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the inner 500-pc region of the galaxy , known as the central molecular zone ( cmz ) , is rich in dense molecular gas , but the current star formation is unusually inactive . other than the few star forming regions such as sgr  b2 @xcite , sgr  a complex @xcite , and sgr  c @xcite , most cmz clouds appear to be inactive in star formation ( e.g. , g0.253 + 0.016 ; * ? ? ? * ; * ? ? ? * ; * ? ? ? * ; * ? ? ? * ; * ? ? ? * ; * ? ? ? this inactivity is in contrast to the general star formation relation that has been established for the galactic disk clouds as well as external galaxies , which presents a good correlation between the amount of dense molecular gas and the star formation rate . the overall star formation rate in the cmz is an order of magnitude lower than the prediction of this correlation @xcite . recent theoretical works by @xcite and @xcite point out that star formation in the cmz could be regulated by the strength of turbulence : the inflowing gas through the galactic bar drives strong turbulence , resulting in episodes of low star formation activity ; when the turbulence finally dissipates , a burst of star formation takes place . such dissipation of the turbulence could be induced by compressive tides during a close passage to the bottom of the gravitational potential near sgr  a * @xcite . in the simulations of @xcite , the massive molecular clouds in the cmz are found to be in several streams of open trajectory centered at sgr  a * with a radius of @xmath3100  pc . the 20 kms@xmath0 cloud , a massive ( @xmath41.6@xmath510@xmath6  @xmath7 ; * ? ? ? * ) molecular cloud named after its radial velocity , appears to be close to pericenter passage with sgr  a * , therefore could be in the process of turbulence dissipation . in this scenario , we expect to observe increasing dense gas fraction and signs of early star formation in the 20 kms@xmath0 cloud . previous interferometric observations have found one region @xcite , and several h@xmath1o masers @xcite in this cloud . however , the maser observations were sensitivity - limited ( usually @xmath430  mjybeam@xmath0 per 0.2  kms@xmath0 channel ) , hence could miss faint sources that trace star formation of lower masses or at early evolutionary stages . here we use interferometric submillimeter observations to trace dense cores , and interferometric h@xmath1o maser observations with a sensitivity of @xmath35 times higher than previous studies to trace embedded protostars . throughout the paper , we adopt a distance to the galactic center of 8.4  kpc @xcite . in 2013 , we observed a mosaic of eight positions in the 20 kms@xmath0 cloud with the submillimeter array ( sma ) in its compact and sub - compact configurations at 230  ghz band . the primary beam size is @xmath356 . all observations share the same correlator setup : the rest frequencies of 216.9220.9  ghz are covered in one sideband , and 228.9232.9  ghz are covered in the other sideband , with a uniform channel width of 0.812  mhz , equivalent to 1.1  kms@xmath0 at 230  ghz . observation details are listed in . the visibility data were calibrated using and inspected and imaged using @xcite and @xcite . continuum was extracted from line free channels and imaged using data from both sidebands . spectral lines were split from the continuum - subtracted visibility data and were imaged separately . we used a robust parameter of 0.5 when cleaning images . the resulting continuum image has a clean beam of 4.9@xmath52.8 ( equivalent to 0.20  pc@xmath50.11  pc ) with a position angle of 5.2 and an rms of 3  mjybeam@xmath0 . typical rms of spectral lines images is @xmath30.13 jybeam@xmath0 per 1.1  kms@xmath0 channel .      in 2013 may , we observed a mosaic of three positions in this cloud with the national radio astronomy observatory ( nrao ) karl g.  jansky very large array ( vla ) in the dnc configuration at k band , with a primary beam size of @xmath32 . the widar correlator was configured to cover the h@xmath1o maser at 22.2  ghz using a 16  mhz subband in dual polarizations , as well as 1.3  cm continuum using eight 128  mhz subbands in full polarizations . for the h@xmath1o maser , the subband was split into 1024 channels with a channel width of 15.6  khz , equivalent to 0.2  kms@xmath0 . observation details are listed in . the visibility data were calibrated and imaged using . continuum was extracted from line free channels of the 128  mhz subbands , leading to a total bandwidth of 0.9  ghz . the robust parameter we used in clean is 0.5 . for the h@xmath1o maser image , the resulting clean beam is 3.5@xmath52.4 ( equivalent to 0.14  pc@xmath50.10  pc ) with a position angle of 5.7 . the rms in each 0.2  kms@xmath0 channel is 6  mjybeam@xmath0 , but can be significantly larger in channels where signals are dynamic range limited . the sma 1.3  mm continuum emission revealed five clumps of 1-pc scales in the 20 kms@xmath0 cloud , labelled as c1c5 in . in the projected plane of the sky they are equally spaced by @xmath31  pc and aligned along the densest part of the cloud . all the clumps appear to have substructures , or cores , among which c4 is the most prominent one which presents at least 6 cores .    after a visual inspection of dust peaks with fluxes above 5@xmath8 levels , we identified 17 cores . then we fitted 2d gaussians to obtain their positions , deconvolved sizes , and primary - beam corrected continuum fluxes . we assumed a gas - to - dust mass ratio of 100 , a dust emissivity index @xmath9 , and applied the dust temperature @xmath10 ( see ) and a mean continuum frequency of 225  ghz , then the core masses were calculated following @xcite . the results are listed in . the vla observations revealed 18 h@xmath1o masers in this cloud , marked by crosses and labeled as w1w18 in . 15 out of 18 are within the sma field , among which 13 spatially coincide with dust emission peaks above 5@xmath8 levels . the velocities of these 13 masers are all within @xmath1120  kms@xmath0 with respect to the core velocities . properties of these h@xmath1o masers are summarized in and their spectra are shown in .    among the previous h@xmath1o maser surveys toward the cmz , @xcite did not find any masers in this cloud using the mopra telescope at a sensitivity of 12  jy , while @xcite detected three masers , using atca at a sensitivity of @xmath120.1  jy : two of them are consistent with w13 and w15 , respectively , within a position uncertainty of 2 ; the other one is in c1 , offset from the masers we detected by @xmath33 . in addition , @xcite serendipitously detected 10 h@xmath1o masers in this cloud while studying oh / ir stars , all of which they concluded to be connected to star formation given the non - detection of oh / ir stars . one of them is consistent with w5 , while the other 9 are scattered in c4 : one is offset from any masers we detected , 8 are likely consistent with w10 , w11w13 , and w15 . we analyze the virial status of the cores . the virial parameter is defined as @xmath13 @xcite , where @xmath14 is the one - dimensional velocity dispersion . for a self - gravitating , non - magnetized core , the critical virial parameter is 2 , above which the core is unbound and may expand , while below which it is bound and may collapse . the n@xmath1h@xmath15 line has a critical density of @xmath410@xmath16  @xmath17 , and is superior for tracing dense gas than the spectral lines in our sma data such as c@xmath18o or h@xmath1co . to derive line widths of the cores , we obtain the sma n@xmath1h@xmath15 32 data ( kauffmann et al .  in prep . ) , then fit gaussians to the mean n@xmath1h@xmath15 spectra of the cores . for the 4 cores without n@xmath1h@xmath15 detections , we fit their mean c@xmath18o or h@xmath1co spectra instead . two cores , c3-p2 and c3-p3 , do not present any of the three dense gas tracers , thus are excluded in the analysis . the results are listed in .    among the 15 cores included in the analysis , 12 have @xmath19 . the two most massive cores , c1-p1 and c4-p1 , have virial parameters as low as @xmath30.2 . three cores have @xmath20 , including c4-p4 which associates with an h@xmath1o maser . the masses themselves have large uncertainties : the errors in the core masses are a factor of 1.21.8 , while the virial masses are sensitive to the selection of line widths and the errors are a factor of 1.72 , according to the simulations of @xcite that take errors in all variables of the mass estimates into account . besides , the magnetic field might be dynamically important on pc scales in the cmz clouds , as suggested by the ordered magnetic vectors in g0.253 + 0.016 @xcite which derive a magnetic flux density of @xmath35  mg from the chandrasekhar - fermi method . within these uncertainties , the 12 cores with @xmath19 are gravitationally bound . h@xmath1o masers in star - forming regions are usually excited in shocked ambient gas @xcite , therefore are used to trace protostellar outflows . however , they can also be excited in the envelope of evolved stars @xcite or excited by shocks created in large - scale dynamic processes ( e.g. , cloud collisions , * ? ? ? we need to exclude these scenarios before using the h@xmath1o masers as star formation indicators . first , we compare the coordinates and velocities of the h@xmath1o masers with the evolved star catalogues in @xcite , @xcite , and @xcite which used oh or sio masers as tracers . two h@xmath1o masers ( w6 , w18 ) are consistent with evolved stars ( red crosses in ) . the other 16 h@xmath1o masers do not have known evolved star counterparts . second , we compare with the class i ch@xmath2oh masers in @xcite , which are collisionally pumped and trace large - scale dynamics . none of the h@xmath1o masers spatially coincide with the class i ch@xmath2oh masers . most of the h@xmath1o masers coincide with the dense cores , instead of uniformly scatter like the class i ch@xmath2oh masers . their velocities are usually offset by @xmath1120  kms@xmath0 from the core velocities , which is easily seen in masers tracing outflows , instead of all showing the same value at the presumable shock interaction velocity . all these facts suggest that the h@xmath1o masers are unlikely connected to large - scale shocks . therefore , the 16 h@xmath1o masers without evolved star counterparts , in particular , the 13 masers coincident with the dense cores , are likely associated with outflows . indeed , we find signatures of outflows associated with these cores traced by sio , so , and hnco lines in the sma images , which we will present in a forthcoming paper . a correlation between luminosities of h@xmath1o masers and protostars ( e.g. , * ? ? ? * ) suggests that the more luminous masers ( @xmath410@xmath21  @xmath22 , see ) may correspond to early b - type stars of @xmath410@xmath23  @xmath22 , while the characteristic luminosity of @xmath310@xmath24  @xmath22 for most masers is usually found in low- or intermediate - mass protostars ( e.g. , * ? ? ? the free - free emission from regions is a reliable star formation tracer . however , only one region has been found in this cloud @xcite , which is verified by our vla 1.3  cm continuum observation ( ) . assuming an electron temperature of 10@xmath25  k , with a continuum flux of 0.12  jy , its ionizing photon rate is 9@xmath510@xmath26  s@xmath0 @xcite , corresponding to an o9 star of 19  @xmath7 @xcite . the non - detection of any other regions at a sensitivity of 100  @xmath27jy may suggest no protostars earlier than b1 above @xmath310  @xmath7 , or simply an early evolutionary phase without any visible ionizing sources . @xcite identified ysos in the cmz using the _ spizter _ 8   and 24   emission , and found three in the vicinity of this cloud , but none of them are within the cloud itself . the only visible infrared point source within the cloud in the _ spitzer _ mid - infrared images is the region . therefore , the deeply embedded protostars are not observable in the infrared bands , probably due to the large column density ( @xmath410@xmath28  @xmath29 ) which obscures the embedded sources . ch@xmath2oh masers have been found in star - forming regions in the galactic disk , and are classified as class i or class ii , depending on whether they are collisionally or radiatively pumped @xcite . recent class i ch@xmath2oh maser observations toward the cmz @xcite suggested that these masers may not trace star formation , but may have cosmic ray or cloud - scale dynamic origins . on the other hand , radiatively pumped class ii ch@xmath2oh masers have been reliable tracers of high - mass star formation @xcite . the methanol multi - beam survey ( mmb , * ? ? ? * ) did not detect any class ii ( 6.7  ghz ) ch@xmath2oh masers at a sensitivity of 0.17  jy in this cloud . by contrast , four h@xmath1o masers in our results ( w3 , w5 , w11 , w15 ) are detectable at 3@xmath8 levels with the same sensitivity .      as discussed above , we find 13 h@xmath1o masers associated with the dense cores and probably trace protostellar outflows , and 5 dense cores without h@xmath1o maser detection but gravitationally bound . in total , they may represent 18 protostellar candidates . this number should be a lower limit because the dense cores could harbor multiple protostars . by contrast , previous observations found @xmath310 h@xmath1o masers most of which are concentrated in c4 , corresponding to 6 protostellar candidates in our sample . in the following we discuss the implication of our results for the evolutionary phase and star formation activity of the 20 kms@xmath0 cloud . the evolutionary phases of star formation traced by h@xmath1o masers is under debate ( e.g. , * ? ? ? * ; * ? ? ? recent follow - up studies of the unbiased mmb survey suggest that while not as well - defined as the other masers , h@xmath1o masers in general trace the protostellar phase when outflows emerge @xcite . on the other hand , these studies seem to converge to the conclusion that class ii ch@xmath2oh masers trace a well - defined evolutionary phase ( e.g. , after protostars start to heat ambient gas and before the uc region phase ) . for the 20 kms@xmath0 cloud , the large population of h@xmath1o masers and the virtually non - detection of class ii ch@xmath2oh masers so far , combining with the non - detection of any prominent uc regions , are likely indicative of two alternative scenarios : an early evolutionary phase when high - mass protostars have not started to heat or ionize the ambient gas , but have developed outflows , similar to the situation in infrared dark clouds ( irdcs ) ; or a cluster of low- or intermediate - mass protostars , whose outflows power the h@xmath1o masers but whose radiation is not enough to create class  ii ch@xmath2oh masers or visible regions . the former may be preferable because the more luminous masers may trace high - mass protostars ( see section  [ subsec : natureofmaser ] ) . no matter which scenario it is , the 18 protostellar candidates traced by h@xmath1o masers and gravitationally bound cores suggest more active star formation than that traced by free - free or infrared emission ( one region ) . sensitive maser and submillimeter observations could be a promising method to trace incipient star formation in other massive clouds in the cmz .    in the orbital models of @xcite , g0.253 + 0.016 is also close to pericenter passage with sgr  a * , but in a different gas stream . g0.253 + 0.016 has a similar mass as the 20 km s@xmath0 cloud ( @xmath31.3@xmath510@xmath6  @xmath7 ; * ? ? ? * ) , but only has one known h@xmath1o maser @xcite and one gravitationally bound dense core associated with the maser @xcite . the progression of star formation activity from g0.253 + 0.016 to sgr  b2 supports the tidal compression and triggered collapse model proposed by @xcite . the 20 kms@xmath0 cloud could be a second case of such triggering . based on the number of masers and the amount of dense gas contained in cores , the current star formation rate in the 20 kms@xmath0 cloud is likely higher than in g0.253 + 0.016 . the difference might be due to variations in the local environment in the streams . we have found 18 protostellar candidates traced by dust emission and h@xmath1o masers in the 20 kms@xmath0 cloud , most of which have been missed by previous infrared or radio continuum studies . systematic studies of other massive clouds in the cmz , using high resolution , sensitive maser and submillimeter observations , will be helpful to establish their star formation status . one such example is the sma legacy survey of the cmz ( pis : c.  battersby & e.  keto ) that will cover all regions in the cmz above a column density threshold of 10@xmath28  @xmath29 using the same setups as our sma observations . a virial analysis shows that among the 17 dense cores traced by dust emission , 12 cores are gravitationally bound . the 13 h@xmath1o masers associated with the dense cores likely trace protostellar outflows . the star formation in the 20 kms@xmath0 cloud appears to be in an early evolutionary phase , before significant heating or ionization of ambient gas . this cloud may follow the tidal compression and triggered collapse model of @xcite and @xcite , similar to the dust ridge clouds . however , its star formation rate is higher than in g0.253 + 0.016 , which likely reflects local differences in initial conditions . we thank the anonymous referee for constructive comments , and elisabeth mills and hauyu baobab liu for helpful discussion . x.l . acknowledges the support of a smithsonian predoctoral fellowship and the program a for outstanding phd candidate of nanjing university . t.p . acknowledges financial support from the _ deutsche forschungsgemeinschaft , dfg _ via the spp ( priority program ) 1573 (  physics of the ism " ) . j.m.d.k . is funded by a gliese fellowship . this work is supported by the si cgps grant on star formation in the central molecular zone of the milky way , the national natural science foundation of china ( grants 11328301 , 11273015 and 11133001 ) , and the national basic research program ( 973 program no . 2013cb834905 ) .                                                          , j.  p. , waters , b. , schiebel , d. , young , w. , & golap , k. 2007 , in astronomical society of the pacific conference series , vol . 376 , astronomical data analysis software and systems xvi , ed . r.  a. shaw , f.  hill , & d.  j. bell , 127    , k. 1991 , in astronomical society of the pacific conference series , vol . 16 , atoms , ions and molecules : new results in spectral line astrophysics , ed . a.  d. haschick & p.  t.  p. ho ( san francisco : asp ) , 119                    , r.  j. , teuben , p.  j. , & wright , m.  c.  h. 1995 , in astronomical society of the pacific conference series , vol . 77 , astronomical data analysis software and systems iv , ed . r.  a. shaw , h.  e. payne , & j.  j.  e. hayes , 433                            cccccccccccc sma & x.  lu & 2013a - s049 & many & 2013 jul 24 & 0.25 & 100400 & s1s8 & 3c84 & neptune & q1 , q2 & 6 antennas + compact & & & & 2013 aug 03 & 0.10 & 100240 & s1s8 & 3c84 & neptune & q1 , q2 & 5 antennas + & & & & 2013 aug 09 & 0.20 & 100240 & s1s8 & 1924@xmath30292 & neptune & q1 , q2 & 5 antennas + sma & & & & & & & & & neptune , & & + subcompact & & & & & & & & & titan & & + & & & nh@xmath2(1,1)(5,5 ) , & & & & & & & & + & & & h@xmath1o maser , & & & & & & & & + & & & 1.3  cm continuum & & & & & & & &    cccrrccc c1-p1 & 17:45:37.58 , -29:03:48.83 & @xmath31 & 48.8 & 648.1 & 3.0 ( n ) & 1309 & 0.17 + c1-p2 & 17:45:38.18 , -29:03:40.31 & @xmath32 & 28.4 & 191.7 & 2.9 ( n ) & 387 & 0.62 + c1-p3 & 17:45:39.17 , -29:03:41.03 & @xmath33 & 5.25 & 112.7 & 4.2 ( n ) & 228 & 2.32 + c2-p1 & 17:45:38.23 , -29:04:26.60 & @xmath34 & 36.0 & 189.0 & 5.8 ( n ) & 382 & 2.55 + c2-p2 & 17:45:38.62 , -29:04:18.69 & @xmath35 & 22.7 & 118.5 & 3.2 ( n ) & 239 & 1.45 + c2-p3 & 17:45:39.04 , -29:04:13.24 & @xmath36 & 34.7 & 87.8 & 2.5 ( n ) & 177 & 1.02 + c3-p1 & 17:45:37.81 , -29:05:02.41 & @xmath37 & 178.0 & 208.1 & 4.3 ( h ) & 420 & 0.92 + c3-p2 & 17:45:37.62 , -29:05:16.65 & @xmath38 & 3.1 & 63.0 & & 127 & + c3-p3 & 17:45:38.28 , -29:04:58.59 & @xmath39 & 90.8 & 112.7 & & 228 & + c4-p1 & 17:45:37.64 , -29:05:43.65 & @xmath40 & 68.0 & 935.4 & 3.6 ( n ) & 1889 & 0.22 + c4-p2 & 17:45:38.23 , -29:05:32.72 & @xmath41 & 30.5 & 393.2 & 3.6 ( c ) & 794 & 0.56 + c4-p3 & 17:45:35.36 , -29:05:55.53 & @xmath42 & 99.0 & 104.1 & 3.8 ( c ) & 210 & 0.87 + c4-p4 & 17:45:36.25 , -29:05:49.03 & @xmath43 & 56.0 & 76.4 & 5.0 ( n ) & 154 & 2.73 + c4-p5 & 17:45:36.74 , -29:05:45.93 & @xmath445.23  @xmath5  @xmath443.07 & & 35.3 & 2.4 ( n ) & 71 & 1.82 + c4-p6 & 17:45:37.16 , -29:05:55.13 & @xmath45 & 6.2 & 37.9 & 2.3 ( c ) & 76 & 1.19 + c5-p1 & 17:45:36.71 , -29:06:17.50 & @xmath46 & 73.4 & 189.7 & 2.7 ( n ) & 383 & 0.54 + c5-p2 & 17:45:36.43 , -29:06:19.55 & @xmath47 & 13.1 & 151.2 & 2.3 ( n ) & 305 & 0.54    ccccccc w1 & 17:45:38.10 , -29:03:41.75 & 14.8 , 19.0 & 431 , 100 & 430 & 7.0 & c1-p2 + w2 & 17:45:37.73 , -29:03:46.29 & 21.5 , 27.8 & 131 , 161 & 775 & 12.6 & c1-p1 + w3 & 17:45:37.48 , -29:03:49.15 & 24.5 , 28.1 & 999 , 1022 & 3797 & 61.8 & c1-p1 + w4 & 17:45:34.63 , -29:04:36.42 & 2.4 & 396 & 670 & 10.9 & + w5 & 17:45:37.76 , -29:05:01.91 & -32.2 , 18.6 & 180 , 13180 & 6822 & 110.0 & c3-p1 + w6 & 17:45:40.75 , -29:05:01.93 & 12.7 & 225 & 357 & 5.8 & + w7 & 17:45:35.85 , -29:05:08.75 & 51.0 & 212 & 186 & 3.0 & + w8 & 17:45:37.68 , -29:05:13.57 & 46.2 & 17 & 18 & 0.3 & c3-p2 + w9 & 17:45:37.52 , -29:05:22.59 & -40.8 , -40.2 & 33 , 35 & 46 & 0.7 & + w10 & 17:45:37.16 , -29:05:41.70 & -18.7 , 10.8 & 54 , 93 & 196 & 3.2 & c4-p1 + w11 & 17:45:37.62 , -29:05:43.92 & 4.5 , 9.9 & 783 , 507 & 3258 & 53.0 & c4-p1 + w12 & 17:45:37.51 , -29:05:43.89 & 13.1 , 16.9 & 110 , 99 & 264 & 4.3 & c4-p1 + w13 & 17:45:37.90 , -29:05:44.24 & -21.5 , 26.4 & 23 , 23 & 55 & 0.9 & c4-p1 + w14 & 17:45:36.72 , -29:05:46.02 & -25.5 , -24.6 & 84 , 171 & 246 & 4.0 & c4-p5 + w15 & 17:45:36.33 , -29:05:49.52 & 5.5 , 13.1 & 972 , 1178 & 2377 & 38.7 & c4-p4 + w16 & 17:45:35.15 , -29:05:53.62 & -4.8 , -4.4 & 26 , 24 & 48 & 0.8 & c4-p3 + w17 & 17:45:37.11 , -29:05:54.38 & -3.8 , -3.1 & 171 , 159 & 302 & 4.9 & c4-p6 + w18 & 17:45:34.77 , -29:06:02.43 & 20.3 , 20.7 & 17 , 16 & 33 & 0.5 &     column density , between 5@xmath510@xmath48  @xmath29 and 4@xmath510@xmath28  @xmath29 in step of 5@xmath510@xmath48  @xmath29 . the green contours present the vla 1.3  cm continuum emission , between 2  mjybeam@xmath0 and 18  mjybeam@xmath0 in step of 4  mjybeam@xmath0 . the cyan and green dotted loops show the fhwms of the sma and vla primary beam responses , respectively . middle : both contours and background image show the sma 1.3  mm continuum emission . the contours are between 5@xmath8 and 65@xmath8 levels in step of 10@xmath8 , where 1@xmath8=3  mjybeam@xmath0 . the five clumps are labeled . the dotted loop shows the fwhm of the sma primary beam response . the synthesized beam of the sma is shown in the lower left corner . the region is marked by a green star . right : same as the middle panel , but only the 5@xmath8 and 15@xmath8 contours are plotted . the ellipses are the results of 2d gaussian fittings.,scaledwidth=100.0% ]    o masers . the two red crosses ( w6 , w18 ) are known oh / ir stars . the blue crosses show h@xmath1o masers without known oh / ir star counterparts . both the contours and the background image show the sma 1.3  mm continuum emission . the loops are the same as in . right : the spectra of the 18 h@xmath1o masers . the features at @xmath318  kms@xmath0 , either in emission or in absorption , are from sidelobes of the strongest maser , w5.,scaledwidth=100.0% ]
we report the discovery of a population of deeply embedded protostellar candidates in the 20 kms@xmath0 cloud , one of the massive molecular clouds in the central molecular zone ( cmz ) of the milky way , using interferometric submillimeter continuum and h@xmath1o maser observations . 
 the submillimeter continuum emission shows five 1-pc scale clumps , each of which further fragments into several 0.1-pc scale cores . 
 we identify 17 dense cores , among which 12 are gravitationally bound . among the 18 h@xmath1o masers detected , 
 13 coincide with the cores and probably trace outflows emanating from the protostars . 
 there are also 5 gravitationally bound dense cores without h@xmath1o maser detection . in total 
 the 13 masers and 5 cores may represent 18 protostars with spectral types later than b1 or potential growing more massive stars at earlier evolutionary stage , given the non - detection in the centimeter radio continuum . in combination with previous studies of ch@xmath2oh masers , 
 we conclude that the star formation in this cloud is at an early evolutionary phase , before the presence of any significant ionizing or heating sources . 
 our findings indicate that star formation in this cloud may be triggered by a tidal compression as it approaches pericenter , similar to the case of g0.253 + 0.016 but with a higher star formation rate , and demonstrate that high angular resolution , high sensitivity maser and submillimeter observations are a promising technique to unveil deeply embedded star formation in the cmz .