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At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

WATER MASERS IN THE ANDROMEDA GALAXY. II. WHERE DO MASERS ARISE?

We present a comparative multiwavelength analysis of water-maser-emitting regions and non-maser-emitting luminous 24 μ m star-forming regions in the Andromeda Galaxy (M31) to identify the sites most likely to produce luminous water masers useful for astrometry and proper motion studies. Included in the analysis are Spitzer 24 μ m photometry, Herschel 70 and 160 μ m photometry, H α emission, dust temperature, and star-formation rate. We find significant differences between the maser-emitting and non-maser-emitting regions: water-maser-emitting regions tend to be more infrared-luminous and show higher star-formation rates. The five water masers in M31 are consistent with being analogs of water masers in Galactic star-forming regions and represent the high-luminosity tail of a larger (and as yet undetected) population. Most regions likely to produce water masers bright enough for proper motion measurements using current facilities have already been surveyed, but we suggest three ways to detect additional water masers in M31: (1) reobserve the most luminous mid- or far-infrared sources with higher sensitivity than was used in the Green Bank Telescope survey; (2) observe early-stage star-forming regions selected by millimeter continuum that have not already been selected by their 24 μ m emission, and (3) reobserve the most luminous mid- or far-infrared sources and rely on maser variability for new detections.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

A search for submillimeter H{sub 2}O masers in active galaxies: the detection of 321 GHz H{sub 2}O maser emission in NGC 4945

We present further results of a search for extragalactic submillimeter H{sub 2}O masers using the Atacama Large Millimeter/submillimeter Array (ALMA). The detection of a 321 GHz H{sub 2}O maser in the nearby type 2 Seyfert galaxy, the Circinus galaxy, has previously been reported, and here the spectral analysis of four other galaxies is described. We have discovered H{sub 2}O maser emission at 321 GHz toward the center of NGC 4945, a nearby type 2 Seyfert. The maser emission shows Doppler-shifted velocity features with velocity ranges similar to those of the previously reported 22 GHz H{sub 2}O masers; however, the non-contemporaneous observations also show differences in velocity offsets. The subparsec-scale distribution of the 22 GHz H{sub 2}O masers revealed by earlier very long baseline interferometry observations suggests that the submillimeter masers could arise in an edge-on rotating disk. The maser features remain unresolved by the synthesized beam of ∼0.″54 (∼30 pc) and are located toward the 321 GHz continuum peak within errors. A marginally detected (3σ) high-velocity feature is redshifted by 579 km s{sup −1} with respect to the systemic velocity of the galaxy. Assuming that this feature is real and arises from a Keplerian rotating disk in this galaxy, it is located at a radius of ∼0.020 pc (∼1.5 × 10{sup 5} Schwarzschild radii), which would enable molecular material closer to the central engine to be probed than the 22 GHz H{sub 2}O masers. This detection confirms that submillimeter H{sub 2}O masers are a potential tracer of the circumnuclear regions of active galaxies, which will benefit from higher angular resolution studies with ALMA.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

WATER MASERS IN THE ANDROMEDA GALAXY. I. A SURVEY FOR WATER MASERS, AMMONIA, AND HYDROGEN RECOMBINATION LINES

We report the results of a Green Bank Telescope survey for water masers, ammonia (1, 1) and (2, 2), and the H66 α recombination line toward 506 luminous compact 24 μ m emitting regions in the Andromeda Galaxy (M31). We include the 206 sources observed in the Darling water maser survey for completeness. The survey was sensitive enough to detect any maser useful for ∼10 μ as yr{sup 1} astrometry. No new water masers, ammonia lines, or H66 α recombination lines were detected individually or in spectral stacks reaching rms noise levels of ∼3 mJy and ∼0.2 mJy, respectively, in 3.1–3.3 km s{sup 1} channels. The lack of detections in individual spectra and in the spectral stacks is consistent with Galactic extrapolations. Contrary to previous assertions, there do not seem to be any additional bright water masers to be found in M31. The strong variability of water masers may enable new maser detections in the future, but variability may also limit the astrometric utility of known (or future) masers because flaring masers must also fade.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Simultaneous Observations of H{sub 2}O and SiO Masers toward OH/IR Stars

We performed simultaneous observations of SiO v = 1, 2, {sup 29}SiO v = 0, J = 1–0, and H{sub 2}O 6{sub 12}→5{sub 23} maser lines toward 252 OH/IR stars using the individual 21 m telescopes of the Korean VLBI Network (KVN). The observations for studying SiO and H{sub 2}O maser properties associated with the different evolutionary stages of OH/IR stars were carried out from 2011 November to 2012 July. Both H{sub 2}O and SiO masers were detected from 50 sources with a detection rate of 20% in one epoch of observation. One-sided SiO maser emissions without H{sub 2}O were detected from 108 sources and H{sub 2}O maser emission was detected from 11 sources, of which the detection rates were 43% and 4%, respectively. The overall detection rate of the SiO maser was 63%, and that of the H{sub 2}O maser was 24%. There were 65 new detections in the SiO maser lines, 22 new detections in the H{sub 2}O maser line, and 4 new detections in the {sup 29}SiO maser line. For the H{sub 2}O and/or SiO maser- detected sources, mutual relations between SiO and H{sub 2}O maser properties (including peak and integrated antenna temperatures, and full widths at zero power, etc.) are investigated based on a statistical analysis. We also investigate these maser properties on an IRAS two-color diagram related to stellar evolutionary sequences. In particular, a large number of SiO v = 2-only detected sources appear among the SiO-only detected sources compared to those of both H{sub 2}O and SiO maser detected sources and also appear in the later evolutionary stages of asymptotic giant branch AGB stars in the IRAS two-color diagram. These results may be associated with the development of a hot and thick dust envelope at later stages of AGB evolution and the different excitation conditions of SiO v = 1 and v = 2 masers. Our observational results will be useful for statistical studies of circumstellar envelopes of OH/IR stars related to their late evolution and future very long baseline interferometry (VLBI) observations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

How to Detect Inclined Water Maser Disks (and Possibly Measure Black Hole Masses)

We describe a method for identifying inclined water maser disks orbiting massive black holes and for potentially using them to measure black hole masses. Owing to the geometry of maser amplification pathways, the minority of water maser disks are observable: only those viewed nearly edge-on have been identified, suggesting that an order of magnitude additional maser disks exist. We suggest that inward-propagating masers are gravitationally deflected by the central black hole, thereby scattering water maser emission out of the disk plane and enabling detection. The signature of an inclined water maser disk would be narrow masers near the systemic velocity that appear to emit from the black hole position, as identified by the radio continuum core. To explore this possibility, we present high-resolution (0.″07–0.″17) Very Large Array line and continuum observations of 13 galaxies with narrow water maser emission and show that three are good inclined-disk candidates (five remain ambiguous). For the best case, CGCG 120−039, we show that the maser and continuum emission are coincident to within 3.5 ± 1.4 pc (6.7 ± 2.7 mas). Subsequent very long baseline interferometric maps can confirm candidate inclined disks and have the potential to show maser rings or arcs that provide a direct measurement of black hole mass, although the mass precision will rely on knowledge of the size of the maser disk.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

SiO MASERS AROUND WX PSC MAPPED WITH THE KVN AND VERA ARRAY (KaVA)

We present the first images of the v = 1 and v = 2 J = 1 → 0 SiO maser lines taken with KaVA, i.e., the combined array of the Korean Very Long Baseline Interferometry (VLBI) Network and the VLBI Exploration of Radio Astrometry (VERA), toward the OH/IR star WX Psc. The combination of long and short antenna baselines enabled us to detect a large number of maser spots, which exhibit a typical ring-like structure in both the v = 1 and v = 2 J = 1 → 0 SiO masers as those that have been found in previous VLBI observational results of WX Psc. The relative alignment of the v = 1 and v = 2 SiO maser spots are precisely derived from astrometric analysis, due to the absolute coordinates of the reference maser spot that were well determined in an independent astrometric observation with VERA. The superposition of the v = 1 and v = 2 maser spot maps shows a good spatial correlation between the v = 1 and v = 2 SiO maser features. Nevertheless, it is also shown that the v = 2 SiO maser spot is distributed in an inner region compared to the v = 1 SiO maser by about 0.5 mas on average. These results provide good support for the recent theoretical studies of the SiO maser pumping, in which both the collisional and the radiative pumping predict the strong spatial correlation and the small spatial discrepancy between the v = 1 and v = 2 SiO maser.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

The Physical Parameters of Clumps Associated with Class I Methanol Masers

We present a study of the association between class I methanol masers and cold dust clumps from the APEX Telescope Large Area Survey of the Galaxy (ATLASGAL) survey. It was found that almost 100% of class I methanol masers are associated with objects listed in the ATLASGAL compact source catalog. We find a statistically significant difference in the flux density, luminosity, number and column density, and temperature distributions of ATLASGAL sources associated with 95/44 GHz methanol masers compared with those ATLASGAL sources devoid of 95 GHz methanol masers. The masers tend to arise in clumps with higher densities, luminosities, and temperatures compared with both the full sample of ATLASGAL clumps, as well as the sample of ATLASGAL sources that were cross-matched with positions previously searched for methanol masers but with no detections. Comparison between the peak position of ATLASGAL clumps and the interferometric positions of the associated class I and II methanol masers reveals that class I masers are generally located at larger physical distances from the peak submillimeter emission than class II masers. We conclude that the tight association between ATLASGAL sources and class I methanol masers may be used as a link toward understanding the conditions of the pumping of these masers and evolutionary stages at which they appear.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

UNUSUAL SHOCK-EXCITED OH MASER EMISSION IN A YOUNG PLANETARY NEBULA

We report on OH maser emission toward G336.644−0.695 (IRAS 16333−4807), which is a H{sub 2}O maser-emitting Planetary Nebula (PN). We have detected 1612, 1667, and 1720 MHz OH masers at two epochs using the Australia Telescope Compact Array, hereby confirming it as the seventh known case of an OH-maser-emitting PN. This is only the second known PN showing 1720 MHz OH masers after K 3−35 and the only evolved stellar object with 1720 MHz OH masers as the strongest transition. This PN is one of a group of very young PNe. The 1612 MHz and 1667 MHz masers are at a similar velocity to the 22 GHz H{sub 2}O masers, whereas the 1720 MHz masers show a variable spectrum, with several components spread over a higher velocity range (up to 36 km s{sup −1}). We also detect Zeeman splitting in the 1720 MHz transition at two epochs (with field strengths of ∼2 to ∼10 mG), which suggests the OH emission at 1720 MHz is formed in a magnetized environment. These 1720 MHz OH masers may trace short-lived equatorial ejections during the formation of the PN.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Long-term Variability of H{sub 2}CO Masers in Star-forming Regions

We present results of a multi-epoch monitoring program on variability of 6 cm formaldehyde (H{sub 2}CO) masers in the massive star-forming region NGC 7538 IRS 1 from 2008 to 2015, conducted with the Green Bank Telescope, the Westerbork Radio Telescope , and the Very Large Array. We found that the similar variability behaviors of the two formaldehyde maser velocity components in NGC 7538 IRS 1 (which was pointed out by Araya and collaborators in 2007) have continued. The possibility that the variability is caused by changes in the maser amplification path in regions with similar morphology and kinematics is discussed. We also observed 12.2 GHz methanol and 22.2 GHz water masers toward NGC 7538 IRS 1. The brightest maser components of CH{sub 3}OH and H{sub 2}O species show a decrease in flux density as a function of time. The brightest H{sub 2}CO maser component also shows a decrease in flux density and has a similar LSR velocity to the brightest H{sub 2}O and 12.2 GHz CH{sub 3}OH masers. The line parameters of radio recombination lines and the 20.17 and 20.97 GHz CH{sub 3}OH transitions in NGC 7538 IRS 1 are also reported. In addition, we observed five other 6 cm formaldehyde maser regions. We found no evidence of significant variability of the 6 cm masers in these regions with respect to previous observations, the only possible exception being the maser in G29.96−0.02. All six sources were also observed in the H{sub 2}{sup 13}CO isotopologue transition of the 6 cm H{sub 2}CO line; H{sub 2}{sup 13}CO absorption was detected in five of the sources. Estimated column density ratios [H{sub 2}{sup 12}CO]/[H{sub 2}{sup 13}CO] are reported.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Two new SiO maser sources in high-mass star-forming regions

Silicon monoxide (SiO) masers are rare in star-forming regions, with the exception of five known SiO maser sources. However, we detected two new SiO maser sources from infrared-loud clumps of the high-mass star-forming regions G19.61−0.23 and G75.78+0.34. High angular resolution observations toward G19.61−0.23 suggest that the deeply embedded young stellar object (YSO) of SMA1 is powering the SiO masers. In addition, the SiO v = 1, J = 1 → 0 line shows four spike features, while the v = 2 maser shows combined features of one spike and broad wing components, implying energetic activities of the YSO of SMA1 in the G19.61−0.23 hot molecular core. The SiO v = 0, J = 2 → 1 emission shows bipolar outflows in the NE–SW direction with respect to the center of the SiO maser source. A high angular resolution map of the SiO v = 1, J = 2 → 1 maser in G75.78+0.34 shows that the SiO maser is associated with the CORE source at the earliest stage of high-mass star formation. Therefore, the newly detected SiO masers and their associated outflows will provide good probes for investigating this early high-mass star formation.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

ON THE RELATIONSHIP OF UC H ii REGIONS AND CLASS II METHANOL MASERS. I. SOURCE CATALOGS

We conducted Very Large Array C-configuration observations to measure positions and luminosities of Galactic Class II 6.7 GHz methanol masers and their associated ultra-compact H ii regions. The spectral resolution was 3.90625 kHz and the continuum sensitivity reached 45 μ Jy beam{sup −1}. We mapped 372 methanol masers with peak flux densities of more than 2 Jy selected from the literature. Absolute positions have nominal uncertainties of 0.″3. In this first paper on the data analysis, we present three catalogs; the first gives information on the strongest feature of 367 methanol maser sources, and the second provides information on all detected maser spots. The third catalog presents derived data of the 127 radio continuum counterparts associated with maser sources. Our detection rate of radio continuum counterparts toward methanol masers is approximately one-third. Our catalogs list properties including distance, flux density, luminosity, and the distribution in the Galactic plane. We found no significant relationship between luminosities of masers and their associated radio continuum counterparts, however, the detection rate of radio continuum emission toward maser sources increases statistically with the maser luminosities.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

Discovery of 22 GHz Water Masers in the Serpens South Region

Using the Karl G. Jansky Very Large Array (VLA), we have conducted a survey for 22 GHz, 6{sub 1,6}–5{sub 2,3} H{sub 2}O masers toward the Serpens South region. The masers were also observed with the Very Long Baseline Array following the VLA detections. We detect for the first time H{sub 2}O masers in the Serpens South region that are found to be associated to three Class 0–Class I objects, including the two brightest protostars in the Serpens South cluster, known as CARMA-6 and CARMA-7. We also detect H{sub 2}O masers associated to a source with no outflow or jet features. We suggest that this source is most probably a background asymptotic giant branch star projected in the direction of Serpens South. The spatial distribution of the emission spots suggest that the masers in the three Class 0–Class I objects emerge very close to the protostars and are likely excited in shocks driven by the interaction between a protostellar jet and the circumstellar material. Based on the comparison of the distributions of bolometric luminosity of sources hosting 22 GHz H{sub 2}O masers and 162 young stellar objects covered by our observations, we identify a limit of L {sub Bol} ≈ 10L {sub ⊙} for a source to host water masers. However, the maser emission shows strong variability in both intensity and velocity spread, and therefore masers associated to lower-luminosity sources may have been missed by our observations. We also report 11 new sources with radio continuum emission at 22 GHz.

47 OTHER INSTRUMENTATION↗
DOE OSTI2017

Class II 6.7 GHz Methanol Maser Association with Young Massive Cores Revealed by ALMA

We explored the implication of the association (or lack of it) of 6.7 GHz class II methanol (CH{sub 3}OH) masers with massive dense cores (MDCs) detected (within a sample of ATLASGAL selected infrared quiet massive clumps) at 0.9 mm with Atacama Large Millimeter/submillimeter array. We found 42 out of the 112 cores (37.5%) detected with the Atacama Compact Array (ACA) to be associated with 6.7 GHz CH{sub 3}OH masers. The lowest mass core with CH{sub 3}OH maser association is ∼12 M{sub ⊙}. The angular offsets of the ACA cores from the 6.7 GHz CH{sub 3}OH maser peak positions range from 0.″17 to 4.″79, with a median value of 2.″19. We found a weak correlation between the 0.9 mm continuum (MDCs) peak fluxes and the peak fluxes of their associated methanol multibeam (MMB) 6.7 GHz CH{sub 3}OH masers. About 90% of the cores associated with 6.7 GHz CH{sub 3}OH masers have masses of >40 M {sub ⊙}. The CH{sub 3}OH maser containing cores are candidates for embedded high-mass protostellar objects in their earliest evolutionary stages. With our ACA 0.9 continuum data compared with the MMB 6.7 GHz CH{sub 3}OH maser survey, we have constrained the cores already housing massive protostars based on their association with the radiatively pumped 6.7 GHz CH{sub 3}OH masers.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

VLA Survey of Dense Gas in Extended Green Objects: Prevalence of 25 GHz Methanol Masers

We present ∼1″–4″ resolution Very Large Array (VLA) observations of four CH{sub 3}OH J{sub 2}--J{sub 1}-E 25 GHz transitions (J = 3, 5, 8, 10) along with the 1.3 cm continuum toward 20 regions of active massive star formation containing Extended Green Objects (EGOs), 14 of which we have previously studied with the VLA in the Class I 44 GHz and Class II 6.7 GHz maser lines. Sixteen regions are detected in at least one 25 GHz line (J = 5), with 13 of 16 exhibiting maser emission. In total, we report 34 new sites of CH{sub 3}OH maser emission and 10 new sites of thermal CH{sub 3}OH emission, significantly increasing the number of 25 GHz Class I CH{sub 3}OH masers observed at high angular resolution. We identify probable or likely maser counterparts at 44 GHz for all 15 of the 25 GHz masers for which we have complementary data, providing further evidence that these masers trace similar physical conditions despite uncorrelated flux densities. The sites of thermal and maser emission of CH{sub 3}OH are both predominantly associated with the 4.5 μm emission from the EGO, and the presence of thermal CH{sub 3}OH emission is accompanied by 1.3 cm continuum emission in 9 out of 10 cases. Of the 19 regions that exhibit 1.3 cm continuum emission, it is associated with the EGO in 16 cases (out of a total of 20 sites), 13 of which are new detections at 1.3 cm. Twelve of the 1.3 cm continuum sources are associated with 6.7 GHz maser emission and likely trace deeply embedded massive protostars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

HIGH-RESOLUTION IMAGING OF WATER MASER EMISSION IN THE ACTIVE GALAXIES NGC 6240 AND M51

We present the results of observations of 22 GHz H{sub 2}O maser emission in NGC 6240 and M51 made with the Karl G. Jansky Very Large Array. Two major H{sub 2}O maser features and several minor features are detected toward the southern nucleus of NGC 6240. These features are redshifted by about 300 km s{sup −1} from the galaxy’s systemic velocity and remain unresolved at the synthesized beam size. A combination of our two-epoch observations and published data reveals an apparent correlation between the strength of the maser and the 22 GHz radio continuum emission, implying that the maser excitation relates to the activity of an active galactic nucleus in the southern nucleus rather than star-forming activity. The star-forming galaxy M51 hosts H{sub 2}O maser emission in the center of the galaxy; however, the origin of the maser has been an open question. We report the first detection of 22 GHz nuclear radio continuum emission in M51. The continuum emission is co-located with the maser position, which indicates that the maser arises from nuclear active galactic nucleus-activity and not from star-forming activity in the galaxy.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

ALMA observation of the 658 GHz vibrationally excited H{sub 2}O maser in Orion KL source I

We present an observational study of the vibrationally excited H{sub 2}O line at 658 GHz (ν{sub 2} = 1, 1{sub 1,0}-1{sub 0,1}) toward Orion KL using the Atacama Large Millimeter/Submillimeter Array (ALMA). This line is clearly detected at the position of the massive protostar candidate, Source I. The spatial structure is compact, with a size of about 100 AU, and is elongated along the northeast–southwest low-velocity (18 km {sup −1}) bipolar outflow traced by 22 GHz H{sub 2}O masers, SiO masers, and thermal SiO lines. A velocity gradient can be seen perpendicular to the bipolar outflow. The overall spatial and velocity structure seems to be analogous to that of the 321 GHz H{sub 2}O maser line previously detected with ALMA and vibrationally excited SiO maser emission. The brightness temperature of the 658 GHz H{sub 2}O line is estimated to be higher than 2 × 10{sup 4} K, implying that it is emitted via maser action. Our results suggest that the 658 GHz H{sub 2}O maser line is emitted from the base of the outflow from a rotating and expanding accretion disk as observed for the SiO masers and the 321 GHz H{sub 2}O maser. We also search for two other H{sub 2}O lines at 646 GHz (9{sub 7,3}-8{sub 8,0} and 9{sub 7,2}-8{sub 8,1}), but they are not detected in Orion KL.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

44 GHZ CLASS I METHANOL (CH{sub 3}OH) MASER SURVEY IN THE GALACTIC CENTER

We report on a large 44 GHz (7{sub 0}–6{sub 1} A {sup +}) methanol (CH{sub 3}OH) maser survey of the Galactic Center. The Karl G. Jansky Very Large Array was used to search for CH{sub 3}OH maser emission covering a large fraction of the region around Sgr A. In 25 pointings, over 300 CH{sub 3}OH maser sources (>10 σ ) were detected. The majority of the maser sources have a single peak emission spectrum with line of sight velocities that range from about −13 to 72 km s{sup −1}. Most maser sources were found to have velocities around 35−55 km s{sup −1}, closely following velocities of neighboring interacting molecular clouds (MCs). The full width at half-maximum of each individual spectral feature is very narrow (∼0.85 km s{sup −1} on average). In the north, where Sgr A East is known to be interacting with the 50 km s{sup −1} MC, more than 100 44 GHz CH{sub 3}OH masers were detected. In addition, three other distinct concentrations of masers were found, which appear to be located closer to the interior of the interacting MCs. It is possible that a subset of masers is associated with star formation, although conclusive evidence is lacking.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

A Search for Water Maser Emission from Brown Dwarfs and Low-luminosity Young Stellar Objects

We present a survey for water maser emission toward a sample of 44 low-luminosity young objects, comprising (proto-)brown dwarfs, first hydrostatic cores (FHCs), and other young stellar objects (YSOs) with bolometric luminosities lower than 0.4 L {sub ⊙}. Water maser emission is a good tracer of energetic processes, such as mass-loss and/or accretion, and is a useful tool to study these processes with very high angular resolution. This type of emission has been confirmed in objects with L {sub bol} ≳ 1 L {sub ⊙}. Objects with lower luminosities also undergo mass-loss and accretion, and thus, are prospective sites of maser emission. Our sensitive single-dish observations provided a single detection when pointing toward the FHC L1448 IRS 2E. However, follow-up interferometric observations showed water maser emission associated with the nearby YSO L1448 IRS 2 (a Class 0 protostar of L {sub bol} ≃ 3.6–5.3 L {sub ⊙}) and did not find any emission toward L1448 IRS 2E. The upper limits for water maser emission determined by our observations are one order of magnitude lower than expected from the correlation between water maser luminosities and bolometric luminosities found for YSOs. This suggests that this correlation does not hold at the lower end of the (sub)stellar mass spectrum. Possible reasons are that the slope of this correlation is steeper at L {sub bol} ≤ 1 L {sub ⊙} or that there is an absolute luminosity threshold below which water maser emission cannot be produced. Alternatively, if the correlation still stands at low luminosity, the detection rates of masers would be significantly lower than the values obtained in higher-luminosity Class 0 protostars.

79 ASTRONOMY AND ASTROPHYSICS↗