. Instead, we narrowed down the selection of filters by aiming to maximize signal-to-noise ratios in brown dwarfs, whose spectral en- ergy distributions peak just longward of 1 µm, or approximately the J band. Complicating this picture are abundant TiO absorption features present in late-type stars, which have been suggested to make variability amplitudes larger at shorter wavelengths such as R or I band (Percy et al. 2001; Maiti 2007). Along with the fact that the longer wavelength near-infrared bands are preferentially affected by atmospheric absorption and variable sky emission, this motivated us to focus mainly on the I band. Practical issues, such as the field of view (FOV) size of detectors, also determined in part the filters available for observation. Many optical imagers (e.g., R, I), tend to have larger FOVs than those operating in the near-infrared (e.g., J , H, K) and are available for longer durations.
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The physical and chemical evolution of star forming regions

The physical and chemical evolution of star forming regions

In th e m ajority of th e work presented in this thesis we have employed theoretical models of th e physics and chemistry in star forming regions in a tte m p ts to explain ex­ isting observational d ata and to infer the physical param eters and evolutionary state of those regions. In some cases, we have been able to suggest from th e m odel results th a t th e observation of other species would be particularly useful to explore the dif­ ferent types of clouds. In this chapter we diverge from this approach; here we explore w hether hom onuclear diatom ic molecules may be detectable in interstellar clouds. T he models th a t we have used (Chapters 3 , 4 , 6 and 7) predict th a t hom onuclear diatom ic molecules may be abundant in dense, low tem p eratu re interstellar clouds (see th e description below). However, a homonuclear diatom ic molecule does not possess a perm anent dipole mom ent and therefore does not have a pure ro tatio n al spectrum . Consequently, low tem perature hom onuclear diatom ics are considered u ndetectable (though, of course, at high tem perature they em it vibrational spectra; H 2 1 - 0 is th e prim e example of this). It is possible to infer abundances for these
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Low-Mass Stars and Their Companions

Low-Mass Stars and Their Companions

One of the other downsides of studying companions to M dwarfs is the di ffi culty in inferring stellar parameters. As can be plainly seen from Equations 1.16 and 1.20, for both RV-detected and transiting planets, the measured quantity of interest (the Doppler amplitude and transit depth) are a function of both planetary and stellar pa- rameters. In both cases, we are only able to understand the planet if we understand its host star: precision planetary astronomy requires precision stellar astronomy. For solar-type stars, we are able to infer stellar parameters at the few percent level through evolutionary models which motivate well-tested relationships between ab- solute magnitude and stellar parameters (Andersen 1991; Casagrande et al. 2010). This is largely possible due to an excellent calibration source located 1 AU away from the Earth. For M dwarfs, we do not have a calibration source. The physics of M dwarf atmospheres is more complicated as well. M dwarfs are defined by the presence of titanium oxide (TiO) bands in their atmospheres (Kuiper 1938; Morgan 1938), but also have molecular bands due to vanadium oxide (VO), carbon monox- ide (CO), and water (H 2 O) (e.g. Mould 1975; Muirhead et al. 2012b). As photons
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Follow-up observations of PTFO 8-8695: a 3 MYR old T Tauri star hosting a Jupiter-mass planetary candidate

Follow-up observations of PTFO 8-8695: a 3 MYR old T Tauri star hosting a Jupiter-mass planetary candidate

In order to create the differential photometry, an initial normalization curve was created by taking the simple mean in magnitude space of all the raw reference light curves. For each reference star light curve, the mode of the residuals against the normalization curve was then subtracted, so that all the reference star light curves were normalized to the same fl ux level. Exposure by exposure, the mode of all the now- normalized reference star magnitudes was found, yielding the differential offset correction needed for each exposure. These differential corrections were subtracted from the raw target light curve to produce the fi nal differentially corrected photometry. The same corrections applied to the original reference stars themselves ( which should yield fl at light curves ) provided an internal consistency check. For a more detailed overview of the differential photometry technique, see van Eyken et al. ( 2011 ) . The LCOGT light curves are tabulated in Table 3 and shown in Figures 3 and 4.
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Water in star-forming regions with Herschel (WISH). III. Far-infrared cooling lines in low-mass young stellar objects

Water in star-forming regions with Herschel (WISH). III. Far-infrared cooling lines in low-mass young stellar objects

The protostellar environment contains many physical compo- nents that can give rise to far-infrared line emission. As illus- trated in Fig. 5 of van Dishoeck et al. (2011), these include (i) the warm quiescent inner part of the envelope passively heated by the luminosity of the source (the “hot core”); (ii) the entrained outflow gas; (iii) UV-heated gas along the cavity walls; (iv) shocks along the outflow cavity walls where the wind from the young star directly hits the envelope; (v) bow shocks at the tip of the jet where it impacts the surrounding cloud; (vi) in- ternal working surfaces within the jet; and (vii) a disk embed- ded in the envelope. In the case of shocks, both C- and J-type shocks are possible. Spatially disentangling all of these compo- nents is not possible with the resolution of Herschel, but our data combined with velocity information from HIFI and physical- chemical models of the molecular excitation provide some in- sight into which components most likely dominate the emission (Visser et al. 2012; Herczeg et al. 2012).
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Atmospheres of brown dwarfs

Atmospheres of brown dwarfs

Perhaps one of the best studied variable objects is Luhman 16B. Luhman 16AB is the third closest system to the sun at only 2pc (Luhman, 2013) being a L7.5 and T0.5 dwarf binary. Gillon et al (2013) reported variability on a 4.87 hour period with strong night to night evolution which was attributed to a fast evolving atmosphere on the cooler T0.5 dwarf. The L7.5 dwarf is not found to be variable. Burgasser et al (2014) performed spectral monitoring of the system, modelling the T dwarf using a two-spot model and inferring a cold covering fraction of ∼30-55% varying by 15-30% over a rotation period. This resulted in a difference of ∼200-400 K between the hot and cold regions. Burgasser et al (2014) interpreted the variations in temperature as changes in the covering fraction of a high cloud deck resulting in cloud holes which expose the deeper, hotter cloud layers. They also suggested the rapidly evolving atmosphere may produce winds as high as 1-3 kms −1 which is consistent with an advection timescale of 1-3 rotation periods. A new analysis of this system, was produced by Crossfield et al (2014) who used Doppler imaging techniques to produce a global surface map, sensitive to a combination of CO equivalent width and surface brightness, of the T dwarf Luhman 16B. The map shows a large, dark mid-latitude region, a brighter area on the opposite hemisphere located near the pole and mottling at equatorial latitudes (Fig. 2 in Crossfield et al 2014). The authors interpreted the map in one of two ways. Either the darker areas represent thicker clouds, obscuring the hotter, inner regions of the atmosphere, and the bright regions correspond to cloud holes providing a view of this warmer interior, or the map shows a combination of surface brightness and chemical abundance variations. They predict that the high latitude bright spot could be similar to polar vortices seen in solar system giant planets, in which case it should be seen in future mapping of this object.
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The formation of brown dwarfs

The formation of brown dwarfs

very low-mass stars, even if they don’t fragment during col- lapse. This is the formation mechanism that has been ex- plored by Padoan & Nordlund (2002). By simulating the de- velopment of interstellar turbulence, they show that a wide range of dense structures is formed. If those structures which are dense and coherent enough to be gravitationally unstable are identified as prestellar cores, they have a mass spectrum very similar to the observed stellar IMF. There is support for this scheme from the observations of Motte, Andr´e & Neri (1998) who show that the mass function for cores does in- deed appear to echo the stellar IMF. However, we note (i) that the core mass function should relate more closely to the system IMF (rather than the stellar IMF), and (ii) that the completeness limit of the core mass function does not extend to brown-dwarf masses. Moreover, the simulations of Padoan & Nordlund do not include gravity, and they use an isother- mal equation of state. Therefore they do not address the re- quirement that dynamically contracting cores must be able to radiate away at least half the gravitational potential energy being released by condensation.
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The production of magnetically dominated star-forming regions

The production of magnetically dominated star-forming regions

We consider the dynamical evolution of an interstellar cloud that is initially in thermal equilibrium in the warm phase and is then subjected to a sudden increase in the pressure of its surroundings. We find that if the initial plasma b of the cloud is of order unity, then there is a considerable period during which the material in the cloud both has a small b and is in the thermally unstable temperature range. These conditions are not only consistent with observations of star-forming regions but also ideally suited to the production of density inhomogeneities by magnetohydrodynamic waves. The end result should be a cloud whose size and average density are typical of Giant Molecular Clouds (GMCs) and that contains denser regions whose densities are in the range inferred for the translucent clumps in GMCs.
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Old star clusters: Bench tests of low mass stellar models

Old star clusters: Bench tests of low mass stellar models

The comparison of star cluster TO and White Dwarf (WD) ages has very recently confirmed the efficiency of a new physical process at work in the interior of WDs. HST observations of the well populated WD cooling sequence in the old metal rich (metallicity about twice solar) open cluster NGC6791 have produced a deep LF that displays a sharp cut-off at low luminosities, caused by the finite age of the cluster, plus a secondary peak at higher luminosities ([3], [4]) as shown in Fig. 5. The secondary peak is best explained by a ∼ 30% fraction of unresolved WD+WD binary systems, the progeny of a ∼ 50% primordial binary fraction. For the distance estimated from isochrone fitting – consistent with the distance modulus more recently obtained from eclipsing binaries hosted by the cluster ([9]) – the age inferred from the absolute magnitude of the faint cut-off is ∼6 Gyr, about 2 Gyr younger than the TO age. This age discrepancy disappears with the inclusion in the WD theoretical calculations of the 22 Ne diffusion in the liquid phase, a physical process previously neglected in WD
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High mass star formation in the nearby region G352 630 1 067  I  parallax

High mass star formation in the nearby region G352 630 1 067 I parallax

Class II methanol masers are known to trace the region close to the high-mass protostar. However, the speci fi c location and association with kinematic structures are still the topic of debate. Some class II methanol masers have been shown to exhibit line, arc, or ring-like distributions and it has been suggested that the methanol masers are associated with either jet / out fl ow-driven shocks ( e.g., Walsh et al. 1998 ) or a rotating disk or torus ( e.g., Norris et al. 1998; Phillips et al. 1998; Bartkiewicz et al. 2009 ) . Measurements of the internal proper motions of methanol masers support an association with rotating disk structures in some sources ( e.g., Sanna et al. 2010a, 2010b; Moscadelli et al. 2011; Sugiyama et al. 2014 ) , and out fl ow motion in other sources ( e.g., Rygl et al. 2010; Sugiyama et al. 2011 ) . For G352.630-1.067, the methanol maser emission is distributed along a linear structure oriented from southwest to northeast ( see Figure 4 ) . The large-scale infrared out fl ows, revealed in the Spitzer and 2MASS images ( see Figure 1 ) , show the same position angle as the methanol maser distribution. So G352.630-1.067 appears to be an example of a class II methanol maser that is closely associated with a jet / out fl ow from the exciting star.
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Low levels of methanol deuteration in the high-mass star-forming region NGC 6334I

Low levels of methanol deuteration in the high-mass star-forming region NGC 6334I

forming regions that not only show D / H ratios which are orders of magnitude higher than that of the ISM, but also display mul- tiply deuterated species. An example of a source exhibiting such high deuterium fractionation is the well-studied low-mass pro- tostellar binary IRAS 16293–2422 (hereafter IRAS 16293) (van Dishoeck et al. 1995; Ceccarelli et al. 1998). This source is es- pecially interesting because it was the first source towards which both doubly as well as triply-deuterated methanol was detected (Parise et al. 2002, 2004). More recently, IRAS 16293 has been studied by Jørgensen et al. (2017, submitted) who have charac- terised the isotope composition of a number of complex organic molecules. They derived D/H ratios, i.e., the column density ra- tio of isotopologues with respect to their hydrogenated counter- parts including the statistical correction for the location of the substituted deuterium, for all detected species in the range 2–
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Weak and compact radio emission in early high mass star forming regions  II  The nature of the radio sources

Weak and compact radio emission in early high mass star forming regions II The nature of the radio sources

It is important to mention that, due to our selection criteria ( see Paper I ) , the sources studied by Sánchez-Monge et al. ( 2013a ) and Cesaroni et al. ( 2015 ) are much brighter at radio wavelengths than the ones from our work, with radio luminosities at 5 GHz of ∼ 10 2 – 10 6 mJy kpc 2 versus 10 −2 – 10 mJy kpc 2 in our sample. In Figure 4, we also show several UC H II regions from Kurtz et al. ( 1994 ) , denoted by the × symbol. These sources seem to be produced by higher free – free emission compared with our sample, suggesting that our sources represent a different population of radio sources. Carral et al. ( 1999 ) , based on selection criteria similar to ours, detected sources with low radio luminosities like the ones in this work. Furthermore, these low radio luminosities are typical of thermal jets, with UV photons that are produced by shocks from collimated winds from the protostar with the surrounding material ( e.g., Anglada 1996 ) . Thus, while our earlier analysis of the cm spectral energy distributions ( SEDs ) suggests a model of pressure-con fi ned H II regions for our compact sources, the Lyman continuum photon rate as a function of the bolometric luminosity shown in Figure 4 does not lend strong support to this model. A further possible explanation for the compact sources with rising spectra, as well as for several elongated sources detected in our survey is that they arise from thermal jets. Consequently, we will explore this scenario in the following section.
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